RbEr₂AsS₇: A Rubidium-Containing Erbium Sulfide Thioarsenate(III) with (S₂)²⁻ Ligands According to RbEr₂S(S₂)[AsS₂(S₂)]

Abstract

The new rubidium-containing erbium sulfide thioarsenate(III) with the structured formula RbEr₂S(S₂)[AsS₂(S₂)] was obtained from the syntheses of elemental erbium (Er), arsenic sesquisulfide (As₂S₃) and rubidium sesquisulfide (Rb₂S₃) with elemental sulfur (S) at 773 K as transparent, orange, needle-shaped crystals. RbEr₂AsS₇ crystallizes monoclinically in the space group C2/c with a = 2339.86(12) pm, b = 541.78(3) pm, c = 1686.71(9) pm and β = 93.109(3) ° for Z = 8. The crystal structure features complex [AsS₂(S₂)]³⁻ anions with two S²⁻ anions and a (S₂)²⁻ disulfide dumbbell coordinating end-on as ligands for each As³⁺ cation. Even outside the ligand sphere of As³⁺, S²⁻ and (S₂)²⁻ can be found as sulfide anions. Two distinct Er³⁺ cations are surrounded by either nine or seven sulfur atoms. The [ErS₉] polyhedra are corner- and face-connected, while the [ErS₇] units share common edges, both building chains along [010]. These different chains undergo edge connectivity with each other, resulting in the formation of corrugated layers, which are held together by Rb⁺ in chains of condensed [RbS₉] polyhedra. So, a three-dimensional network is generated, offering empty channels along [010] apt to take up the As³⁺ lone-pair cations. Wavelength-dispersive X-ray spectroscopy verified a molar Rb:Er:As:S ratio of approximately 1:2:1:7 and diffuse reflectance spectroscopy showed the typical f–f transitions of Er³⁺, while the optical band gap was found to be 2.42 eV.

1. Introduction

Since lanthanoid-containing thiophosphates show a huge variety of compositions, structures and chemical and physical properties, and arsenic is the next heaviest congener of phosphorus in the periodic system of the elements (PSE), it appears reasonable to assume that similar structures and compositions could be expected for thioarsenates. For a better overview, the system of alkali metal-containing lanthanoid thiophosphates can be subdivided with regard to the oxidation state of phosphorus. In the case of P⁵⁺, different compositions of A_xLn_y[PS₄]_z (A = Li, K, Rb Cs; Ln = lanthanoids) are known, e.g., Li₆Ln₃[PS₄]₅ with Ln = Y, Gd, Dy and Lu [1]; K₃Ln[PS₄]₂ with Ln = La, Ce and Nd [2,3,4]; Rb₃Sm[PS₄]₂ [5]; and Cs₃Ln₅[PS₄]₆ with Ln = La and Ce [6]. All these ortho-thiophosphates are based on the same thiophosphate(V) anion [PS₄]³⁻, where P⁵⁺ is tetrahedrally coordinated by four S²⁻ anions. At this point, it should be emphasized that no lanthanoid-containing ortho-thiophosphates are known so far with A = Na, which is surprising, since they occur with both direct group neighbors, A = Li and K. Instead, hypo-thiodiphosphates NaLn[P₂S₆] with Ln = La–Pr, Sm, Tb, Yb and Lu are formed with phosphorus in the oxidation state +IV, featuring a P–P single bond [7,8,9]. Thus, since for A = Li, only ortho-thiophosphates(V), and for A = Na, only hypo-thiodiphosphates(IV) exist, potassium is the first of the alkali metals to form both ortho-thio- and hypo-thiodiphosphates, followed by A = Rb and Cs, with KLn[P₂S₆] (Ln = La–Pr and Sm), RbLa[P₂S₆] and CsCe[P₂S₆] as examples, in addition to the earlier mentioned ortho-thiophosphates [3,7,9,10]. Moreover, even mixed valent compounds, such as Rb₄Ln₂[P₂S₆][PS₄]₂ with Ln = La–Nd, Sm and Gd containing both [P₂S₆]⁴⁻ and [PS₄]³⁻ anions with phosphorus in oxidation states of +IV and +V are known [11]. It can thus be stated that, within the quaternary system of alkali metal-containing lanthanoid(III) thiophosphates, phosphorus occurs preferentially in the +V and +IV oxidation states, but never with +III. For the ternary system of lanthanoid(III) thiophosphates, this statement can even be restricted to the ortho-thiophosphates, which contain only P⁵⁺ cations according to Ln[PS₄] [12,13,14].

For the analogous systems of alkali metal-containing lanthanoid(III) thioarsenates, the question arises of whether derivatives with As⁵⁺ are preferentially formed, especially since the ternary lanthanoid(III) ortho-thioarsenates(V) Ln[AsS₄] have not been successfully prepared so far, or whether As³⁺ prevails and thus a stronger relationship to thioantimonates(III) occurs. Initial research in the system K/Ln/As/S (Ln = Nd, Sm, Gd, Dy) showed that tetrahedrally coordinated As⁵⁺ cations are likely to be preferred [15,16]. Also, for cesium, representatives of the thioarsenates(V) with the composition Cs₃Ln[AsS₄]₂ (Ln = La–Nd and Sm), the structure of which is dominated by isolated tetrahedral [AsS₄]³⁻ anions, have been prepared successfully in the past [17]. However, lanthanoid(III)-containing thioarsenates can also be synthesized with As³⁺ cations coordinated pyramidally by three S²⁻ anions, e.g., in Ln₃S₂Cl₂[AsS₃] with Ln = La and Pr or CsCeCl₂[AsS₃] [18,19], featuring discrete ψ¹-tetrahedral [AsS₃]³⁻ pyramids with a stereochemically active lone pair of electrons at each As³⁺ cation. However, Cs₄Pr₂[AsS₃]₂[As₂S₅] was the first member of the alkali metal-containing lanthanoid(III) thioarsenates(III) to be synthesized without halide participation, containing both isolated [AsS₃]³⁻ anions as well as [As₂S₅]⁴⁻ units with a sulfide bridge between them [20]. Additionally, trivalent phosphorus in discrete or condensed ψ¹-tetrahedral [PS₃]³⁻ anions is still unknown within the systems under consideration here. So, at first glance, As⁵⁺ seems to be preferred in the quaternary systems of alkali metal-containing lanthanoid(III) thioarsenates without extra halide anions and hypo-thioarsenates(IV) with tetravalent arsenic displaying the [As₂S₆]^4- anion with a As–As single bond have been never observed in these systems. In the special case of europium, the exclusive presence of divalent Eu²⁺ cations in KEu[AsS₃] and KEu[AsS₄] have been found, and the crystal structures show on the one hand, pyramidally coordinated As³⁺, and tetrahedrally coordinated As⁵⁺ cations on the other [16,21]. Because of these results, the authors postulated a less oxidizing arsenic-rich flux to stabilize As³⁺ in the A/Eu/As/S (A = Li, K–Cs) systems [21]. Moreover, Eu₃[AsS₄]₃ is the only reported ternary compound with divalent europium, but neither Ln[AsS₄] nor Ln[AsS₃] as ternaries with trivalent lanthanoids have been observed yet.

According to the successful attempts of our former research in the systems of alkali metals, lanthanoids, arsenic and sulfur (A/Ln/As/S, A = Rb and Cs) with respect to the formation of A₃Ln[AsS₄]₂ compounds with Ln = La–Nd and Sm [17], we have now tried to prepare Rb₃Er[AsS₄]₂ in analogy to K₃Gd[AsS₄]₂ [15]. But the simple change in the synthesis temperature and different sizes of the monovalent counter cations led to single crystals of the new erbium-rich compound RbEr₂AsS₇ with the structured formula RbEr₂S(S₂)[AsS₂(S₂)], which includes a rare asymmetric [AsS₂(S₂)]³⁻ anion not documented previously in the lanthanoid-thioarsenates chemistry.

2. Results and Discussion

2.1. Structure Description for RbEr₂AsS₇ (≡ RbEr₂S(S₂)[AsS₂(S₂)])

The new rubidium-containing erbium thioarsenate(III) RbEr₂AsS₇ crystallizes monoclinically in the space group C2/c with the lattice parameters a = 2339.86(12) pm, b = 541.78(3) pm, c = 1686.71(9) pm and β = 93.109(3)° for Z = 8 (

Table 1. Crystallographic data for RbEr₂AsS₇ and their determination.

Empirical Formula	RbEr2AsS7
Structured formula	RbEr2S(S2)[AsS2(S2)]
Crystal system	monoclinic
Space group	C2/c (no. 15)
Lattice constants
a/pm	2339.86(12)
b/pm	541.78(3)
c/pm	1686.71(9)
β/°	93.109(3)
Unit cell volume, Vuc/nm3	2.135
Number of formula units	Z = 8
Diffractometer	κ-CCD (Bruker-Nonius)
Radiation	Mo-Kα (λ = 71.07 pm)
Structure solution and refinement	SHELX-97 [22,23,24]
Index range, ±hmax/±kmax/±lmax	30/7/21
Number of e− per unit cell, F(000)	2544
Absorption coefficient, µ/mm−1	24.52
Number of collected/unique reflections	28015/2449
Rint/Rσ	0.068/0.028
R1/wR2 for all reflections	0.026/0.068
Goodness of fit (GooF)	1.030
Residual electron density, ρmax/min/10−6 pm3	1.64/−1.51
CSD number	2219896

). The structure is composed of only one crystallographically unique rubidium and arsenic cation position each, but two erbium and seven sulfur sites, which are all located at the general Wyckoff position 8f (

Table 2. Fractional atomic coordinates and coefficients of the equivalent isotropic displacement parameters for RbEr₂AsS₇ with all atoms at the symmetry-free Wyckoff site 8f.

Atom	x/a	y/b	z/c	Ueq/pm2
Rb	0.42638(3)	0.26898(9)	0.41040(4)	299(2)
Er1	0.31925(1)	0.24529(4)	0.08041(2)	150(1)
Er2	0.19026(1)	0.25347(4)	0.22655(2)	149(1)
As	0.06627(3)	0.28248(9)	0.33685(4)	183(2)
S1	0.07440(7)	0.2712(3)	0.20711(9)	213(3)
S2	0.16020(7)	0.2460(3)	0.38410(9)	156(3)
S3	0.43645(7)	0.2077(3)	0.14952(9)	224(3)
S4	0.42774(7)	0.2307(3)	0.02700(9)	227(4)
S5	0.30438(7)	0.2523(3)	0.24007(9)	144(3)
S6	0.20702(7)	0.4488(3)	0.07613(9)	195(4)
S7	0.20447(7)	0.0591(3)	0.07745(9)	201(4)
LP	0.0282	0.2077	0.4047	–

).

According to the data from the powder X-ray diffraction measurements (PXRD) this compound can also be synthesized from a new mixture of Er:Rb₂S₃:As₂S₃:S with a molar ratio of 4:1:1:8, leading to an almost phase-pure microcrystalline powder, the diffraction data of which match well with those calculated from the single-crystal measurements (Figure 1). The experimental powder diffraction data show several impurities with weak intensities, which could not all be clearly identified. Whereas the excess reflection at 2θ = 27° can be assigned to the main reflection of Rb₂S₃ [25], the second one at 2θ = 35° could not be assigned to a third phase. Washing the crude product with different solvents (e.g., DMSO or DMF) should be tested in the future to avoid possible side phases.

As can be seen from the structured formula RbEr₂S(S₂)[AsS₂(S₂)] (Table 1) and as illustrated in Figure 2, the central structure feature is an isolated complex [AsS₂(S₂)]³⁻ anion, which is built from two S²⁻ anions and one (S₂)²⁻ dumbbell end-on coordinating as a ligand to the central As³⁺ cation. The corresponding As–S distances d(As–S) = 220–232 pm (

Table 3. Selected interatomic distances in RbEr₂AsS₇ (≡ RbEr₂S(S₂)[AsS₂(S₂)]).

Atom Pair	d/pm	Atom Pair	d/pm
Er1–S5	273.4(1)	Rb–S4	334.5(2)
Er1–S4	274.0(2)	Rb–S1	334.7(2)
Er1–S2	280.6(1)	Rb–S4	334.8(2)
Er1–S2	281.4(1)	Rb–S1	336.6(2)
Er1–S6	284.5(2)	Rb–S3	343.3(2)
Er1–S7	286.7(2)	Rb–S7	345.8(2)
Er1–S6	287.7(2)	Rb–S4	352.4(2)
Er1–S7	289.2(2)	Rb–S6	358.9(2)
Er1–S3	292.8(2)	Rb–S5	394.0(2)
Er2–S5	266.8(2)	As–S1	220.7(2)
Er2–S1	271.5(2)	As–S2	230.5(2)
Er2–S5	276.2(1)	As–S3	231.6(2)
Er2–S7	276.3(2)	As–LP	154
Er2–S5	277.5(1)
Er2–S2	278.6(2)	S3–S4	206.9(2)
Er2–S6	279.6(2)	S6–S7	211.2(2)

) fall into the range of commonly known distances, as, for example, in orpiment As₂S₃ [26]. The angles ∢(S–As–S) = 95–102° within this anion, however, deviate significantly from those of an ideal tetrahedron (

Table 4. Selected interatomic angles in RbEr₂AsS₇ (≡ RbEr₂S(S₂)[AsS₂(S₂)]).

Atom Triple	∢/°
S1–As–S2	102.04(6)
S1–As–S3	97.49(6)
S2–As–S3	94.69(6)
As–S3–S4	99.06(7)

). This may be due to the stereochemically active lone pair of electrons at the As³⁺ cation on the one hand and the steric influence of the (S₂)²⁻ ligand on the other. This type of complex [AsS₂(S₂)]³⁻ anion has not been reported yet in comparable solids, so, as is known so far, RbEr₂S(S₂)[AsS₂(S₂)] is the first lanthanoid-containing thioarsenate(III) with this structural unit.

For the charge balance, this complex [AsS₂(S₂)]³⁻ unit is surrounded by five rubidium and three erbium cations (Figure 2b). The end-on coordinating disulfide dumbbell ⁻(S3)–(S4)⁻ at the As³⁺ cation is still side-on coordinating the (Er1)³⁺ cation together with two further isolated disulfide dumbbells ⁻(S6)–(S7)⁻ and three S²⁻ anions, forming an [ErS₉] polyhedron (Figure 3a, left). The two ⁻(S6)–(S7)⁻ dumbbells form a rectangular face, via which two [ErS₉] polyhedra are linked. These become additionally corner-connected to their neighboring polyhedra of the same kind and thus form infinite chains ${}_{\infty }{}^1\left\{\right(\mathrm{E}\mathrm{r}1\left){\mathrm{S}}_{3/1}^{\mathrm{t}}{\mathrm{S}}_{2/2}^{\mathrm{c}}{\mathrm{S}}_{4/2}^{\mathrm{f}}\right\}$ along [010] (Figure 3a, right), with the (Er1)–S distances ranging from 273 to 292 pm (Table 3). The (Er2)³⁺-centered [(Er2)S₇] polyhedra shown in Figure 3b (left) are composed by a side-on coordinating disulfide dumbbell ⁻(S6)–(S7)⁻ and five S²⁻ anions, including the isolated (S5)²⁻ anion, which is neither part of the coordination environment of As³⁺ nor part of a disulfide dumbbell, and is therefore explicitly emphasized in the structured formula. These distorted pentagonal bipyramids [ErS₇] become linked by their two (S5)²⁻ edges to build up infinite chains ${}_{\infty }{}^1\left\{\right(\mathrm{E}\mathrm{r}2\left){\mathrm{S}}_{4/1}^{\mathrm{t}}{\mathrm{S}}_{3/3}^{\mathrm{e}}\right\}$, which also run along [010] (Figure 3b, right). Furthermore, the two erbium-centered polyhedral chains become edge-connected via S2 and S5 and face-connected via S5, S6 and S7 with each other along [001], and thus form corrugated layers ${}_{\infty }{}^2\left\{{\left[\mathrm{E}{\mathrm{r}}_2{\mathrm{S}}_7\right]}^{4-}\right\}$, spreading out parallel to the (100) plane (Figure 4). The (Er2)–S separations of d((Er2)–S) = 267–280 pm cover the range of the known distances also found in D-type or F-type Er₂S₃ [27,28], for example. As Figure 2 implies and Figure 5 clearly shows, the isolated [AsS₂(S₂)]³⁻ units are part of these corrugated layers ${}_{\infty }{}^2\left\{{\left[\mathrm{E}{\mathrm{r}}_2{\mathrm{S}}_7\right]}^{4-}\right\}$ via the connection of three Er³⁺-centered polyhedra, resulting in a two-dimensional anionic layer structure ${}_{\infty }{}^2\left\{{\left[\mathrm{E}{\mathrm{r}}_2{\mathrm{AsS}}_7\right]}^{-}\right\}$ oriented parallel to (100) and separated by Rb⁺ cations.

The unique Rb⁺ cations are located in the boundary planes of the corrugated Er³⁺-centered polyhedral sulfur layers, at the centroid of a rectangle spanned by S1∙∙∙S1∙∙∙S4∙∙∙S4. This rectangle is capped by a side-on coordinated disulfide dumbbell ⁻(S3)–(S4)⁻ and a triangle spanned by S6∙∙∙S5∙∙∙S7, resulting in [RbS₉] polyhedra with d(Rb–S) = 335–394 pm (Figure 5, left). It is worth noting that these polyhedra, on the one hand, form undulated chains by edge connections in ${}_{\infty }{}^1\left\{\mathrm{R}\mathrm{b}{\mathrm{S}}_{4/1}^{\mathrm{t}}{\mathrm{S}}_{2/2}^{\mathrm{e}}{\mathrm{S}}_{3/3}^{\mathrm{e}}\right\}$ along [010] and are, in addition, bridged by the end-on coordinating disulfide dumbbells ⁻(S6)–(S7)⁻ (Figure 5b). As illustrated in Figure 4, the (S₂)²⁻ ligands of the complex thioarsenate(III) anions contribute to the formation of the [RbS₉] polyhedra. In this context, using the numerical calculation program LPLoc [29], the position, radius and distance of the stereochemically active lone pairs (LPs) of the As³⁺ cations were calculated to determine their possible location within the structure. Their radius was calculated to be 115 pm, their atomic coordinates and distances to As³⁺ are given in Table 2 and Table 3. The calculation confirms the assumption that the lone pairs are oriented in the direction of the low points in the Rb⁺-centered polyhedral chains (Figure 5b). With respect to the earlier mentioned two-dimensional structure built of ${}_{\infty }{}^2\left\{{\left[\mathrm{E}{\mathrm{r}}_2{\mathrm{AsS}}_7\right]}^{-}\right\}$ layers, the overall structure is extended by the Rb⁺-centered polyhedra to become a three-dimensional network with empty channels along [010] (Figure 6), if the weak ionic Rb–S bonds are additionally considered.

The crystal structure of the compound is characterized by the rare, non-tetrahedral [AsS₄]³⁻, or, more specifically, the [AsS₂(S₂)]³⁻ anion. This type of complex anion was previously unknown in lanthanoid compounds and could only be realized in solid compounds with tin so far, namely, KSnAsS₅ and Cs₂SnAs₂S₉ or molecular [Pt₃(AsS₄)]³⁻ and [Pd₃(AsS₄)₃]³⁻ anions [30,31,32]. Interestingly, in Cs₂SnAs₂S₉, this [AsS₂(S₂)]³⁻ anion occurs next to a [AsS(S₂)₂]³⁻ anion that derives from the pyramidal [AsS₃]³⁻ anion through the substitution of two S²⁻ ligands with disulfide dumbbells (S₂)²⁻. From the literature, also for As⁵⁺, a complex [AsS₃(S₂)]³⁻ with one disulfide dumbbell as a ligand instead of a spherical S²⁻ anion is known [33]. So, disulfide dumbbells as ligands coordinating to As³⁺ or As⁵⁺ cations are not something new, but very rare, especially in solids, and were previously unknown in lanthanoid-containing systems. The question arises of why within a system, that previously seemed to favor the formation of tetrahedral [AsS₄]³⁻ anions with pentavalent arsenic, the appearance of the rare [AsS₂(S₂)]³⁻ anion is favored in RbEr₂S(S₂)[AsS₂(S₂)]. At this point, the chemistry of arsenic seems to align preferentially with the chemistry of antimony, which prefers the oxidation state +III and for which this type of complex anion is also known, i.e., an analogous [SbS₂(S₂)]³⁻ anion with similar composition is the structural element in Ba₃Sb₂S₇ [34,35].

2.2. Diffuse Reflectance Spectroscopy (DRS)

Diffuse reflectance UV/Vis measurements were performed to determine the optical band gap of this orange-colored compound. The collected data were transformed using the Kubelka-Munk function and plotted in Figure 7 [36]. The optical band gap can be determined by the intersection of the tangents and was found to be 2.42 eV. Furthermore, some bands appear, which are clearly able to be assigned to the f–f transitions of Er³⁺ cations by means of the Dieke diagram.

3. Experimental Section

3.1. Solid-State Synthesis

All preparations and further manipulations were carried out under inert conditions in an argon-filled glove box (GS Mega E-line, Glovebox Systemtechnik, Malsch, Germany). Elemental erbium (Er: 99.9%, ChemPur, Karlsruhe, Germany), arsenic sesquisulfide (As₂S₃: 99.999%, Alfa Aesar, Schwerte, Germany), rubidium sesquisulfide (Rb₂S₃) and elemental sulfur (S: 99+%, ChemPur) were mixed in a molar ratio of 1:1:1:6 expected to yield Rb₃Er[AsS₄]₂. The mixture was filled into a fused silica glass ampoule and sealed under dynamic vacuum. The vessel was placed in a computer-controlled muffle furnace (Nabertherm, Lilienthal, Germany) and heated to 773 K within 20 h. After 96 h at 773 K, it was cooled to 523 K at a rate of −2 K/h. This temperature was kept for another 120 h, before cooling down to 373 K at −2 K/h again. At 373 K, the furnace was turned off and allowed to reach room temperature quickly. The trisulfide reagent Rb₂S₃ required for the synthesis was prepared in analogy to Cs₂S₃ from its elements in liquid ammonia [17]. For this purpose, a stoichiometric mixture of elemental rubidium (Rb: 99.5%, ChemPur) and sulfur (S: 99+%, ChemPur) were filled into a pressure-resistant viewing-glass vessel in an argon-filled glove box. This specific container was built with a brass casing with a window, which allows for the observation of the filling level and the reaction’s progress. In the brass casing, a thick and stable glass tube (wall thickness: 2 mm) can be placed and closed with a sealing ring made of polytetrafluoroethylene (PTFE) and a connection to a DN 16 small flange. Due to this construction, the vessel is at least resistant against pressure until approximately 12 bar. After filling the solid reactants in that vessel, it was evacuated under exclusion of air at the tensi-eudiometer and subsequently about 6 mL ammonia (99.9999%, Linde Gas GmbH, Pullach, Germany) was condensed into it with the aid of an isopropanol dry ice slush until it was filled to about 50 vol-% with liquid NH₃ [37]. After two days, the conversion of the alkali metal was successfully reached and the synthesis of the yellow-orange colored trisulfide Rb₂S₃ was completed. Due to the moisture sensitivity of this product, it was stored and handled exclusively in a glove box under dry argon.

3.2. Single-Crystal X-ray Diffraction (SCXRD)

The reaction product contained transparent, orange, needle-shaped crystals, which were stable with regard to moist air and water for several weeks. Suitable single crystals were selected for X-ray diffraction measurements (SCXRD) performed on a Bruker-Nonius κ-CCD diffractometer (Karlsruhe, Germany) using monochromatized Mo-K_α radiation (λ = 71.07 pm). For the structure solution and refinement, the program package SHELX-97 was used [22,23,24]. The monoclinic crystal structure of what turned out to be RbEr₂AsS₇ was solved by direct method calculations using the program SHELXS-97 and refined with SHELXL-97 by full matrix least-squares iterations on F². Detailed crystallographic data are given in Table 1 and further information of the crystal structure can be obtained from the Cambridge Crystallographic Data Center and the Fachinformationszentrum Karlsruhe service www.ccdc.cam.ac.uk/structures (CSD number: 2219896).

3.3. Wavelength-Dispersive X-ray Spectroscopy (WDXS)

Elemental analysis was performed with an electron-beam X-ray microprobe system (SX-100, Cameca, Gennevilliers, France ) equipped with five X-ray wavelength-dispersive spectrometers (WDS). WDXS results for RbEr₂AsS₇ confirmed a molar Rb/Er/As/S ratio of approximately 1.0:2.0:1.0:7.1, which agrees well with the single-crystal X-ray structure analysis. For these measurements, single crystals of RbEr₂AsS₇ were placed onto a conductive carbon pad (Plano G3347, Wetzlar, Germany) and sputtered with carbon.

3.4. Powder-Crystal X-ray Diffraction (PXRD)

Powder X-ray diffraction patterns were recorded at room temperature on a microcrystalline powder sample (synthesized from a new mixture of Er/Rb₂S₃/As₂S₃/S with the appropriate molar ratio of 4:1:1:8) on a Stoe Stadi-P (Darmstadt, Germany) diffractometer equipped with a linear PSD Detector and Cu-K_α radiation (λ = 15.42 nm) and matched nicely with the simulated diffractograms from the single-crystal data (Figure 1).

3.5. Diffuse Reflectance Spectroscopy (DRS)

Diffuse reflectance spectroscopy (DRS) on a UV/Vis spectrometer (J&W Tidas, Essingen, Germany) using a finely ground powder without inert gas atmosphere were performed for selecting reflection data and calculating the optical band gap, using the Kubelka-Munk function [36].

4. Conclusions

The presented new compound RbEr₂AsS₇ with the structured formula RbEr₂S(S₂)[AsS₂(S₂)] represents a hitherto unknown composition in the system of lanthanoid-containing thioarsenates(III) with alkali metal participation. Its crystal structure could be solved from single-crystal X-ray diffraction data, while its composition was confirmed through WDXS measurements. Attempts of phase-pure syntheses using different stoichiometric mixtures of Rb₂S₃, Er As₂S₃ and S led to microcrystalline powder samples, which, according to PXRD, fit well with the data calculated from the SCXRD data. A central building block of the structure is an isolated [AsS₂(S₂)]³⁻ anion, which can be derived from an [AsS₃]³⁻ ψ¹-tetrahedron through the substitution of one S²⁻ ligand with an end-on coordinating (S₂)²⁻ dumbbell. At the same time, the structure contains S²⁻ or (S₂)²⁻ anions that do not belong to the coordination sphere of the As³⁺ cation, but contribute to the formation of the coordination polyhedra about the Er³⁺ and Rb⁺ cations, respectively. The Er³⁺ cations are surrounded by either seven or nine sulfur atoms, forming, in total, a two-dimensional structure of corrugated layers ${}_{\infty }{}^2\left\{{\left[\mathrm{E}{\mathrm{r}}_2{\mathrm{S}}_7\right]}^{4-}\right\}$ parallel to the (100) plane, apt to accommodate the As³⁺ lone-pair cations according to ${}_{\infty }{}^2\left\{{\left[\mathrm{E}{\mathrm{r}}_{2}As{\mathrm{S}}_7\right]}^{-}\right\}$. These layers are at least separated by Rb⁺ cations, but, with regard to the weak ionic Rb–S bonds, they could also be described as linked by chains of condensed [RbS₉] polyhedra along [010], in which the isolated disulfide dumbbells additionally act as a bridging feature. The resulting three-dimensional network ${}_{\infty }{}^3\left\{{\left[\mathrm{RbE}{\mathrm{r}}_2{\mathrm{S}}_7\right]}^{3-}\right\}$ then shows empty channels along [010] for taking up the As³⁺ cations with their lone pairs. Using the LPLoc program to determine the orientation of the stereochemically active lone pairs of the As³⁺ cations, it suggests an alignment of them pointing into the centers of these empty channels.

Since the original target compound would have been Rb₃Er[AsS₄]₂ with pentavalent arsenic and tetrahedral [AsS₄]³⁻ anions, as it was successful for the monoclinic series Rb₃Ln[AsS₄]₂ (space groups P2₁/c for Ln = La and Ce; P2₁ for Ln = Pr, Nd and Sm; and C2/c for Ln = Tb), the question arises of why, in the analogous system of Rb/Er/As/S, the oxidation to As⁵⁺ did not occur and the prevalence of As³⁺ leaves sulfur behind, ready to oxidize S²⁻ to (S₂)²⁻ (≡ ⁻S–S⁻) inside and outside its coordination sphere. The mentioned results for this Rb₃Ln[AsS₄]₂ series will be published in 2024 or later [38].

In conclusion, the new erbium-rich thioarsenate(III) RbEr₂S(S₂)[AsS₂(S₂)] is the first lanthanoid-containing compound with isolated [AsS₂(S₂)]³⁻ polyhedra. This complex thioarsenate(III) anion [AsS₂(S₂)]³⁻ was known in the past in KSnAsS₅ and Cs₂SnAs₂S₉ or the molecular [Pd₃(AsS₄)₃]³⁻ and [Pt₃(AsS₄)]³⁻ anions, but not in lanthanoid-containing systems [30,31,32]. Also, the occurrence of S²⁻ and (S₂)²⁻ outside the ligand sphere of As³⁺ represents a new phenomenon in the structural diversity of alkali metal-containing lanthanoid(III) thioarsenates(III).