Crystals

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4 pages, 192 KiB

Open AccessEditorial

Compounds with Polar Metallic Bonding

by Constantin Hoch

Crystals 2019, 9(5), 267; https://doi.org/10.3390/cryst9050267 - 22 May 2019

Cited by 1 | Viewed by 2445

Abstract

Recently, I witnessed a discussion amongst solid state chemists whether the term polar intermetallic bonding was necessary or dispensable, whether a conceptual discernation of this special class of intermetallic compounds was indicated or spurious [...] Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

Research

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17 pages, 3550 KiB

Open AccessArticle

Ammoniates of Zintl Phases: Similarities and Differences of Binary Phases A₄E₄ and Their Corresponding Solvates

by Corinna Lorenz, Stefanie Gärtner and Nikolaus Korber

Crystals 2018, 8(7), 276; https://doi.org/10.3390/cryst8070276 - 29 Jun 2018

Cited by 14 | Viewed by 4521

Abstract

The combination of electropositive alkali metals A (A = Na-Cs) and group 14 elements E (E = Si-Pb) in a stoichiometric ratio of 1:1 in solid state reactions results in the formation of polyanionic salts, which belong to a class of intermetallics for [...] Read more.

The combination of electropositive alkali metals A (A = Na-Cs) and group 14 elements E (E = Si-Pb) in a stoichiometric ratio of 1:1 in solid state reactions results in the formation of polyanionic salts, which belong to a class of intermetallics for which the term Zintl compounds is used. Crystal structure analysis of these intermetallic phases proved the presence of tetrahedral tetrelide tetraanions [E₄]⁴⁻ precast in solid state, and coulombic interactions account for the formation of a dense, three-dimensional cation-anion network. In addition, it has been shown that [E₄]⁴⁻ polyanions are also present in solutions of liquid ammonia prepared via different synthetic routes. From these solutions crystallize ammoniates of the alkali metal tetrahedranides, which contain ammonia molecules of crystallization, and which can be characterized by X-ray crystallography despite their low thermal stability. The question to be answered is about the structural relations between the analogous compounds in solid state vs. solvate structures, which all include the tetrahedral [E₄]⁴⁻ anions. We here investigate the similarities and differences regarding the coordination spheres of these anions and the resulting cation-anion network. The reported solvates Na₄Sn₄·13NH₃, Rb₄Sn₄·2NH₃, Cs₄Sn₄·2NH₃, Rb₄Pb₄·2NH₃ as well as the up to now unpublished crystal structures of the new compounds Cs₄Si₄·7NH₃, Cs₄Ge₄·9NH₃, [Li(NH₃)₄]₄Sn₄·4NH₃, Na₄Sn₄·11.5NH₃ and Cs₄Pb₄·5NH₃ are considered for comparisons. Additionally, the influence of the presence of another anion on the overall crystal structure is discussed by using the example of a hydroxide co-crystal which was observed in the new compound K_4.5Sn₄(OH)_0.5∙1.75 NH₃. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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14 pages, 4176 KiB

Open AccessCommunication

Ba₄[Mn₃N₆], a Quasi-One-Dimensional Mixed-Valent Nitridomanganate (II, IV)

by Alexander Ovchinnikov, Matej Bobnar, Yurii Prots, Walter Schnelle, Peter Höhn and Yuri Grin

Crystals 2018, 8(6), 235; https://doi.org/10.3390/cryst8060235 - 25 May 2018

Cited by 6 | Viewed by 4046

Abstract

The mixed-valent nitridomanganate Ba₄[Mn₃N₆] was prepared using a gas–solid high temperature route. The crystal structure was determined employing high resolution synchrotron powder diffraction data: space group Pbcn, a = 9.9930(1) Å, b = 6.17126(8) Å, c [...] Read more.

The mixed-valent nitridomanganate Ba₄[Mn₃N₆] was prepared using a gas–solid high temperature route. The crystal structure was determined employing high resolution synchrotron powder diffraction data: space group Pbcn, a = 9.9930(1) Å, b = 6.17126(8) Å, c = 14.4692(2) Å, V = 892.31(2) Å³, Z = 4. The manganese atoms in the structure of Ba₄[Mn₃N₆] are four-fold coordinated by nitrogen forming infinite corrugated chains of edge-sharing [MnN₄] tetrahedra. The chains demonstrate a complete charge order of Mn species. Magnetization measurements and first principle calculations indicate quasi-one dimensional magnetic behavior. In addition, chemical bonding analysis revealed pronounced Mn–Mn interactions along the chains. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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10 pages, 2427 KiB

Open AccessArticle

Optimization of Ca₁₄MgSb₁₁ through Chemical Substitutions on Sb Sites: Optimizing Seebeck Coefficient and Resistivity Simultaneously

by Yufei Hu, Kathleen Lee and Susan M. Kauzlarich

Crystals 2018, 8(5), 211; https://doi.org/10.3390/cryst8050211 - 13 May 2018

Cited by 9 | Viewed by 4051

Abstract

In thermoelectric materials, chemical substitutions are widely used to optimize thermoelectric properties. The Zintl phase compound, Yb₁₄MgSb₁₁, has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca₁₄AlSb₁₁ with space [...] Read more.

In thermoelectric materials, chemical substitutions are widely used to optimize thermoelectric properties. The Zintl phase compound, Yb₁₄MgSb₁₁, has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca₁₄AlSb₁₁ with space group I4₁/acd. Its iso-structural analog, Ca₁₄MgSb₁₁, was discovered to be a semiconductor and have vacancies on the Sb(3) sites, although in its nominal composition it can be described as consisting of fourteen Ca²⁺ cations with one [MgSb₄]⁹⁻ tetrahedron, one Sb₃⁷⁻ linear anion and four isolated Sb³⁻ anions (Sb(3) site) in one formula unit. When Sn substitutes Sb in Ca₁₄MgSb₁₁, optimized Seebeck coefficient and resistivity were achieved simultaneously although the Sn amount is small (<2%). This is difficult to achieve in thermoelectric materials as the Seebeck coefficient and resistivity are inversely related with respect to carrier concentration. Thermal conductivity of Ca₁₄MgSb_11-xSn_x remains almost the same as Ca₁₄MgSb₁₁. The calculated zT value of Ca₁₄MgSb_10.80Sn_0.20 reaches 0.49 at 1075 K, which is 53% higher than that of Ca₁₄MgSb₁₁ at the same temperature. The band structure of Ca₁₄MgSb₇Sn₄ is calculated to simulate the effect of Sn substitutions. Compared to the band structure of Ca₁₄MgSb₁₁, the band gap of Ca₁₄MgSb₇Sn₄ is smaller (0.2 eV) and the Fermi-level shifts into the valence band. The absolute values for density of states (DOS) of Ca₁₄MgSb₇Sn₄ are smaller near the Fermi-level at the top of valence band and 5p-orbitals of Sn contribute most to the valence bands near the Fermi-level. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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18 pages, 4065 KiB

Open AccessArticle

Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄: Two More Germanides among Polar Intermetallics

by Riccardo Freccero, Pavlo Solokha, Davide Maria Proserpio, Adriana Saccone and Serena De Negri

Crystals 2018, 8(5), 205; https://doi.org/10.3390/cryst8050205 - 5 May 2018

Cited by 13 | Viewed by 3939

Abstract

In this study, two novel Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄ polar intermetallics were prepared by direct synthesis of pure constituents. Their crystal structures were determined by single crystal X-ray diffraction analysis: Lu₅Pd₄ [...] Read more.

In this study, two novel Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄ polar intermetallics were prepared by direct synthesis of pure constituents. Their crystal structures were determined by single crystal X-ray diffraction analysis: Lu₅Pd₄Ge₈ is monoclinic, P2₁/m, mP34, a = 5.7406(3), b = 13.7087(7), c = 8.3423(4) Å, β = 107.8(1), Z = 2; Lu₃Pd₄Ge₄ is orthorhombic, Immm, oI22, a = 4.1368(3), b = 6.9192(5), c = 13.8229(9) Å, Z = 2. The Lu₅Pd₄Ge₈ analysed crystal is one more example of non-merohedral twinning among the rare earth containing germanides. Chemical bonding DFT studies were conducted for these polar intermetallics and showing a metallic-like behavior. Gathered results for Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄ permit to described both of them as composed by [Pd–Ge]^δ– three dimensional networks bonded to positively charged lutetium species. From the structural chemical point of view, the studied compounds manifest some similarities to the Zintl phases, containing well-known covalent fragment i.e., Ge dumbbells as well as unique cis-Ge₄ units. A comparative analysis of molecular orbital diagrams for Ge₂^6– and cis-Ge^10– anions with COHP results supports the idea of the existence of complex Pd–Ge polyanions hosting covalently bonded partially polarised Ge units. The palladium atoms have an anion like behaviour and being the most electronegative cause the noticeable variation of Ge species charges from site to site. Lutetium charges oscillate around +1.5 for all crystallographic positions. Obtained results explained why the classical Zintl-Klemm concept can’t be applied for the studied polar intermetallics. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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21 pages, 3383 KiB

Open AccessArticle

Mixed Sr and Ba Tri-Stannides/Plumbides A^II(Sn_1−xPb_x)₃

by Michael Langenmaier, Michael Jehle and Caroline Röhr

Crystals 2018, 8(5), 204; https://doi.org/10.3390/cryst8050204 - 4 May 2018

Cited by 2 | Viewed by 4060

Abstract

The continuous substitution of tin by lead (M

^{IV}

) allows for the exploration geometric criteria for the stability of the different stacking variants of alkaline-earth tri-tetrelides

A^{II} M_{3}^{IV}

. A series of ternary Sr and Ba mixed tri-stannides/plumbides [...] Read more.

The continuous substitution of tin by lead (M

^{IV}

) allows for the exploration geometric criteria for the stability of the different stacking variants of alkaline-earth tri-tetrelides

A^{II} M_{3}^{IV}

. A series of ternary Sr and Ba mixed tri-stannides/plumbides A

^{II}

(Sn

_{1 - x}

Pb

_{x}

)

_{3}

(A

^{II}

= Sr, Ba) was synthesized from stoichiometric mixtures of the elements. Their structures were determined by means of single crystal X-ray data. All structures exhibit close packed ordered

A M_{3}

layers containing M kagomé nets. Depending on the stacking sequence, the resulting M polyanion resembles the oxygen substructure of the hexagonal (face-sharing octahedra, h stacking, Ni

_{3}

Sn-type, border compound BaSn

_{3}

) or the cubic (corner-sharing octahedra, c stacking, Cu

_{3}

Au-type, border compound SrPb

_{3}

) perovskite. In the binary compound BaSn

_{3}

(Ni

_{3}

Sn-type) up to 28% of Sn can be substituted against Pb (hP8, P $6_{3} /$ mmc, x = 0.28(4): a = 726.12(6), c = 556.51(6) pm, R1 = 0.0264). A further increased lead content of 47 to 66% causes the formation of the BaSn

_{2.57}

Bi

_{0.43}

-type structure with a

{(hhhc)}_{2}

stacking [hP32, P $6_{3} /$ mmc, x = 0.47(3): a = 726.80(3), c = 2235.78(14) pm, R1 = 0.0437]. The stability range of the BaPb

_{3}

-type sequence

{(hhc)}_{3}

starts at a lead proportion of 78% (hR36, R $\bar{3}$ m, a = 728.77(3), c = 2540.59(15) pm, R1= 0.0660) and reaches up to the pure plumbide BaPb

_{3}

. A second new polymorph of BaPb

_{3}

forms the Mg

_{3}

In-type structure with a further increased amount of cubic sequences [

{(hhcc)}_{3}

; hR48, a = 728.7(2), c = 3420.3(10) pm, R1 = 0.0669] and is thus isotypic with the border phase SrSn

_{3}

of the respective strontium series. For the latter, a Pb content of 32% causes a small existence region of the PuAl

_{3}

-type structure [hP24, P $6_{3} /$ mmc, a = 696.97(6), c = 1675.5(2) pm, R1 = 0.1182] with a

{(hcc)}_{2}

stacking. The series is terminated by the pure c stacking of SrPb

_{3}

, the stability range of this structure type starts at 75% Pb (cP4, Pm $\bar{3}$ m; a = 495.46(9) pm, R1 = 0.0498). The stacking of the close packed layers is evidently determined by the ratio of the atomic radii of the contributing elements. The Sn/Pb distribution inside the polyanion (’coloring’) is likewise determined by size criteria. The electronic stability ranges, which are discussed on the basis of the results of FP-LAPW band structure calculations are compared with the Zintl concept and Wade’s/mno electron counting rules. Still, due to the presence of only partially occupied steep M-p bands the compounds are metals exhibiting pseudo band gaps close to the Fermi level. Thus, this structure family represents an instructive case for the transition from polar ionic/covalent towards (inter)metallic chemistry. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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13 pages, 3801 KiB

Open AccessArticle

Rhombohedral Distortion of the Cubic MgCu₂-Type Structure in Ca₂Pt₃Ga and Ca₂Pd₃Ga

by Asa Toombs and Gordon J. Miller

Crystals 2018, 8(5), 186; https://doi.org/10.3390/cryst8050186 - 26 Apr 2018

Cited by 4 | Viewed by 5418

Abstract

Two new fully ordered ternary Laves phase compounds, Ca₂Pt₃Ga and Ca₂Pd₃Ga, have been synthesized and characterized by powder and single-crystal X-ray diffraction along with electronic structure calculations. Ca₂Pd₃Ga was synthesized as [...] Read more.

Two new fully ordered ternary Laves phase compounds, Ca₂Pt₃Ga and Ca₂Pd₃Ga, have been synthesized and characterized by powder and single-crystal X-ray diffraction along with electronic structure calculations. Ca₂Pd₃Ga was synthesized as a pure phase whereas Ca₂Pt₃Ga was found as a diphasic product with Ca₂Pt₂Ga. Electronic structure calculations were performed to try and understand why CaPt₂ and CaPd₂, which crystalize in the cubic MgCu₂-type Laves phase structure, distort to the ordered rhombohedral variant, first observed in the magneto-restricted TbFe₂ compound, with the substitution of twenty-five percent of the Pt/Pd with Ga. Electronic stability was investigated by changing the valence electron count from 22e⁻/f.u. in CaPd₂ and CaPt₂ (2x) to 37e⁻/f.u. in Ca₂Pd₃Ga and Ca₂Pt₃Ga, which causes the Fermi level to shift to a more energetically favorable location in the DOS. The coloring problem was studied by placing a single Ga atom in each of four tetrahedra of the cubic unit cell of the MgCu₂-type structure, with nine symmetrically inequivalent models being investigated. Non-optimized and optimized total energy analyses of structural characteristics, along with electronic properties, will be discussed. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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20 pages, 27981 KiB

Open AccessArticle

Crystal Structure, Spectroscopic Investigations, and Physical Properties of the Ternary Intermetallic REPt₂Al₃ (RE = Y, Dy–Tm) and RE₂Pt₃Al₄ Representatives (RE = Tm, Lu)

by Fabian Eustermann, Simon Gausebeck, Carsten Dosche, Mareike Haensch, Gunther Wittstock and Oliver Janka

Crystals 2018, 8(4), 169; https://doi.org/10.3390/cryst8040169 - 16 Apr 2018

Cited by 10 | Viewed by 5850

Abstract

The REPt₂Al₃ compounds of the late rare-earth metals (RE = Y, Dy–Tm) were found to crystallize isostructural. Single-crystal X-ray investigations of YPt₂Al₃ revealed an orthorhombic unit cell (a = 1080.73(6), b = 1871.96(9), c [...] Read more.

The REPt₂Al₃ compounds of the late rare-earth metals (RE = Y, Dy–Tm) were found to crystallize isostructural. Single-crystal X-ray investigations of YPt₂Al₃ revealed an orthorhombic unit cell (a = 1080.73(6), b = 1871.96(9), c = 413.04(2) pm, wR2 = 0.0780, 942 F² values, 46 variables) with space group Cmmm (oC48; q²pji²hedb). A comparison with the Pearson database indicated that YPt₂Al₃ forms a new structure type, in which the Pt and Al atoms form a [Pt₂Al₃]^δ⁻ polyanion and the Y atoms reside in the cavities within the framework. Via a group-subgroup scheme, the relationship between the PrNi₂Al₃-type structure and the new YPt₂Al₃-type structure was illustrated. The compounds with RE = Dy–Tm were characterized by powder X-ray diffraction experiments. While YPt₂Al₃ is a Pauli-paramagnet, the other REPt₂Al₃ (RE = Dy–Tm) compounds exhibit paramagnetic behavior, which is in line with the rare-earth atoms being in the trivalent oxidation state. DyPt₂Al₃ and TmPt₂Al₃ exhibit ferromagnetic ordering at T_C = 10.8(1) and 4.7(1) K and HoPt₂Al₃ antiferromagnetic ordering at T_N = 5.5(1) K, respectively. Attempts to synthesize the isostructural lutetium compound resulted in the formation of Lu₂Pt₃Al₄ (Ce₂Ir₃Sb₄-type, Pnma, a = 1343.4(2), b = 416.41(8), c = 1141.1(2) pm), which could also be realized with thulium. The structure was refined from single-crystal data (wR2 = 0.0940, 1605 F² values, 56 variables). Again, a polyanion with bonding Pt–Al interactions was found, and the two distinct Lu atoms were residing in the cavities of the [Pt₃Al₄]^δ^– framework. X-ray photoelectron spectroscopy (XPS) measurements were conducted to examine the electron transfer from the rare-earth atoms onto the polyanionic framework. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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Review

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26 pages, 4591 KiB

Open AccessEditor’s ChoiceReview

The Crystal Orbital Hamilton Population (COHP) Method as a Tool to Visualize and Analyze Chemical Bonding in Intermetallic Compounds

by Simon Steinberg and Richard Dronskowski

Crystals 2018, 8(5), 225; https://doi.org/10.3390/cryst8050225 - 18 May 2018

Cited by 239 | Viewed by 18564

Abstract

Recognizing the bonding situations in chemical compounds is of fundamental interest for materials design because this very knowledge allows us to understand the sheer existence of a material and the structural arrangement of its constituting atoms. Since its definition 25 years ago, the [...] Read more.

Recognizing the bonding situations in chemical compounds is of fundamental interest for materials design because this very knowledge allows us to understand the sheer existence of a material and the structural arrangement of its constituting atoms. Since its definition 25 years ago, the Crystal Orbital Hamilton Population (COHP) method has been established as an efficient and reliable tool to extract the chemical-bonding information based on electronic-structure calculations of various quantum-chemical types. In this review, we present a brief introduction into the theoretical background of the COHP method and illustrate the latter by diverse applications, in particular by looking at representatives of the class of (polar) intermetallic compounds, usually considered as “black sheep” in the light of valence-electron counting schemes. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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