Complex rare-earth tetrelides, RE5(SixGe(1-x))4: new materials for magnetic refrigeration and a superb playground for solid state chemistry

A "giant magnetocaloric effect" discovered in 1997 for Gd5Si2Ge2 near room temperature has triggered optimism that environmentally-friendly, solid-state magnetic refrigeration may be viable to replace gas-compression technology in the near future. Gd5Si2Ge2 is one member of an extensive series of rare-earth compounds, RE5(SixGe(1-x))4. Due to the complexity of their structures and flexibility associated with chemical compositions, this series is an attractive "playground" to study the interrelationships among composition, structure, physical properties and chemical bonding. This tutorial review, which is directed toward students and researchers interested in structure-property relationships in solids, summarizes recent efforts concerning the synthesis, structure, physical properties, chemical bonding and chemical modifications of RE5(SixGe(1-x))4. A brief history of refrigerants, to present certain motivating factors for this research effort, as well as a brief overview of the magnetocaloric effect serves to introduce this review.     

Gd5-xYxTt4 (Tt = Si or Ge): effect of metal substitution on structure, bonding, and magnetism

A crystallographic study and theoretical assessment of the Gd/Y site preferences in the Gd 5- x Y x Tt 4 ( Tt = Si, Ge) series prepared by high-temperature methods is presented. All structures for the Gd 5- x Y x Si 4 system belong to the orthorhombic, Gd 5Si 4-type (space group Pnma). For the Gd 5- x Y x Ge 4 system, phases with x < 3.6 and x >or= 4.4 adopt the orthorhombic, Sm 5Ge 4-type structure. For the composition range of 3.6 相似文献   

Crystal structure, coloring problem and magnetism of Gd(5-x)Zr(x)Si4

Ternary Gd(5-x)Zr(x)Si(4) silicides were synthesized by arc melting of the constituent elements and subsequent heat treatments. The Gd(5-x)Zr(x)Si(4) phases adopt the orthorhombic Gd(5)Si(4)-type (space group Pnma) structure for x≤ 0.25 and the tetragonal Zr(5)Si(4)-type (space group P4(1)2(1)2) structure for x≥ 1.0, respectively. The samples with intermediate compositions contain two phases. Single-crystal X-ray diffraction reveals a preferential site occupancy for Zr on the three metal sites in the order of M3 > M2 > M1. Size arguments based on the local coordination environments suggest that the larger Gd atoms preferentially occupy the larger M1 site, while the smaller Zr atoms tend to occupy the smaller metal sites, M2 and M3. Tight-binding linear-muffin-tin orbital calculations illustrate a role of the metal-silicon bonds in the metal site occupation. An increase in the valence electron concentration through the Zr substitution weakens the Si-Si interactions but enhances the metal-silicon and metal-metal interactions. The Curie temperature of Gd(5-x)Zr(x)Si(4) decreases gradually with the increasing Zr content.     

Chemical pressure and rare-earth orbital contributions in mixed rare-earth silicides La(5-x)Y(x)Si4 (0 ≤ x ≤ 5)

A crystallographic study and theoretical analysis of the structural and La/Y site preferences in the La(5-x)Y(x)Si(4) (0 ≤ x ≤ 5) series prepared by high-temperature methods is presented. At room temperature, La-rich La(5-x)Y(x)Si(4) phases with x ≤ 3.0 exhibit the tetragonal Zr(5)Si(4)-type structure (space group P4(1)2(1)2, Z = 4, Pearson symbol tP36), which contains only Si-Si dimers. On the other hand, Y-rich phases with x = 4.0 and 4.5 adopt the orthorhombic Gd(5)Si(4)-type structure (space group Pnma, Z = 4, Pearson symbol oP36), also with Si-Si dimers, whereas Y(5)Si(4) forms the monoclinic Gd(5)Si(2)Ge(2) structure (space group P2(1)/c, Z = 4, Pearson symbol mP36), which exhibits 50% "broken" Si-Si dimers. Local and long-range structural relationships among the tetragonal, orthorhombic, and monoclinic structures are discussed. Refinements from single crystal X-ray diffraction studies of the three independent sites for La or Y atoms in the asymmetric unit reveal partial mixing of these elements, with clearly different preferences for these two elements. First-principles electronic structure calculations, used to investigate the La/Y site preferences and structural trends in the La(5-x)Y(x)Si(4) series, indicate that long- and short-range structural features are controlled largely by atomic sizes. La 5d and Y 4d orbitals, however, generate distinct, yet subtle effects on the electronic density of states curves, and influence characteristics of Si-Si bonding in these phases.     

Phase transformation driven by valence electron concentration: tuning interslab bond distances in Gd5GaxGe4-x

X-ray single crystal and powder diffraction studies on the Gd(5)Ga(x)()Ge(4)(-)(x)() system with 0 < or = x < or = 2.2 reveal dependence of interslab T-T dimer distances and crystal structures themselves on valence electron concentration (T is a mixture of Ga and Ge atoms). While the Gd(5)Ga(x)()Ge(4)(-)(x)() phases with 0 < or = x < or = 0.6 and valence electron concentration of 30.4-31 e(-)/formula crystallize with the Sm(5)Ge(4)-type structure, in which all interslab T-T dimers are broken (distances exceeding 3.4 A), the phases with 1 < or = x < or = 2.2 and valence electron concentration of 28.8-30 e-/formula adopt the Pu(5)Rh(4)- or Gd(5)Si(4)-type structures with T-T dimers between the slabs. An orthorhombic Pu(5)Rh(4)-type structure, which is intermediate between the Gd(5)Si(4)- and Sm(5)Ge(4)-type structures, has been identified for the Gd(5)GaGe(3) composition. Tight-binding linear-muffin-tin-orbital calculations show that substitution of three-valent Ga by four-valent Ge leads to larger population of the antibonding states within the dimers and, thus, to dimer stretching and eventually to dimer cleavage.     

Isolation of the new cubic phases RE4FeGa(12-x)Ge(x) (RE = Sm,Tb; x = 2.5) from molten gallium: single-crystal neutron diffraction study of the Ga/Ge distribution

The compounds RE4FeGa(12-x)Ge(x) (RE = Sm, Tb) were discovered in reactions employing molten Ga as a solvent at 850 degrees C. However, the isostructural Y4FeGa(12-x)Ge(x) was prepared from a direct combination reaction. The crystal structure is cubic with space group Imm, Z = 2, and a = 8.657(4) A and 8.5620(9) A for the Sm and Tb analogues, respectively. Structure refinement based on full-matrix least squares on F(o)2 resulted in R1 = 1.47% and wR2 = 4.13% [I > 2(I)] for RE = Sm and R1 = 2.29% and wR2 = 7.12% [I > 2(I)] for RE = Tb. The compounds crystallize in the U4Re7Si6 structure type, where the RE atoms are located on 8c (1/4, 1/4, 1/4) sites and the Fe atoms on 2a (0, 0, 0) sites. The distribution of Ga and Ge in the structure, investigated with single-crystal neutron diffraction on the Tb analogue, revealed that these atoms are disordered over the 12d (1/4, 0, 1/2) and 12e (x, 0, 0) sites. The amount of Ga/Ge occupying the 12d and 12e sites refined to 89(4)/11 and 70(4)/30%, respectively. Transport property measurements indicate that these compounds are metallic conductors. Magnetic susceptibility measurements and M?ssbauer spectroscopy performed on the Tb analogue show a nonmagnetic state for Fe, while the Tb atoms carry a magnetic moment corresponding to a mu(eff) of 9.25 mu(B).     

La-Fe-M(M=Al, Si)化合物磁热性能研究进展

介绍了La-Fe-M(M=AI,Si)化合物在磁热性能研究方面的最新进展。具有NaZn13型晶体结构,含高浓度Fe的La-Fe—M(M=AI,Si)化合物为良好的软磁材料;用少量的Co替代化合物中Si,Al元素可以将化合物的居里温度提高至室温;对La(Fe1-yCoy)xSi13-x化合物,适量的Si,Co组合可使化合物在室温产生可与Gd5Si2Ge2比拟的磁热效应;加入适量的间隙原子H,也可使La(FexSi1-x)13在室温的磁热性能远远大于金属Gd;对含Si量低及含Si量高的La(FexSi1-x)13化合物在相转变点附近由温度和磁场诱导相变的本质做了详细阐述。     

Synthesis and properties of new multinary silicides R5Mg5Fe4Al(x)Si(18-x) (R = Gd, Dy, Y, x ≈ 12) grown in Mg/Al flux

Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R(5)Mg(5)Fe(4)Al(x)Si(18-x) (x ≈ 12). These phases have a new structure type in tetragonal space group P4/mmm (a = 11.655(2) ?, c = 4.0668(8) ?, Z = 1 and R(1) = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and (27)Al and (29)Si MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at T(N) = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field.     

Electron-deficient Eu(6.5)Gd(0.5)Ge6 intermetallic: a layered intergrowth phase of the Gd5Si4- and FeB-type structures

A novel electron-poor Eu(6.5)Gd(0.5)Ge? compound adopts the Ca?Sn?-type structure (space group Pnma, Z = 4, a = 7.5943(5) ?, b = 22.905(1) ?, c = 8.3610(4) ?, and V = 1454.4(1) ?3). The compound can be seen as an intergrowth of the Gd?Si?-type (Pnma) R?Ge? (R = rare earth) and FeB-type (Pnma) RGe compounds. The phase analysis suggests that the Eu(7-x)Gd(x)Ge? series displays a narrow homogneity range of stabilizing the Ca?Sn? structure at x ≈ 0.5. The structural results illustrate the structural rigidity of the 2(∝)[R?X?] slabs (X = p-element) and a possibility for discovering new intermetallics by combining the 2(∝)[R?X?] slabs with other symmetry-approximate building blocks. Electronic structure analysis suggests that the stability and composition of Eu(6.5)Gd(0.5)Ge? represents a compromise between the valence electron concentration, bonding, and existence of the neighboring EuGe and (Eu,Gd)?Ge? phases.     

Polygallide RE2MGa9Ge2 (RE = Ce, Sm; M = Ni, Co) phases grown in molten gallium

The quaternary intermetallics Ce2CoGa9Ge2, Ce2NiGa9Ge2, and Sm2NiGa9Ge2 were prepared by reacting elemental metals in excess of gallium at 850 degrees C. The title compounds crystallize in the tetragonal space group P4/nmm in the Sm2Ni(Si(1-x)Ni(x))Al4Si6 structure type with cell parameters a = 5.9582(5) A, c = 15.0137(18) A, and a = 5.9082(17) A, c = 14.919(6) A, Z = 2, for Ce2CoGa9Ge2 and Sm2NiGa9Ge2, respectively. The structures are composed of covalently bonded three-dimensional networks of [CoGa9Ge2] in which the rare-earth metals fill the voids forming a 2D square net. The structures of RE2MGa9Ge2 are Ga-rich and possess extensive Ga-Ga bonding even though the Ga atoms do not form a network on their own. Magnetic susceptibility measurements for Ce2CoGa9Ge2 and Ce2NiGa9Ge2 show Curie-Weiss paramagnetism, consistent with presence of Ce(3+) ions. Magnetocrystalline anisotropy was observed for Ce2NiGa9Ge2, with the magnetically easy axis lying along the [001] crystallographic direction. A transition to an antiferromagnetic state was observed below 4 K in the easy direction of magnetization. In the magnetically hard direction of the basal plane, paramagnetic behavior was observed down to 1.8 K.     

MA(delta)Sb(2-delta) (M = Zr, Hf; A = Si, Ge): a new series of ternary antimonides and not "beta-ZrSb2"

The ternary antimonides ZrSi(delta)Sb(2-delta), HfGe(delta)Sb(2-delta), and ZrGe(delta)Sb(2-delta) were prepared by annealing of the elements in stoichiometric ratios below 800 degrees C. ZrSi(delta)Sb(2-delta) was earlier erroneously described as the binary "beta-ZrSb(2)", which does not exist as such, because the incorporation of tetrel atoms is necessary for the formation of this structure. ZrSi(delta)Sb(2-delta) has a small yet significant phase width with at least 0.066(7) < or = delta < or = 0.115(3), whereas the Ge analogues exist with larger tetrel concentration, i.e., ZrGe(0.211(5))Sb(1.789) and HfGe(0.205(6))Sb(1.795). The whole series of title compounds crystallizes in the Co(2)Si type (space group Pnma), with lattice dimensions of, e.g., for ZrGe(0.211(5))Sb(1.789), a = 730.4(1) pm, b = 395.13(6) pm, c = 957.6(2) pm, V = 0.27635(7) nm(3), Z = 4. The anionic substructure comprises infinite ribbons formed by the atom sites Q1 and Sb2, with Q1 being mixed occupied by Si or Ge and Sb atoms. These ribbons exhibit Q1-Q1 single bonds and Q1-Sb2 "half" bonds. Assuming the validity of the 8 - N rule, one can assign seven valence-electrons to Sb2 but only five to Q1, which might explain the preference of the tetrel atoms for the latter site.     

RE2MAl6Si4 (RE = Gd, Tb, Dy; M = Au, Pt): layered quaternary intermetallics featuring CaAl2Si2-type and YNiAl4Ge2-type slabs grown from aluminum flux

Six new intermetallic aluminum silicides--Gd(2)PtAl(6)Si(4), Gd(2)AuAl(6)Si(4), Tb(2)PtAl(6)Si(4), Tb(2)AuAl(6)Si(4), Dy(2)PtAl(6)Si(4), and Dy(2)AuAl(6)Si(4)--have been obtained from reactions carried out in aluminum flux. The structure of these compounds was determined by single-crystal X-ray diffraction. They form in space group Rthremacr;m with cell constants of a = 4.1623(3) A and c = 51.048(5) A for the Gd(2)PtAl(6)Si(4) compound. The crystal structure is comprised of hexagonal nets of rare earth atoms alternating with two kinds of layers that have been observed in other multinary aluminide intermetallic compounds (CaAl(2)Si(2) and YNiAl(4)Ge(2)). All six RE(2)MAl(6)Si(4) compounds show antiferromagnetic transitions at low temperatures (T(N) < 20 K); magnetization studies of the Dy compounds show metamagnetic behavior with reorientation of spins at 6000 G. Band structure calculations indicate that the AlSi puckered hexagonal sheets in this structure are electronically distinct from the other surrounding structural motifs.     

Alkaline-earth metal mercury intermetallics A11-xHg54+x (A = Ca, Sr)

Re-examination of the mercury-rich regions of the Ca-Hg and Sr-Hg phase diagrams has shown that the phases previously identified as "AHg 3.6" should be reformulated as A(11-x) Hg(54+x) (A = Ca, Sr). The crystal structures for representative members of these A 11- x Hg 54+ x phases were determined from single-crystal X-ray diffraction data (Pearson symbol hP65, space group P6; a = 13.389(1) A, c = 9.615(1) A for Ca(10.92(2))Hg(54.08) (x = 0.08(2)); a = 13.602(2) A, c = 9.818(1) A for Sr(10.48(4))Hg(54.52) ( x = 0.52(4))) and confirmed by powder Rietveld refinements ( R B = 0.020 for Ca(10.7(2))Hg(54.3) and 0.014 for Sr(10.7(3))Hg(54.3)). Diverse coordination polyhedra surround the A (CN14-16, multiply capped pentagonal or hexagonal prisms as well as Friauf polyhedra) and Hg atoms (CN11-13, pentacapped trigonal prisms and icosahedra). Partial disorder of Hg into one of the A sites accounts for the nonstoichiometry in the A(11-x)Hg(54+ x) phases. If this disordered A site is completely occupied by Hg atoms, the composition is constrained to a maximum of x = 2 in A(11-x)Hg(54+ x), corresponding to a small homogeneity range of "A(0.14-0.17)Hg(0.86-0.83)"; the true homogeneity range is likely narrower. The structure can be regarded as being built up from a stacking of triangular nets with hexagonal voids that are filled with single atoms or various clusters. In particular, the presence of triangular Hg 3 clusters in ordered orientations distinguishes this structure from that of the related Gd 14Ag 51-type structure, in which triangular Ag 3 clusters are in disordered orientations. Band structure calculations reveal a small degree of electron transfer from the A to Hg atoms, supporting the presence of a partially anionic mercuride substructure.     

On the distribution of tetrelide atoms (Si, Ge) in Gd5(SixGe1−x)4

A crystallographic study of the Si/Ge site preferences in the Si-rich regime of Gd₅(Si_xGe_1−x)₄ and a crystal chemical analysis of these site preferences for the entire range is presented. The room temperature crystal structure of Gd₅Si₄ as well as four pseudobinary phases, Gd₅(Si_xGe_1−x)₄ for x?0.6, is reported. All structures are orthorhombic (space group Pnma), Gd₅Si₄-type and show decreasing volume as the Si concentration increases. Refinements of the site occupancies for the three crystallographic sites for Si/Ge atoms in the asymmetric unit reveal a nonrandom, but still incompletely ordered arrangement of Si and Ge atoms. The distribution of Si and Ge atoms at each site impacts the fractions of possible homonuclear and heteronuclear Si-Si, Si-Ge and Ge-Ge dimers in the various structures. This distribution correlates with the observed room temperature crystal structures for the entire series of Gd₅(Si_xGe_1−x)₄.     

REMGa3Ge and RE3Ni3Ga8Ge3 (M = Ni, Co; RE = rare-earth element): new intermetallics synthesized in liquid gallium. X-ray, electron, and neutron structure determination and magnetism

New quaternary intermetallic phases REMGa(3)Ge (1) (RE = Y, Sm, Tb, Gd, Er, Tm; M = Ni, Co) and RE(3)Ni(3)Ga(8)Ge(3) (2) (RE = Sm, Gd) were obtained from exploratory reactions involving rare-earth elements (RE), transition metal (M), Ge, and excess liquid Ga the reactive solvent. The crystal structures were solved with single-crystal X-ray and electron diffraction. The crystals of 1 and 2 are tetragonal. Single-crystal X-ray data: YNiGa(3)Ge, a = 4.1748(10) A, c = 23.710(8) A, V = 413.24(2) A(3), I4/mmm, Z = 4; Gd(3)Ni(3)Ga(8)Ge(3), a = 4.1809(18) A, c = 17.035(11) A, V = 297.8(3) A(3), P4/mmm, Z = 1. Both compounds feature square nets of Ga atoms. The distribution of Ga and Ge atoms in the REMGa(3)Ge was determined with neutron diffraction. The neutron experiments revealed that in 1 the Ge atoms are specifically located at the 4e crystallographic site, while Ga atoms are at 4d and 8g. The crystal structures of these compounds are related and could be derived from the consecutive stacking of disordered [MGa](2) puckered layers, monatomic RE-Ge planes and [MGa(4)Ge(2)] slabs. Complex superstructures with modulations occurring in the ab-plane and believed to be associated with the square nets of Ga atoms were found by electron diffraction. The magnetic measurements show antiferromagnetic ordering of the moments located on the RE atoms at low temperature, and Curie-Weiss behavior at higher temperatures with the values of mu(eff) close to those expected for RE(3+) free ions.     

Structure and properties of Gd3Ge4: the orthorhombic RE3Ge4 structures revisited (RE = Y, Tb-Tm)

The new binary compound Gd(3)Ge(4) has been synthesized and its structure has been determined from single-crystal X-ray diffraction. Gd(3)Ge(4) crystallizes in the orthorhombic space group Cmcm (No. 63) with unit cell parameters a = 4.0953(11) A, b = 10.735(3) A, c = 14.335(4) A, and Z = 4. Its structure can be described as corrugated layers of germanium atoms with gadolinium atoms enclosed between them. The bonding arrangement in Gd(3)Ge(4) can also be derived from that of the known compound GdGe (CrB type) through cleavage of the (infinity)(1)[Ge(2)] zigzag chains in GdGe and a subsequent insertion of an extra germanium atom between the resulting triangular fragments. Formally, these characteristics represent isotypism with the Er(3)Ge(4) type (Pearson's oC28). However, re-examination of the crystallography in the whole RE(3)Ge(4) series (RE = Y, Tb-Tm) revealed discrepancies and called into question the accuracy of the originally determined structures. This necessitated a new rationalization of the bonding, which is provided in the context of a comparative discussion concerning both the original and revised structure models, along with an analysis of the trends across the series. The temperature dependence of the magnetic susceptibility of Gd(3)Ge(4) shows that it is paramagnetic at room temperature and undergoes antiferromagnetic ordering below 29 K. Magnetization, resistivity, and calorimetry data for several other members of the RE(3)Ge(4) family are presented as well.     

Extreme differences in oxidation states: synthesis and structural analysis of the germanide oxometallates A10[Ge9]2[WO4] as well as A(10+x)[Ge9]2[W(1–x)Nb(x)O4] with A = K and Rb containing [Ge9]4- polyanions

Semitransparent dark-red or ruby-red moisture- and air-sensitive single crystals of A(10+x)[Ge(9)](2)[W(1-x)Nb(x)O(4)] (A = K, Rb; x = 0, 0.35) were obtained by high-temperature solid-state reactions. The crystal structure of the compounds was determined by single-crystal X-ray diffraction experiments. They crystallize in a new structure type (P2(1)/c, Z = 4) with a = 13.908(1) ?, b = 15.909(1) ?, c = 17.383(1) ?, and β = 90.050(6)° for K(10.35(1))[Ge(9)](2)[W(0.65(1))Nb(0.35(1))O(4)]; a = 14.361(3) ?, b = 16.356(3) ?, c = 17.839(4) ?, and β = 90.01(3)° for Rb(10.35(1))[Ge(9)](2)[W(0.65(1))Nb(0.35(1))O(4)]; a = 13.8979(2) ?, b = 15.5390(3) ?, c = 17.4007(3) ?, and β = 90.188(1)° for K(10)[Ge(9)](2)WO(4); and a = 14.3230(7) ?, b = 15.9060(9) ?, c = 17.8634(9) ?, and β = 90.078(4)° for Rb(10)[Ge(9)](2)WO(4). The compounds contain discrete Ge(9)(4-) Wade's nido clusters and WO(4)(2-) (or NbO(4)(3-)) anions, which are packed according to a hierarchical atom-to-cluster replacement of the Al(2)Cu prototype and are separated by K and Rb cations, respectively. The alkali metal atoms occupy the corresponding tetrahedral sites of the Al(2)Cu prototype. The amount of the alkali metal atoms on these diamagnetic compounds corresponds directly to the amount of W substituted by Nb. Thus, the transition metals W and Nb appear with oxidation numbers +6 and +5, respectively, in the vicinity of a [Ge(9)](4-) polyanion. The crystals of the mixed salts were further characterized by Raman spectroscopy. The Raman data are in good agreement with the results from the X-ray structural analyses.     

平面五配位硅、 锗XBe5H6(X=Si,Ge)团簇

采用密度泛函理论PBE0方法, 在aug-cc-pVTZ水平上理论预测了含平面五配位硅和锗原子的XBe₅H₆ (X=Si, Ge)团簇. 势能面系统搜索及高精度量化计算表明, 它们均为全局极小结构. XBe₅H₆(X=Si, Ge)团簇整体呈完美的扇形结构: Si/Ge原子被5个金属Be原子配位; 4个H原子以桥基方式与Be原子相键连, 剩余的2个 H原子以端基方式与两端的Be原子成键. 化学键分析表明, XBe₅H₆(X=Si, Ge) 团簇中XBe₅单元具有完全离域的1个π及3个σ键, 外围铍氢间形成4个Be—H—Be 三中心二电子(3c-2e)键及2个定域的Be—H键. XBe₅单元上离域的2π及6σ电子赋予体系π和σ双重芳香性, 并使Si/Ge原子满足八隅律（或八电子规则）. 能量分解-化学价自然轨道分析揭示, Si/Ge和Be₅H₆之间主要为电子共享键.     

Structure and thermoelectric properties of the new quaternary bismuth selenides A(1-x)M(4-x)Bi(11+x)Se21 (A = K and Rb and Cs; M = Sn and Pb)--members of the grand homologous series Km(M6Se8)m(M(5+n)Se(9+n))

Several members of the new family A(1-x)M(4-x)Bi(11+x)Se21 (A = K, Rb, Cs; M = Sn, Pb) were prepared by direct combination of A2Se, Bi2Se3, Sn (or Pb), and Se at 800 degrees C. The single-crystal structures of K(0.54)Sn(3.54)Bi(11.46)Se21, K(1.46)Pb(3.08)Bi(11.46)Se21, Rb(0.69)Pb(3.69)Bi(11.31)Se21, and Cs(0.65)Pb(3.65)Bi(11.35)Se21 were determined. The compounds A(1-x)M(4-x)Bi(11+x) Se21 crystallize in a new structure type with the monoclinic space group C2/m, in which building units of the Bi2Te3 and NaCl structure type join to give rise to a novel kind of three-dimensional anionic framework with alkali-ion-filled tunnels. The building units are assembled from distorted, edge-sharing (Bi,Sn)Se6 octahedra. Bi and Sn/Pb atoms are disordered over the metal sites of the chalcogenide network, while the alkali site is not fully occupied. A grand homologous series Km(M6Se8)m(M(5+n)Se(9+n)) has been identified of which the compounds A(1-x)M(4-x)Bi(11+x)Se21 are members. We discuss here the crystal structure, charge-transport properties, and very low thermal conductivity of A(1-x)M(4-x)Bi(11+x)Se21.     

Synthesis, Structure, and Bonding of Two Lanthanum Indium Germanides with Novel Structures and Properties

The new tetragonal phases La(3)In(4)Ge and La(3)InGe are obtained from high-temperature reactions of the elements in welded Ta followed by annealing. The structures of both were established by single-crystal X-ray diffraction in tetragonal space group I4/mcm (Z = 4 and 16, a = 8.5165(3) and 12.3083(2) ?, c = 11.9024(4) and 16.0776(4) ?, respectively). La(3)In(4)Ge contains layers or slabs of three-connected indium built of puckered 8-rings and 4-rings, or of squashed tetrahedra ("butterflies") interlinked at all vertices, and these are separated by layers of La and isolated Ge. The phase is deficient of being a Zintl phase by three electrons per formula unit and is better described in terms of an alternate optimized and delocalized bonding picture and an open-shell metallic behavior for the In slabs. The more complex La(3)InGe, isostructural with Gd(3)Ga(2), is also layered. This phase contains pairs of mixed-occupancy (0.75 In, 0.25 Ge) sites separated by 3.020 ?, as well as isolated In and Ge atoms. The former appear to be fully reduced closed-shell atoms (relative to the bonded Ga dimers in Gd(3)Ga(2)) that are held in somewhat close proximity by cation matrix effects. The compound appears to be semiconducting and thus is a classical Zintl phase, (La(+3))(3)In(-5)Ge(-4) in the simplest oxidation state notation. High Coulomb energies are presumably important for the nature of the bonding and the stabilities of both compounds.