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Article

Synthesis and Crystal Structures of Two Crystalline Silicic Acids: Hydrated H-Apophyllite, H16Si16O40 • 8–10 H2O and H-Carletonite, H32Si64O144

by
Bernd Marler
* and
Isabel Grosskreuz
Department of Geology, Mineralogy, and Geophysics, Ruhr University Bochum, 44801 Bochum, Germany
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(4), 326; https://doi.org/10.3390/cryst14040326
Submission received: 9 March 2024 / Revised: 25 March 2024 / Accepted: 28 March 2024 / Published: 30 March 2024
(This article belongs to the Section Inorganic Crystalline Materials)
Browse Figures
Versions Notes

Abstract

:
Hydrated H-Apophyllite (HH-Apo) and H-carletonite (H-Car) were synthesized at 0 °C by leaching an apophyllite and a carletonite single crystal in a large surplus of 1.2 molar hydrochloric acid. The XRD powder patterns of HH-Apo and H-Car were indexed with space group symmetries of P4/ncc and I4/mcm and lattice parameters of a = 8.4872(2) Å, c = 16.8684(8) Å and a = 13.8972(3) Å, c = 20.4677(21) Å, respectively. The crystal structures were solved based on model building of the structures of the precursors and a physico-chemical characterization. Rietveld structure refinements confirmed the structure models. HH-Apo and H-Car are among the very few crystalline silicic acids whose structures have been determined and confirmed based on a structure refinement. The structure of HH-Apo contains thin silicate monolayers that can be regarded as constructed by rings of interconnected [SiO3OH] tetrahedra which form a puckered silicate layer. A sheet of water molecules is intercalated between the silicate layers. There are no direct hydrogen bonds between the silanol groups, but there are hydrogen bonds of different strengths between the terminal O atoms of the silicate layers and the intercalated water molecules. The 1H MAS NMR spectrum presents a strong signal at 4.9 ppm related to the aforementioned bonds and interactions between the water molecules, as well as a small signal at 22.5 ppm corresponding to an extremely strong hydrogen bond with d(O...O) ≈ 2.2 Å. The structure of H-Car is free of structural water and consists exclusively of microporous silicate double-layers with 4-connected [SiO4] and 3-connected [SiO3OH] tetrahedra in a ratio of 1:1 and a thickness of 9.2 Å. Neighboring layers are connected to each other by medium–strong hydrogen bonds with O...O distances of 2.56 Å. The structure of HH-Apo decays within several hours while H-Car is stable. A topotactic condensation reaction applied to H-Car forms an irregularly condensed silicate which still contains the layers in a distorted form as building blocks.

1. Introduction

The designation “crystalline silicic acid” is used in the literature for layered silicates with a general composition of HxSiyO2y+x/2 • w H2O. Crystalline silicic acids (CSAs) are prepared via acid leaching of layer silicates [1,2]. Synthetic materials, as well as minerals, have been used as parent materials, e.g., alkaline and alkali earth silicates like apophyllite, magadiite, kenyaite, α-Na2Si2O5, etc. The ion exchange of Na+, K+, or Ca2+ cations against H+ leads to silicic acids like H-magadiite [3], H-kenyaite [4], or disilicic acid I [5]. Pioneering work on the synthesis of crystalline silicic acids has been performed by Schwarz and Menner [5], Iler [6], Lagaly et al. [7,8], and Frondel [1]. More recently, layered silicates containing organic cations like MCM-69(p) [9] or RUB-15 [10] have also been used as precursors to produce the silicic acids H-MCM-69 and H-RUB-15, respectively. Table 1 lists some known CSAs. Only one natural silicic acid is known: silhydrite, 3 SiO2 • H2O [11,12].
The crystal structures of most crystalline silicic acids, including the ones of H-apophyllite and H-carletonite [13], however, remained unsolved due to their instability or structural disorder. So far, the structures of only three silicic acids have been solved: H-LDS [14], H-RUB-18 [15], and disilicic acid I [16], whereas the structure of H-gillespite (leached gillespite) was partly solved [17]. In some additional cases, one may speculate on the structure of the silicate layer of the CSAs, assuming that the topology of the precursor layer is retained during leaching. Examples include H-kanemite [18], which is similar to H-LDS and probably contains the kan layer; H-MCM-69 (cas layer) [9]; H-RUB-15 (sod layer) [10]; and H-magadiite (mag layer) [3]. For the designations of the layer types used in this contribution, please refer to the Database of Hydrous Layer Silicates [19].
Generally, the crystal structures of crystalline silicic acids (CSA) are characterized by a low degree of structural order, since only weak interactions exist between neighboring silicate layers. Moreover, no crystal large enough for conventional single-crystal structure analysis has ever been synthesized. Consequently, only powder diffraction diagrams of low resolution can be recorded, severely impeding the structure analysis. Reciprocal space and direct space methods fail because Sheldrick’s rule is not satisfied and because of the disorder. Still, knowledge of their crystal structures would be very helpful for the development of applications and improved synthesis strategies.
Table 1. List of crystalline silicic acids. The different layer types are designated by an acronym which refers to a known material (mineral, zeolite, or layer silicate) that contains this layer as a layer-like building unit. cas: cesium aluminosilicate zeolite, cri: cristobalite, kan: kanemite, mag: magadiite, sod: sodalite. Layer types are designated by three small, underlined letters to distinguish layer types from zeolite structure types.
Table 1. List of crystalline silicic acids. The different layer types are designated by an acronym which refers to a known material (mineral, zeolite, or layer silicate) that contains this layer as a layer-like building unit. cas: cesium aluminosilicate zeolite, cri: cristobalite, kan: kanemite, mag: magadiite, sod: sodalite. Layer types are designated by three small, underlined letters to distinguish layer types from zeolite structure types.
NameChem. CompositionParent MaterialStructure
Disilicic acid I [5,16]H2Si2O5α-Na2Si2O5cri layer
Disilicic acid II [5]H2Si2O5α-Na2Si2O6, Unknown
Disilicic acid III [20,21]H2Si2O5KHSi2O6kan layers
Disilicic acid IV [22]H2Si2O5β-Na2Si2O6,Probably kan layers
Disilicic acid V [23]H2Si2O5K2Si2O5Unknown
Disilicic acid VI [23]H2Si2O5Li2Si2O5 • 2.5 H2OUnknown
Disilicic acid (VII) [23]H2Si2O5Makatite Unknown
H-LDS [14]H2Si2O5K-LDSkan layers
H-kanemite [18]H2Si2O5KanemiteProbably kan layers
H-silinaite [24]H2Si2O5Silinaite, NaLiSi2O5 • 2H2OUnknown
Unnamed [25]H2Si2O5 • 0.7 H2ONa2CuSi4O10Unknown
Unnamed [26]H2Si3O7Na2CuSi6O14 • 5 H2OUnknown
Unnamed [23]H2Si4O9K2Si4O9Unknown
H-RUB-18 [15]H2Si4O9Na-RUB-18Stacking disordered rwr layers
β-H-RUB-18 [15] H2Si4O9Na-RUB-18 Unknown
H-octosilicate [27]H2Si4O9Na2[Si8O16(OH)2] • 8 H2OProbably rwr layers
Unnamed [7]H2Si4O9 • 1.1 H2ONa2Si4O9*5 H2OUnknown
Leached hydr. gillespite [13,17]H4Si4O10 • 0.5 H2OGillespite, BaFeSi4O10Stacking disordered aco layers
Unnamed [23]H2Si8O17 • 0.5 H2ONa2Si8O17 • x H2OUnknown
Unnamed [6]H2Si8O17 •1.1 H2OK2Si8O17 • x H2OUnknown
H-magadiite [3]H2Si14O29 • 5.4 H2OMagadiiteUnknown
Unnamed [28]H2Si14O29 • 5 H2OK2Si14O29 • 5 H2OUnknown
H-kenyaite [4]H2Si20O41 • x H2OKenyaiteUnknown
Unnamed [29]H2Si20O41 • x H2OK2Si20O41 • x H2OUnknown
H-MCM-69 [9]ca. [H2Si6O13] MCM-69(p)Probably cas layers
H-RUB-15 [10][H16Si24O40] • x H2ORUB-15Probably sod layers
Silhydrite [12]ca. 3 SiO2 • H2O-Unknown
Hydrated H-apophylliteH16Si16O40
• 8–10 H2O		Apophyllite, (K,Na)Ca4[Si8O20(F(/OH)]•8 H2OThis study
H-carletoniteH32Si64O144Carletonite, KNa4Ca4[Si8O18(CO3)4(OH,F)]•H2OThis study
To a small extent, CSAs find applications in the pharmaceutical industry or as adsorbents for gas chromatography. The acid H2Si20O41 • x H2O, for example, exhibits some outstanding gas adsorption properties [30].
The CSAs can be modified in many ways. An overview of the properties and modifications of CSAs was provided by Lagaly and Schwieger [31]. These modifications comprise (i) the intercalation of polar organic molecules like formamide, urea, various amines, dimethylsulfoxide, various alcohols, ethylene glycol, or glycerol; (ii) the ion exchange of H ⇔ alkali cations or organic cations, (iii) the intercalation of organic surfactants, or even the delamination of CSAs through the use of surfactants; (iv) esterification with longer-chain alcohols, forming alkoxy groups at the inner surface of the CSA; (v) grafting reactions to generate covalent bonds between the silanol groups of the layer and additional compound. Typically, chlorosilanes (e.g., (CH3)2SiCl2) or disilizanes, e.g., [(CH3)3Si]NH, are used. A more recent example was presented by Nomi et al. The authors used 4-phosphono-phenylsilane to modify the inter-layer region of H-RUB-18 via a grafting reaction [32].
Moreover, the CSAs can be used as precursors to produce crystalline microporous materials by either condensing the silicate layers topotactically through heating or by expanding the inter-layer region between layers by introducing covalently bonded inorganic linkers.
When striving for a topotactic condensation of a layered silicate, the CSAs serve as intermediates to form a microporous silica framework in which acid leaching of a precursor—a layer silicate containing Na+ or an organic cation—produces a CSA. The subsequent intercalation via a suitable organic molecule helps to control the stacking sequence of layers through the condensation process with the aim of producing a structurally ordered framework silicate. The condensation is achieved by heating the intercalated CSA to about 500–600 C. Table 2 lists some successful transformations of this kind.
The periodic expansion of a layered silicate containing, e.g., mww-type or ferrierite-type layers by interconnecting neighboring silicate layers via small “pillars” was introduced by Tatsumi et al. (2007–2009), who used the new method of an “interlayer expansion reaction” [36,37,38]. Later, other research groups performed similar reactions e.g., [39]. Typically, diethoxydimethylsilane or dichlorodimethylsilane are used as silylation agents to react with the silanol groups of neighboring layers. The expansion reaction proceeds only under acidic conditions—e.g., HCl or HNO3 has to be added. During the silylation, the acid enhances the extraction of the (organic) cation residing in the interlayer region, supports the hydrolysis of the silylating agent (C2H5-O-Si(CH3)2-O-C2H5 + 2 H2O => HO-Si(CH3)2-OH + 2 C2H5OH), and protonates the terminal ≡Si-O¯ groups of the layers (=> ≡Si-OH). Eventually, a reaction occurs:
  • ≡Si-OH.…HO-Si(CH3)2-OH….HO-Si≡ => ≡Si-O-Si(CH3)2-O-Si≡            + 2 H2O
    (Layer)  (Silylating agent)  (Layer)  (Interconnected pair of layers)
Because of the protonation of the ≡Si-O¯ groups, the layered precursor may—at least locally and in a state of transition—be considered a CSA. Already, in 2002, Thiesen et al. used a very similar silylating agent, dimethyldimethoxysilane (DMDMS), among other alkylmethoxysilanes on a kenyaite-like hydrous potassium silicate [40]. In this case, the authors chose a two-step procedure: the corresponding silicic acid was prepared and subsequently modified with dimethyldimethoxysilane. The authors state: “almost all surface silanol groups reacted with these molecules.” The study, however, did not reveal whether the silylation reaction attached the molecule only to one particular silicate layer or whether it connected neighboring layers.
To improve and extend condensation and expansion reactions via CSAs as described above, it is very helpful to analyze the structures of CSAs if possible. Here, we report on the synthesis of two crystalline silicic acids and their crystal structures which have been evaluated for the first time: H-apophyllite and H-carletonite. A part of this study was presented as a poster at the 23rd Annual Conference of the German Crystallographic Society [41]. These CSAs were first synthesized in 1979 [1] and 1980 [13], respectively, but at that time, their structures could not be determined. A detailed study on some properties of H-apophyllite was published in 1998 [42]. The structures of the precursors apophyllite, (K,Na)Ca4[Si8O20(F/OH)] • 8 H2O, [43], and carletonite, KNa4Ca4[Si8O18(CO3)4(OH,F)] • H2O [44] are displayed in Figure 1. Apophyllite is tetragonal and constructed from corrugated silicate mono-layers which contain exclusively 3-connected [SiO3O−] tetrahedra (Q3 silicon). To compensate for the negative charge of the silicate layers and the additional OH−/F− anions, Na+, K+, and Ca2+ cations occupy the inter-layer region, together with structural water. In contrast, carletonite contains thicker, more complex silicate double-layers which consist of 4-connected [SiO4] tetrahedra (Q4 silicon) and [SiO3O−] tetrahedra (Q3 silicon) in a ratio of 1:1. Carletonite, too, is tetragonal and contains one silicate layer, three carbonate layers, OH−/F− anions, and Na+, K+, and Ca2+ cations in one unit cell. Very little water is present. The alkali and alkaline earth cations located between the respective silicate layers stabilize the structures via ionic bonds. Different from many layer silicates (clay minerals, mica) which form pronounced plate-like crystals, apophyllite and carletonite grow as isometric crystals due to the strong interactions along the stacking direction of the layers.

2. Materials and Methods

2.1. Synthesis

Hydrated H-apophyllite and H-carletonite (abbreviated in the following as HH-Apo and H-Car) were synthesized by leaching natural apophyllite, KCa4[Si8O20(OH/F)] • 8 H2O, from Poona, India, and carletonite, KNa4Ca4[Si8O18(CO3)4(OH,F)] • H2O, in a large surplus of an acid for 3 days. The synthesis experiments were performed at 0 °C and 20 °C, leaching the precursor either as a powdered material or as a single crystal. For the leaching, four different acidic solutions were used: 0.1 M, 1.2 M, and 10 M hydrochloric acid and 10 M acetic acid. For the treatments at 0 °C, the solid precursor and the acid were kept in a closed glass bottle which was placed in an ice-water mixture for 3 days without agitation. Table 3 lists the synthesis conditions and the obtained products.

2.2. General Characterization

For the characterization and the structure analysis, CSAs obtained from single crystals were used. The apophyllite crystal converted into hydrated H-apophyllite shown in Figure 2 was a colorless, translucent cube with edge lengths of 6 mm. The carletonite crystals were of a dark blue color and had an isometric but irregular shape and diameters of approx. 8 mm.
The thermal properties were investigated via simultaneous DTA/TG measurements using a Bähr STA-503 thermal analyzer. The samples were heated in synthetic air from 30 to 1000 °C with a heating rate of 10 °C/min.
Fourier transform infrared (FTIR) spectroscopy was performed using a Nicolet 6700 FT-IR spectrometer equipped with an ATR unit. The first spectrum of hydrated H-apophyllite was recorded immediately (10 min) after the sample was separated from the acid and dried in an air flow. All spectra were recorded in air between 400 and 4000 cm−1 with a resolution of 4 cm−1. Successive spectra were recorded after 1, 2, and 24 h. Since H-carletonite is stable at ambient conditions (at least for several years), only one measurement was performed.
Solid-state MAS NMR spectra were recorded at room temperature using a Bruker ASX-400 spectrometer using standard Bruker MAS probes. In order to average the chemical shift anisotropies, the samples were spun about the magic angle. In all cases, tetramethylsilane (TMS) was used as a chemical shift standard. In the case of carletonites, 29Si high-power decoupling (hpdec) measurements were performed to calculate the silicon Q3:Q4 ratio based on the signal intensities. Cross polarization (1H-29Si CP) measurements were performed to reduce the time of data collection.

2.3. Structure Analysis

The crystals of HH-Apo and H-Car were too small and had too low of a scattering power to analyze their structure using conventional single-crystal experiments. Instead, powder XRD data were recorded using a Siemens D5000 powder diffractometer in modified Debye–Scherrer geometry using CuKα1 radiation (λ = 1.54059 Å). The samples were sealed in borosilicate glass capillaries with a diameter of 0.3 mm to avoid loss or uptake of water by the samples. The diffractometer was equipped with a curved germanium (111) primary monochromator and a Braun linear position-sensitive detector (2θ coverage = 6°). Several ranges were recorded. In the case of HH-Apo, data were collected only up to 60° 2θ and within a short time (one hour) because of ongoing condensation. The structure models were refined using the FullProf 2K program [45,46] with scattering factors as implemented. No absorption correction was necessary. In both cases, six additional parameters had to be used to account for the anisotropic peak halfwidths.
Soft distance restraints were used for the atoms of the silicate layers: d(Si-O) = 1.610 (5) Å, d(Si...Si) = 3.10 (3) Å, and d(O...O) = 2.62 (3) Å. The isotropic displacement parameters B(iso) for identical elements like silicon (H-Car) or oxygen atoms were constrained to be equal. The hydrogen atoms of the water molecules could not be located. The omission of hydrogen atoms—although very weak scatterers—has, nevertheless, a significant negative influence on the Rietveld refinement. To compensate for this, the contribution of the hydrogen atoms was covered by using the scattering factor of the oxygen anion (O2−) to represent a water molecule, assuming that water has predominantly ionic bonds between O and H.
The atomic coordinates and unit cell dimensions of a hypothetical “condensed H-carletonite” were optimized via distance-least-squares (DLS) refinement [47]. The optimization was performed using prescribed distances of Si-O = 1.594 Å, O-O = 2.603 Å, and Si...Si = 3.079 Å; prescribed angles of Si-O-Si = 150 °C; and regular O-Si-O tetrahedra angles = 109.5°. The prescribed distances were taken from Baur and Fischer ([48], Table 1), derived as the average values for zeolites with pure SiO2 frameworks.

3. Results and Discussion

3.1. Synthesis of Hydrated H-Apophyllite and H-Carletonite

Among the synthesis runs, only the one at 0 °C with 1.2 molar HCl led to well-crystalline-hydrated H-apophyllite. As a by-product, a small amount of K2SiF6 was formed. Leaching experiments with 10 molar acetic acid or 0.1 molar hydrochloric acid led to nearly unaltered apophyllite. The leaching run with 10 molar HCl formed an amorphous product. An isometric single crystal of apophyllite with a diameter of approx. 6 mm transformed into an aggregate of very thin, colorless HH-Apo crystals of rectangular shape (Figure 2). These plate-like crystals were separated from the liquid, washed with icy water, and quickly dried in an air stream at room temperature. If a powdered apophyllite sample was used instead of a single crystal, only amorphous material was obtained (see Table 4).
The exchange of Na+, K+, and Ca2+ against H+ during the leaching of apophyllite leads to an expansion of the structure. The c-value representing the stacking direction of layers of HH-Apo (16.87 Å) is considerably larger than the value of the parent phase (15.77 Å). The Ca2+ cations of apophyllite form strong ionic bonds with the terminal Si-O- groups of the layers, keeping them close to each other. In HH-Apo, the hydrated layers are interconnected only by weak hydrogen bonds. An expansion of apophyllite crystals during acid leaching has already been reported by Aldushin et al. [49], who used atomic force microscopy to continuously observe the change of the crystal morphology.
H-Car was obtained at 0 °C and at 20 °C from carletonite single crystals or powdered material (see Table 4). After leaching, the fine-grained crystals were separated from the liquid washed, with distilled water, and dried overnight at room temperature.
In contrast to apophyllite, carletonite with a repeat unit of layers of 16.695 Å (one layer per unit cell) [44] shrank during acid leaching to a repeat unit of 10.234 Å (two layers per unit cell with c = 20.468 Å) since cations and water molecules are removed between silicate layers to form (dehydrated) H-Car.

3.2. Characterization

Thermal analyses (DTA and TG) of freshly prepared hydrated H-apophyllite shows (see Figure 3, left) a first step of weight loss (21.1%) between room temperature and ca. 125 °C, which is assigned to adhesive water (slightly moist sample) and structural water. It was not possible to start the analysis with a dry sample, since during the drying process, condensation would take place. Therefore, the amount of structural water cannot be determined based on the TG measurement. The second step, which extends to a quite high temperature (weight loss: 9.6% corresponding to 13.0% of the dry and dehydrated HH-Apo), is assigned to water formed by the condensation of Si-OH groups. The calculated weight loss of idealized anhydrous HH-Apo (H16Si16O40 => 16 SiO2 + 8 H2O) would be 12.2%. The two main steps of weight loss are accompanied by two endothermic peaks with maxima at 100 °C and 205 °C. After heating to 1025 °C, the remaining silica material is amorphous.
The heating of HH-Car in the dry air led to a total loss of water of 6.5% (Figure 3, right). The condensation of the silanol groups occurs in two steps: one at a very low temperature (minimum of the corresponding DTA curve at ca. 80 °C) with a weight loss of 2.0%, and another between ca. 300 °C and 600 °C, resulting in a weight loss of 4.5%. The minimum of the corresponding DTA curve appears at ca. 320 °C. The calculated percentage of water which can be generated via condensation according to the formula of H-Car, as determined by the structure analysis (H32Si64O144 => Si64O128 + 16 H2O), is 6.9%.
The ATR-FTIR spectrum of freshly prepared HH-Apo is presented in Figure 4 (top), with a tentative assignment of absorption bands. The spectrum indicates that the fresh material contains a large amount of water represented by very broad and intense absorption bands around 3300 cm−1, ν(H2O), and around 1640 cm−1, δ(H2O). Such signals are typical for water molecules interconnected by hydrogen bonds of different strengths, similar to liquid water.
The observed bands in the region between 400 and 1300 cm−1 are tentatively assigned according to Huang et al. [50] as follows: the bands at 1167 and 1068 cm−1 represent the asymmetric stretching vibration of Si-O-Si units, and the bands at 965 and 929 cm−1 can be ascribed to the asymmetric stretching vibration of terminal Si-OH groups. Due to the high amount of silanol groups in the structure, these signals are clearly detectable. The two weak bands at about 750 cm−1 are assigned to symmetric stretching vibrations of the silicate layer, the band at 668 cm−1 is ascribed to the symmetric stretching vibration of Si-OH groups, and the signal at 411 cm−1 represents the bending vibrations of Si-O-Si units. Other weak signals cannot be assigned to specific units of the structure.
The FTIR spectrum indicates that H-Car (Figure 4, bottom) contains considerably less water than HH-Apo (see the OH stretching vibrations around 3200 cm−3, which are believed to stem from Si-OH groups possessing hydrogen bonds of different strengths between each other). In particular, the OH bending vibration at 1630 cm−1 is very weak. The observed bands in the fingerprint region between 400 and 1300 cm−1 are quite similar to those of HH-Apo but are less numerous. The bands are assigned in analogy to HH-Apo as follows: the strong band at 1079 cm−1 represents the asymmetric stretching vibration of Si-O-Si units, and the medium strong bands at 943 cm−1 and at 668 cm−1 are assigned to the symmetric and asymmetric stretching vibration of terminal Si-OH groups, respectively. The band at 810 cm−1 can be ascribed to symmetric S-O-Si stretching vibrations of the silicate layer. The signal at 431 cm−1 is related to the bending vibrations of Si-O-Si units. No other significant signals are visible.
The 29Si MAS NMR spectrum of freshly prepared HH-Apo (Figure 5, left) shows only one broad signal at δ −103.0 ppm. The signal is assigned to Q3-type silicon, i.e., the silanol-groups typical of layered silicates. Any signal in the range of approx. −105 to −120 ppm (typical for Q4-type silicon) is missing. The 29Si NMR spectrum, however, changes with time. Therefore, only a short measurement was possible (66 min). The corresponding decay of the structure of HH-Apo is presented in Section 3.4.
In contrast, the 29Si MAS NMR spectrum of H-Car (Figure 5, right) shows two sharp signals at δ = −106.0 ppm (Q3-type silicon) and −114.8 ppm (Q4-type silicon) of approximately similar intensity. The intensity ratio is a first indication that the car layer possessing a Q3-:Q4- ratio of 1 has survived the acid leaching.
The 1H spectrum of freshly prepared HH-Apo (Figure 6, left) displays two signals: a strong one at 4.9 ppm attributed to the protons of the water molecules and the silanol groups, and a weak, sharp signal at 22.5 ppm corresponding to a very strong hydrogen bond.
Based on correlations connecting the isotropic chemical shift value of a 1H NMR signals with the O...O distance (in pm) of hydrogen-bonded OH groups or H2O molecules [51,52], a shift of 22.5 ppm corresponds to an O...O distance of about 2.22 Å. This distance correlates approximately with the distance between the oxygen atoms of the two nearest water molecules of the square of four possible water positions, as determined by the structural analysis (see Section 3.3.4). Depending on the particular topology of the silicate layer and the distance between neighboring layers, strong hydrogen bonds with distances between the oxygen atoms of the terminal silanol groups as short as 2.3 Å ([53], Table 3) have been observed in several layered silicates. Still, strong hydrogen bonds did not involve water molecules.
The 1H NMR spectrum of H-Car (Figure 6, right) shows a sharp, strong signal of the silanol groups at 4.8 ppm and an asymmetric, broad signal between ca. 5 and 17 ppm due to hydrogen bonds of different strengths.

3.3. Structural Analysis

3.3.1. Analyzing the PXRD Patterns

The powder XRD diagram of HH-Apo and H-Car show fairly sharp reflections with indices (hk0) and broadened reflections with indices (h0l) and (hkl), indicating a well-ordered geometry within the a-b-plane (layers) but a certain degree of stacking disorder along the c-axis. The fact that (00l) reflections are also broadened might be due to the plate-like morphology of the crystals. A deviation from a strictly regular layer stacking along [001] with slight shifts that are perpendicular to the stacking direction is typical for layered silicates with only weak bonding interactions between neighboring silicate layers. Still, the reflections were unambiguously indexed based on tetragonal lattices. Refinement of the lattice parameters led to a = 8.4872(2) Å, c = 16.8684(8) Å (HH-Apo), and a = 13.8972(3) Å, c = 20.4677(21) Å in the space group symmetry I4/mcm (H-Car). In the case of HH-Apo, systematic extinctions revealed two c-glide planes, limiting the number of possible space groups to P4cc, P4/mcc and P4/ncc. It was, however, not possible to unambiguously confirm or exclude the presence of an n-glide plane because of the small number of possibly observable reflections. The structure, therefore, was refined in all three space groups.
For comparison, the unit cell dimensions of apophyllite (space group symmetry P4/mnc) are a = 8.965 Å and c = 15.767 Å; and the structure of carletonite, again, is tetragonal, with symmetry P4/mbm and lattice parameters a = 13.178 Å and c = 16.695 Å.

3.3.2. Structure Determination

The structures of hydrated H-apophyllite and H-carletonite were solved via model building based on a comparison with the structures of the precursors and the results of the general characterization. The particular synthesis procedure as used to obtain HH-Apo and H-Car, the NMR spectra (Figure 5 and Figure 6) and the FTIR spectra (Figure 4) were indicative of layered silicates. Since the precursors and the corresponding leached materials possess similar lattice parameters (a) perpendicular to the stacking direction of layers, it was assumed that the silicate layers are intact in both HH-Apo and H-Car. Consequently, the structure models were constructed, with silicate layers having the same topology as the layers of apophyllite and carletonite, respectively. The models, however, had to obey the particular space group symmetry and the unit cell dimensions, which deviate from those of the precursors. The distances between successive layers were calculated based on the c parameters. The respective stacking of silicate layers with approximate shift vectors between successive layers were arranged to meet the space group symmetries of HH-Apo and H-Car, respectively. In the case of HH-Apo, the thermal analysis and the FTIR spectrum indicate that it accommodates structural water between the silicate layers. The structure model was, therefore, completed by oxygen atoms placed at random in the inter-layer region to account for the water molecules. In the starting model, the dummy oxygen atoms representing water molecules were given a very high displacement parameter. Since the thickness of an aco layer is only 5.0 Å, while the c parameter is 16.8684 Å, it was concluded that the unit cell of HH-Apo contains two silicate layers and two additional water sheets with a thickness of approx. 3.3 Å.

3.3.3. Structure Refinement

The somewhat vague starting model of the structure of HH-Apo was, nonetheless, sufficient to refine the structure. The true positions of the O atoms of the water molecules were determined via an iterative process of refining the atomic coordinates and calculating difference Fourier maps. The best refinement of the structure was obtained in space group P4/ncc, which converged to RF = 0.028, Chi2 = 1.23 and led to meaningful bond lengths and angles. In space group P4/ncc, the structure of HH-Apo has only one symmetrically independent Si site, as indicated by the 29Si MAS NMR spectrum. The refinement of the structure of H-Car in space group I4/mcm converged to residual values of RF = 0.025 and Chi2 = 4.14, confirming the structure model. The details of the data collection and the results of the structure refinement are summarized in Table 5.

3.3.4. Descriptions of the Structures

  • The structure of Hydrated H-apophyllite
According to the structure analysis, TG, and NMR spectra, the unit cell content of hydrated H-Apophyllite is H16Si16O40 • 8–10 H2O. There is only one symmetrically independent Si site, three oxygen sites, and one water site in the structure. The distances between the atoms vary in the following ranges: d(Si-O) = 1.57 Å to 1.61 Å, d(Si...Si) = 3.08 Å to 3.15 Å, d(O...O) = 2.52 Å to 2.71 Å. The atomic coordinates, displacement parameters, and occupancy factors are presented in Table 6.
The structure model of HH-Apo is characterized by a thin silicate monolayer (aco layer) consisting of 4- and 8-rings and having a thickness of 5.0 Å (not counting the van-der-Waals radii of the oxygen atoms). The layer is made up of 4-rings, which point alternatively up and down and form a puckered silicate layer with only 3-connected [SiO3OH] tetrahedra. Exclusively Q3-type Si atoms (Figure 7) characterize the structure. The acronym “aco” [19] is derived from the zeolite framework type ACO, which contains aco layers as layer-like building blocks [54]. Interconnecting the aco layers generates the complete ACO framework. The stacking of layers along the c-axis in HH-Apo differs from the stacking in apophyllite, with every second layer being shifted by 1/2a + 1/2b (compare also Figure 1).
A sheet of hydrogen-bonded water molecules arranged in partially occupied 4-rings is intercalated between the silicate layers (Figure 7). There are no direct intra-layer hydrogen bonds between the silanol groups, and the shortest distance between such groups is 3.75 Å. Also, the inter-layer distances -OH...HO- are too large (>4.0 Å) to form hydrogen bonds. Instead, there are medium–strong hydrogen bonds between the silanol groups of the silicate layers and adjacent water molecules (d(O...O) ≈ 2.6 Å). Weak hydrogen bonds interconnect the water molecules with silanol groups of the opposite silicate layer (d(O...O) = 3.04 Å), establishing only weak interactions between the building blocks stacked along the c-axis.
According to the structural analysis, the water site resembling a square of four possible water positions (Figure 8, right) has an occupancy factor of 0.622(5), indicating that the water positons are only partly occupied. Because of the limited diffraction dataset, this occupancy factor is also of limited reliability. The shortest interatomic distances between the oxygen atoms of the water molecules are 1.98(2) Å (edge of the square), while the diagonal distance has a length of 2.80 Å. Given an occupancy factor of 0.5, every second water position would be occupied, allowing for molecules to be located in opposite positions at a distance of 2.80 Å but statistically distributed about the two possible pairs in the 4-ring throughout the crystal. An ordered arrangement of water molecules would require a superstructure with a doubling of either the a or the c parameter. This cannot be confirmed by the diffraction data set, which is of limited quality. One may speculate that since the occupancy factor is slightly higher than 0.5 and since the 1H NMR spectrum indicates the presence of very short hydrogen bonds with an O...O distance of 2.22 Å, a few more water molecules are “squeezed” into the water sheet in the freshly prepared HH-Apo. The number of water molecules in the structure is, therefore, specified as 8 to 10 per unit cell.
The hydrous leached gillespite (HL-Gil) analyzed by A. Pabst with the composition of H16Si16O40 + 2 H2O [17] is a less-hydrated relative of HH-Apo, H16Si16O40 • 8–10 H2O. HL-Gil and HH-Apo contain the same aco silicate layer and the same chemical composition per unit cell, except for the number of water molecules. A comparison of the lattice parameters, HH-Apo: a = 8.4872(2) Å, c = 16.8684(8) Å and HL-Gil: a = 7.64 Å, c = 15.10 Å [17], shows the aco layer in HL-Gil to be more corrugated (i.e., “compressed” along the a and b axes) than that in HH-Apo, possibly because of fewer ≡Si–OH...H2O interactions and stronger intra-layer ≡Si–OH...HO-Si≡ interactions of the terminal silanol groups.
  • The structure of H-carletonite
The structure of H-Car is free of structural water and consist only of so-called car silicate layers (see Figure 9) built from 4-, 6-, and 8-rings. These layers are microporous double-layers with a thickness of 9.2 Å and represent two aco layers which are fused to each other. The fusion generates a layer with 4-connected [SiO4] and 3-connected [SiO3OH] tetrahedra in a ratio of 1:1. Channel-like pores (Figure 9, right) limited by 8-rings of SiO4 tetrahedra and running along [001] possess free diameters of 3.3 Å × 3.3 Å. The shortest distances between the oxygen atoms of the terminal silanol groups of a given layer have a length of 4.08 Å, excluding intra-layer hydrogen bonds. Neighboring layers, however, are connected to each other by medium–strong hydrogen bonds with O...O distances of 2.56 Å (see Figure 9). The unit cell comprises two car layers with only two symmetrically independent Si sites and six independent oxygen sites in the structure. The distances between atoms vary, in the ranges of d(Si-O) = 1.60 Å to 1.67 Å, d(Si...Si) = 3.11 Å to 3.17 Å, and d(O...O) = 2.42 Å to 2.85 Å. The atomic coordinates, displacement parameters, and occupancy factors are listed in Table 7.

3.4. Condensation of Silicate Layers

The silicate layers of HH-Apo and H-Car can (formally) be condensed through a reaction of the terminal silanol groups according to ≡Si-OH + HO-Si≡ → ≡Si-O-Si≡ + H2O to form microporous framework structures. In both cases, adjacent layers have to be shifted by a/2 + b/2 relative to each other in order to mutually interconnect all silanol groups of neighboring layers.
In the case of HH-Apo, this formal and regular condensation would generate the framework structure of the known zeolite type ACO [54]. However, HH-Apo is unstable, even at room temperature. Accompanied by a rapid loss of water (see Figure 10), the structural order decreases, eventually forming a nearly amorphous material (Figure 11).
The 29Si MAS NMR spectrum of HH-Apo recorded 48 h after preparation clearly shows a Q4-type Si signal at −112.0 ppm (generated by the condensation) in addition to a Q3-type signal of about the same intensity at −103 ppm (Figure 12). It was not possible to achieve the condensation that forms the ACO zeolite framework. Instead, an irresistible condensation occurs starting very soon after the HH-Apo has been removed from the cold (0 °C) acid, randomly interconnecting neighboring silanol groups to Si-O-Si bridges. Finally, a completely disordered silicic acid is formed with 4- and still some 3-connected SiO4 tetrahedra.
Unlike HH-Apo, H-Car is stable for several years at room temperature. The car layers in H-Car can formally be interconnected (condensed) to form a fully connected microporous framework composed of SiO2. The tetragonal framework is characterized by channel-like pores running along [001]. The channel cross-section is limited by 8-rings of SiO4 tetrahedra with free diameters of approx. 3.3 Å × 3.3 Å. The channel walls are completely made up of 6-rings. This hypothetical “condensed carletonite” represents an all-silica zeolite possessing a one-dimensional pore system (Figure 13). The distance-least-squares refinement on this framework with space group symmetry I4/mcm converged to an R value of 0.00298, demonstrating that this type of framework topology consisting of car layers as building blocks is feasible. Nearly undistorted angles and ideal interatomic distances (Si-O: 1.5917–1.5936 Å, O...O = 2.5935–2.6050 Å, and Si...Si: 3.0915–3.1440 Å) were calculated.
To achieve this condensation, layered H-Car was heated in air at 600 °C for 18 h. The calcined material (calc-H-Car) was of a low structural order. A comparison of the PXRD patterns of these two materials showed a few moderately sharp (hk0) reflections of calc-H-Car, proving that car layers still exist in a distorted form as building blocks in an irregularly condensed silicate (see Figure 14).
To obtain a structurally ordered “condensed carletonite,” H-Car (0.1 g) was intercalated with ethylenediamine (en) prior to the condensation experiment by refluxing the sample in en (20 mL) at 120 °C for 2 h. The presence of suitable organic molecules/cations between the silicate layers of CSAs is helpful in controlling the stacking sequence of the layers. This synthesis route has already been used to successfully condense CSAs (see Table 2).
The en-containing sample, en-H-Car, was heated in air at 600 °C for 18 h to achieve condensation. The transformed material, denoted calc-en-H-Car, was analyzed using diffraction and NMR spectroscopy. The PXRD pattern of the calc-en-H-Car was indexed based on the symmetry (I4mc) and the unit cell dimensions of the DLS-optimized “condensed carletonite.” A subsequent LeBail fit led to the parameters a = 13.464 Å and c = 8.807 Å. Strong, moderately sharp reflections are of the (hk0) type; all other reflections are broad and very weak, indicating that the structure is fairly ordered in the a-b plane (the plane of the car layers) but disordered with respect to layer stacking (see Figure 15).
The 29Si MAS NMR spectrum of calc-en-H-Car (Figure 16) presents one broad and asymmetric peak with a barycenter at −109.5 ppm that can best be fitted by two signals: one assigned to Q4-type silicon at −110.9 ppm with an analytically integrated intensity of 85.5%, and another signal assigned to Q3-type silicon at −102.0 ppm with 14.5% intensity. Although the condensation of silanol groups led to a considerable increase in Q4-type silicon compared to H-Car that has a Q4:Q3 ratio of 50%:50%, the condensation is not complete and resulted in a disturbed structure.
The 1H spectrum of calc-en-H-Car (not shown) contains only one broad signal centered at 4.6 ppm, indicative of hydrogen bonds of different strengths between the remaining silanol groups. The condensation of H-Car improved when using ethylenediamine-loaded en-H-Car as the precursor instead of pure H-Car, but the structure of the condensed material was still disordered. A structure analysis, therefore, failed.

4. Conclusions

HH-Apo and H-Car are rare examples of crystalline silicic acids that allow us to solve and refining their crystal structures. The classical Rietveld analyses conducted in this study can only describe an idealized (or average) structure. The fact that both materials, HH-Apo and H-Car, show PXRD patterns with moderately sharp and broadened reflections is indicative of a slight stacking disorder of the layer arrangement. It is obvious that the structural analysis of HH-Apo is of only restricted quality due to the relatively fast decay of the structure. Only a limited diffraction dataset was successfully recorded. However, since only very few structural analyses of CSAs are available in the literature, the analyses presented in this paper are a useful replenishment, shedding light on this class of materials, their unique properties, and their applications as precursors in condensation reactions.
Structural characterization of crystalline silicic acids is important in order to realize the layer arrangement and hydrogen bonding system. This is particularly relevant if the CSAs are made to serve as parent materials for the synthesis of porous silicates that might be useful as molecular sieves (silica zeolites) or functionalized IEZs.
HH-Apo’s instability is probably caused by the thin silicate mono-layers constructed of only 3-connected [SiO3OH] tetrahedra, while stabilizing 4-connected [SiO4] tetrahedra are missing. It is assumed that the low thickness and the flexibility of the layers provoke a random condensation of silanol groups, uncompelled even at room temperature.
In contrast to HH-Apo, H-Car is stable. It is, nevertheless, difficult to form the related microporous silica framework through condensation. The reason for the observed partly randomly occurring condensation is attributed to the necessary shift in layers. For the complete condensation of layers to form the ordered structure of “condensed carletonite,” every second car layer has to be shifted by a/2 + b/2. This corresponds to a length of 9.9 Å, which is likely too long under the conditions chosen here (intercalation of ethylenediamine). The intercalated organic molecule must support a shift that rearranges the layer stacking favorably and allows for subsequent regular condensation. This has successfully been demonstrated, as shown in Table 2. In some fortunate cases, a layer of silicate in its as-made form already contains a suitable molecule to achieve perfect condensation by merely heating the precursor, e.g., layered PREFER => zeolite ferrierite [55] or layered Nu-6(1) => zeolite Nu-6(2) [56]. Transformation of H-Car into an ordered zeolite structure is possible because of stable car layers if such a suitable molecule/cation is recognized. Many more experiments are necessary to discover the proper molecule needed to transform H-Car into “condensed carletonite,” which is beyond the scope of this study. So far, there has been no straightforward procedure to identify such proper molecules to be intercalated between layers of a particular topology and specific interlayer region.

Author Contributions

B.M.: conceptualization, formal analysis, investigation, funding acquisition, writing—final version. I.G.: investigation, writing—original draft, visualization. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Deutsche Forschungsgemeinschaft under grant number MA 6641/3-1.

Data Availability Statement

Cif files have been deposited at the Cambridge Crystallographic Data Centre (CCDC) with numbers: 2341770 (hydrated H-apophyllite) and 2341771 (H-carletonite).

Acknowledgments

The authors thank Alexandra Bruns, Bochum, Germany, for help with the synthesis experiments. Michael Fechtelkord, Bochum, Germany, and A. Bruns kindly recorded the NMR spectra. We also thank the anonymous reviewers for their instructive comments, which helped to improve the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 14 00326 g001
Figure 1. The structures of apophyllite (left) and carletonite (right) projected on (010). [SiO4] = blue tetrahedra, CO3 = brown triangles, O (bridging) = red, O (terminal) = blue, K = purple, Ca = orange, Na = yellow, OH/F = gray, water = light blue.
Figure 1. The structures of apophyllite (left) and carletonite (right) projected on (010). [SiO4] = blue tetrahedra, CO3 = brown triangles, O (bridging) = red, O (terminal) = blue, K = purple, Ca = orange, Na = yellow, OH/F = gray, water = light blue.
Crystals 14 00326 g001
Crystals 14 00326 g002
Figure 2. Thin, plate-like HH-Apo crystals obtained via acid leaching of a single crystal.
Figure 2. Thin, plate-like HH-Apo crystals obtained via acid leaching of a single crystal.
Crystals 14 00326 g002
Crystals 14 00326 g003
Figure 3. DTA (blue) and TG (black) curves of HH-Apo (left) and H-Car (right).
Figure 3. DTA (blue) and TG (black) curves of HH-Apo (left) and H-Car (right).
Crystals 14 00326 g003
Crystals 14 00326 g004
Figure 4. The ATR-FTIR spectra of freshly prepared HH-Apo (top) and H-Car (bottom).
Figure 4. The ATR-FTIR spectra of freshly prepared HH-Apo (top) and H-Car (bottom).
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Crystals 14 00326 g005
Figure 5. 29Si MAS NMR spectra of freshly prepared HH-Apo (left) and of H-Car (right).
Figure 5. 29Si MAS NMR spectra of freshly prepared HH-Apo (left) and of H-Car (right).
Crystals 14 00326 g005
Crystals 14 00326 g006
Figure 6. The 1H spectra of HH-Apo (left) and H-Car (right).
Figure 6. The 1H spectra of HH-Apo (left) and H-Car (right).
Crystals 14 00326 g006
Crystals 14 00326 g007
Figure 7. The structure of hydr. H-Apophyllite projected on (100). Blue tetrahedra represent the silicate layers, red spheres represent bridging oxygen atoms, small blue spheres represent terminal OH groups, and large light blue spheres represent the water molecules.
Figure 7. The structure of hydr. H-Apophyllite projected on (100). Blue tetrahedra represent the silicate layers, red spheres represent bridging oxygen atoms, small blue spheres represent terminal OH groups, and large light blue spheres represent the water molecules.
Crystals 14 00326 g007
Crystals 14 00326 g008
Figure 8. Left: the aco silicate layer (from the top); right: the sheet of intercalated water molecules.
Figure 8. Left: the aco silicate layer (from the top); right: the sheet of intercalated water molecules.
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Crystals 14 00326 g009
Figure 9. The structure of H-Car projected on (110) (left) and on (001) (right) showing the microporosity (8-rings). Hydrogen bonds = light blue lines.
Figure 9. The structure of H-Car projected on (110) (left) and on (001) (right) showing the microporosity (8-rings). Hydrogen bonds = light blue lines.
Crystals 14 00326 g009
Crystals 14 00326 g010
Figure 10. ATR-FTIR spectra of hydr. H-Apophyllite: freshly prepared (blue, bottom), after 1 h (orange, middle), and after 24 h (purple, top).
Figure 10. ATR-FTIR spectra of hydr. H-Apophyllite: freshly prepared (blue, bottom), after 1 h (orange, middle), and after 24 h (purple, top).
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Crystals 14 00326 g011
Figure 11. Powder-XRD patterns of hydr. H-Apophyllite: freshly prepared (blue, top), after 12 h (red, middle), and after ca. 48 h (green, bottom).
Figure 11. Powder-XRD patterns of hydr. H-Apophyllite: freshly prepared (blue, top), after 12 h (red, middle), and after ca. 48 h (green, bottom).
Crystals 14 00326 g011
Crystals 14 00326 g012
Figure 12. 29Si MAS NMR spectra of HH-Apo after 16 h (left) and 48 h (right) at room temperature.
Figure 12. 29Si MAS NMR spectra of HH-Apo after 16 h (left) and 48 h (right) at room temperature.
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Crystals 14 00326 g013
Figure 13. Hypothetical structure of ordered “condensed carletonite.”
Figure 13. Hypothetical structure of ordered “condensed carletonite.”
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Crystals 14 00326 g014
Figure 14. Comparison of the PXRD patterns of H-Car and calc-H-Car.
Figure 14. Comparison of the PXRD patterns of H-Car and calc-H-Car.
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Crystals 14 00326 g015
Figure 15. Comparison of the PXRD patterns of H-Car (top), calc-en-H-Car (middle), and calc-H-Car (bottom).
Figure 15. Comparison of the PXRD patterns of H-Car (top), calc-en-H-Car (middle), and calc-H-Car (bottom).
Crystals 14 00326 g015
Crystals 14 00326 g016
Figure 16. 29Si MAS NMR spectrum of calc-en-H-Car. The green and the purple curves represent the calculated intensities of the two signals at −102.0 ppm and at −110.9 ppm, respectively. The red curve represents the sum of the intensities.
Figure 16. 29Si MAS NMR spectrum of calc-en-H-Car. The green and the purple curves represent the calculated intensities of the two signals at −102.0 ppm and at −110.9 ppm, respectively. The red curve represents the sum of the intensities.
Crystals 14 00326 g016
Table 2. Topotactic condensation of layered silicates via intercalated CSAs.
Table 2. Topotactic condensation of layered silicates via intercalated CSAs.
Layered ►►
Precursor		CSA ►►Intercalated CSA ►►Framework SilicateRef.
(Na-)magadiite H-magadiite Methylformamide-magadiite RWZ-1, a high
density zeolite		[33]
(Na-)octosilicate H-octosilicate Methylformamide-RUB-18 RWR-type zeolite[34]
(TMA-)RUB-15 H-RUB-15 Acetic acid-RUB-15Silica sodalite,
a clathrasil		[35]
TMA = tetramethylammonium.
Table 3. Synthesis of HH-Apo and H-Car (HAc = acetic acid).
Table 3. Synthesis of HH-Apo and H-Car (HAc = acetic acid).
Temp.Acid Apophyllite PowderApophyllite
Single Crystal		Carletonite PowderCarletonite
Single Crystal
0 °C0.1 M HCl-Nearly unchanged --
0 °C1.2 M HClNearly unchangedHydrated-H-apophylliteH-carletoniteH-carletonite
20 °C1.2 M HClAmorphousAmorphousH-carletoniteH-carletonite
0 °C10 M HClAmorphousAmorphous--
20 °C10 M HClAmorphousAmorphous--
20 °C10 M HAcNearly unchangedNearly unchanged--
Table 4. Recording conditions for MAS NMR spectra.
Table 4. Recording conditions for MAS NMR spectra.
Technique1H-29Si CP29Si Single Puls29Si Hpdec 29Si Hpdec1H 1H 1H
Sample HH-Apo
fresh		HH-Apo
decayed		H-Carcalc-en-H-CarHH-Apo
fresh		H-CarCalc-en-H-Car
StandardTMSTMSTMSTMSTMSTMSTMS
Frequency (MHz)79.4979.4979.4979.49400.15400.15400.35
Pulse width (10−6 s)25252525424
Contact time (10−3 s)4------
Recycle time (s)515060180101010
Spinning rate (kHz)4.04.04.05.07.512.512.5
No. of scans8004008001800128128320
Table 5. Experimental and crystallographic parameters for the structure analysis of the two CSAs.
Table 5. Experimental and crystallographic parameters for the structure analysis of the two CSAs.
MaterialHydrated H-ApophylliteH-Carletonite
Unit cell contentH16Si16O40 • 8–10 H2O H32Si64O144
DiffractometerSiemens D5000 using Braun position-sensitive detector
Wavelength1.54059 Å (Cu Kα1 Radiation)
Sample holderGlass capillary
2Θ range of data used [°]8.0–60.07.0–89.9
Step size [°2Θ]0.007900.00789
No. contributing reflections175637
No. geometric restraints1225
No. structural parameters1724
No. profile parameters1616
FWHM at ca. 25°2Θ [°2Θ]0.28–0.430.23–0.59
RF0.0280.025
Rwp0.1510.090
χ21.234.14
Space groupP 4/n c c (No. 126)I 4/m c m (No. 140)
a [Å] 8.4872(2)13.8972(3)
c [Å]16.8684(8)20.4677(21)
VUC [Å3]1215.08(7)3953.0(4)
Density (calc.) [g/cm3]1.757 (10 H2O)1.736
Table 6. Atomic coordinates, displacement parameters, and occupancy factors of HH-Apo.
Table 6. Atomic coordinates, displacement parameters, and occupancy factors of HH-Apo.
AtomScat. Fac.XyzBisoOcc.
Si1Si0.0172(3)0.1417(4)0.1758(2)3.88(12)1.00000
O1O0.0874(10)−0.0874(10)0.750005.32(13)1.00000
O2O0.0482(5)0.3232(4)0.1848(6)5.32(13)1.00000
O3O0.4102(9)−0.0982(11)0.4001(4)5.32(13)1.00000
OW1O2−0.9061(16)0.803(2)0.0467(8)5.32(13)0.622(5)
Table 7. Atomic coordinates, displacement parameters, and occupancy factors of H-Car.
Table 7. Atomic coordinates, displacement parameters, and occupancy factors of H-Car.
AtomScat. Fac.XyzBisoOcc.
Si1Si0.09572(15)0.24623(14)0.42227(9)0.67(2)1.000
Si2Si0.23410(13)0.10375(13)0.35202(10)0.67(2)1.000
O1O0.1483(2)0.1557(2)0.3913(3) 0.86(4)1.000
O2O0.2530(3)0.0092(2)0.3937(2)0.86(4)1.000
O3O0.2076(3)0.0849(3)0.27396(15)0.86(4)1.000
O4O0.1650(2)0.6650(2)0.3554(3)0.86(4)1.000
O5O0.1597(3)0.6597(3)0.0964(2)0.86(4)1.000
O6O0.0857(6)0.2265(5)0.50000 0.86(4)1.000
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MDPI and ACS Style

Marler, B.; Grosskreuz, I. Synthesis and Crystal Structures of Two Crystalline Silicic Acids: Hydrated H-Apophyllite, H16Si16O40 • 8–10 H2O and H-Carletonite, H32Si64O144. Crystals 2024, 14, 326. https://doi.org/10.3390/cryst14040326

AMA Style

Marler B, Grosskreuz I. Synthesis and Crystal Structures of Two Crystalline Silicic Acids: Hydrated H-Apophyllite, H16Si16O40 • 8–10 H2O and H-Carletonite, H32Si64O144. Crystals. 2024; 14(4):326. https://doi.org/10.3390/cryst14040326

Chicago/Turabian Style

Marler, Bernd, and Isabel Grosskreuz. 2024. "Synthesis and Crystal Structures of Two Crystalline Silicic Acids: Hydrated H-Apophyllite, H16Si16O40 • 8–10 H2O and H-Carletonite, H32Si64O144" Crystals 14, no. 4: 326. https://doi.org/10.3390/cryst14040326

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