Lattice vibration spectra. XXVI. Far-infared spectra of the ternary skutterudites CoP3−xAsx, CoAs3−xSbx, and MGe1.5Y1.5 (M = Co,Rh, Ir; Y = S,Se)

The infrared spectra of the skutterudite solid solutions CoP_3?xAs_x and CoAs_3?xSb_x and the ordered ternary skutterudites MGe_1.5Y_1.5 (M = Co, Rh, Ir; Y = S, Se) have been studied. In the system CoP_3?xAs_x a complete mixed-crystal series has been obtained. In the system CoAs_3?xSb_x a miscibility gap has been found. The infrared spectra of the mixed crystals are influenced by large plasmon-phonon interactions with anomalous temperature shifts of the resonance frequencies. Substitution of arsenic in CoAs₃ by small amounts of phosphorus or antimony results in an additional short-waved mode at 362 and 342 cm^?1, respectively, which can be assigned to an internal vibration of the four-membered anionic rings not allowed in the binary skutterudites. The infrared spectra of the ordered ternary skutterudites MGe_1.5Y_1.5 (M = Co, Ir) show a large number of the total of 64 ir allowed lattice modes, whereas the spectra of the rhodium compounds are mainly of the free carrier type. RhGe_1.5S_1.5 and the firstly obtained RhGe_1.5Se_1.5 exhibit small deviations from the formerly claimed pseudo-cubic cell. The lattice constants (space group R3) are a = 828.2(1) pm, α = 89.95(1)° and a = 854.6(1) pm, α = 89.86(1)°, respectively.     

Structure and thermoelectric properties of the ordered skutterudite CoGe1.5Te1.5

The skutterudite-related material CoGe_1.5Te_1.5 has been synthesised and structurally characterised by powder neutron diffraction. Analysis of the high-resolution powder neutron diffraction data indicates that the structure of CoGe_1.5Te_1.5 retains the a⁺a⁺a⁺ tilt system of the ideal skutterudite structure, while the anions are ordered in layers perpendicular to the [111] direction of the skutterudite unit cell. This anion ordering results in a lowering of the symmetry from cubic to rhombohedral (space group , a=12.3270(5) and c=15.102(1) Å at 293 K). The electrical transport properties have been investigated using four-probe resistivity and Seebeck coefficient measurements. The temperature dependence of the electrical resistivity and the magnitude of the Seebeck coefficient indicate that CoGe_1.5Te_1.5 is an n-type semiconductor.     

Solvothermal synthesis of three new dimeric thiogermanates (enH)4Ge2S6, [Mn(en)3]2Ge2S6 and [Ni(en)3]2Ge2S6 from germanium dioxide and sulfur powder

Three new thiogermanates (enH)₄Ge₂S₆ (1) and [M(en)₃]₂Ge₂S₆ (M=Mn (2), Ni (3); en=ethylenediamine) were synthesized using GeO₂ and S₈ as starting materials in molar ratio of 1:0.5 under solvothermal conditions. These compounds suggest that the dimeric [Ge₂S₆]⁴⁻ anion is likely to be the main germanium-containing species in en system and it also might be preferred as counter anions by the transition metal complex cations in crystallization. The cations of [Mn(en)₃]²⁺ and [Ni(en)₃]²⁺ are even better mineralizers than the protonated amine of [enH]⁺. The crystal systems of [Ge₂S₆]⁴⁻ compounds are related to entities of cations and intermolecular reactions between cations and [Ge₂S₆]⁴⁻ anions. The compounds remove ethylenediamine and H₂S molecules in multi steps when being heated under nitrogen stream.     

Phase relationships in the systems MSCr2S3In2S3 (M = Co,Cd, Hg)

The phase diagrams of the quaternary systems MSCr₂S₃In₂S₃, with M = Co, Cd, and Hg, were studied with the help of X-ray powder photographs of quenched samples, high-temperature X-ray diffraction patterns, DTA and TG measurements, and far-infrared spectra. Because indium sulfides do react with silica tubes, alumina crucibles must be used for annealing the samples. Complete series of mixed crystals are formed among the spinel-type compounds MCr₂S₄, MIn₂S₄ (M = Cd, Hg), and In₂S₃. HgIn₂S₄ is decomposed at temperatures above 300°C. In the sections CoCr₂S₄CoIn₂S₄ and CoCr₂S₄In₂S₃ relatively large miscibility gaps exist due to the change from normal to inverse spinel structure. But the interchangeability of both systems increases with increasing temperature, and at temperatures above 1000°C, complete series of solid solutions are formed, which can be quenched to ambient temperature. Superstructure ordering like that of ordered α-In₂S₃ has been found in the In-rich region of the MIn₂S₄In₂S₃ solid solutions. The unit cell dimensions of all stoichiometric and phase boundary compounds, e.g., Cd_1.15In_1.9S₄, including the chromium spinels MCr₂S₄ (M = Mn, Zn) and ZnCr₂Se₄, are given and discussed in terms of possible deviations from stoichiometry.     

Synthesis and structure of a new family of phases, A2MGe5O12: A = Rb,Cs; M = Be,Mg, Co,Zn

A new family of eight germanate phases, A₂MGe₅O₁₂: A = Rb, Cs; M = Be, Mg, Co, Zn, has been synthesized. They are cubic with a in the range 13.7 to 14.0 Å, Z = 8, and space group $\text{I}\text{4}\text{3d}$. These phases, named the β phases, are isostructural with KBSi₂O₆ which has a structure related to that of pollucite, CsAlSi₂O₆. The structure of one, Rb₂ZnGe₅O₁₂, has been refined to an R value of 0.079 using X-ray powder diffraction data. Several of the new phases are polymorphic. Cs₂ZnGe₅O₁₂, Cs₂CoGe₅O₁₂, and Rb₂MgGe₅O₁₂ form low-temperature, δ polymorphs which have primitive cubic unit cells. Rb₂ZnGe₅O₁₂ forms a low-temperature, ε polymorph which is probably a tetragonal distortion of the β structure.     

New Ternary Compounds MxTa11−xGe8 (M=Ti, Zr, Hf): Structure and Stabilization

The title compounds M_xTa_11−xGe₈ (M=Ti, Zr, Hf) were prepared from the pure elements by arc-melting and subsequent induction heating at temperatures between 1200°C and 1400°C. X-ray powder diffraction studies of the samples were performed using the Guinier technique and the respective powder patterns were refined with a structure model based on the orthorhombic Cr₁₁Ge₈-structure type (oP76, Pnma). The homogeneity ranges of the compounds were determined to be 0.9<x<1.3 (M=Ti), 0.7<x<1.3 (M=Zr) and 0.7<x<2.4> (M=Hf) by means of electron probe microanalysis. Chemical bonding, electronic structure and site preferences are discussed based on extended Hückel calculations performed on hypothetical binary Ta₁₁Ge₈.     

New Compounds of the ThCr2Si2-Type and the Electronic Structure of CaM2Ge2 (M: Mn-Zn)

Two new compounds were synthesized by heating mixtures of the elements at 975-1025 K and characterized by single-crystal X-ray methods. CaZn₂Si₂ (a=4.173(2) Å, c=10.576(5) Å) and EuZn₂Ge₂ (a=4.348(2) Å, c=10.589(9) Å) crystallize in the ThCr₂Si₂-type structure (space group I4/mmm; Z=2). Magnetic susceptibility measurements of EuZn₂Ge₂ show Curie-Weiss behavior with a magnetic moment of 7.85(5)μ_B/Eu and a paramagnetic Curie temperature of 10(1) K. EuZn₂Ge₂ orders antiferromagnetically at T_N=10.0(5) K and undergoes a metamagnetic transition at a low critical field of about 0.3(2) T. The saturation magnetization at 2 K and 5.5 T is 6.60(5) μ_B/Eu. ¹⁵¹Eu Mössbauer spectroscopic experiments show one signal at 78 K at an isomer shift of −11.4(1) mm/s and a line width of 2.7(1) mm/s compatible with divalent europium. At 4.2 K full magnetic hyperfine field splitting with a field of 26.4(4) T is detected. The already known compounds CaM₂Ge₂ (M: Mn-Zn) also crystallize in the ThCr₂Si₂-type structure. Their MGe₄ tetrahedra are strongly distorted with M=Ni and nearly undistorted with M=Mn or Zn. According to LMTO electronic band structure calculations, the distortion is driven by a charge transfer from M-Ge antibonding to bonding levels.     

Synthese et structure du sulfate double MSbF2SO4 (M = Rb,Cs)

The crystal structure of RbSbF₂SO₄ has been determined on a single crystal (R = 0.078 for 710 reflections). The structure shows sulfate anions distorted by the SOSb bonds. The antimony atom is from an SbF₂ unit. This antimony dihalogen is from the family of the $\text{1}\text{1}$ compounds which are in MX₃SbX₃ (M = Al, Ga, In) (X = Cl, Br) systems.     

Insights on the Structural Chemistry of Hydrocalumite and Hydrotalcite-like Materials: Investigation of the Series Ca2M(OH)6Cl·2H2O (M: Al, Ga, Fe, and Sc) by X-Ray Powder Diffraction

Hydrotalcite and hydrocalumite are two close minerals belonging to the layered double hydroxide family. Both structures are based on positive brucite-like layers alternating with layers containing anions and water molecules. Most of synthetic (LDHs) are hydrotalcite-like materials. On the other hand, the hydrocalumite structure type is rare for those less broad in composition, typically Ca²⁺ and Al³⁺ in the hydroxide layers. In order to get further insight into the conditions of stabilization of this structure type, we have undertaken the synthesis and the structural characterization by powder X-ray diffraction of the series Ca₂M³⁺(OH)₆Cl·2H₂O(M³⁺:Al³⁺, Ga³⁺, Fe³⁺ and Sc³⁺). The incorporation of Sc³⁺ ions is quite original. All phases crystallize in the rhombohedral space group R-3 resulting in a three-layers polytype. The main consequence of the replacement of Al³⁺ cations by large M³⁺ cations is a compression of the octahedral layers like it proceeds in hydrotalcite-like materials. The existence of Sc-containing phase allows us to say that it is the size of Ca²⁺ ions and the pronounced anisotropy of coordination spheres around Ca²⁺ and M³⁺ which are responsible for the ordered distribution of cations in hydrocalumite-like materials.     

Etude cristallochimique et spectroscopique des fluoroniobates MNbF6 avec M = Mg,Ca, Mn,Fe, Co,Ni, Zn,et Cd

The compounds MNbF₆ were synthesized for M = Mg, Ca, Mn, Fe, Co, Ni, Zn, and Cd. They have a ReO₃-type structure, or one derived from it, and are isostructural with MNbF₇ compounds. The insertion of fluoride anions in the ReO₃-type structure is discussed. Studies of the spectroscopic uv and visible behavior were carried out, and assignments of absorption bands of solid samples were made in terms of crystal field theory, assuming an octahedral surrounding of the niobium ion and of the bivalent cation.     

Dielectric properties of some MM′O4 and MTiM′O6 (M=Cr, Fe, Ga; M′=Nb, Ta, Sb) rutile-type oxides

We describe an investigation of the structure and dielectric properties of MM′O₄ and MTiM′O₆ rutile-type oxides for M=Cr, Fe, Ga and M′=Nb, Ta and Sb. All the oxides adopt a disordered rutile structure (P4₂/mnm) at ambient temperature. A partial ordered trirutile-type structure is confirmed for FeTaO₄ from the low temperature (17 K) neutron diffraction studies. While both the MM′O₄ oxides (CrTaO₄ and FeTaO₄) investigated show a normal dielectric property MTiM′O₆ oxides for M=Fe, Cr and M′=Nb/Ta/Sb display a distinct relaxor/relaxor-like response. Significantly the corresponding gallium analogs, GaTiNbO₆ and GaTiTaO₆, do not show a relaxor response at T<500 K.     

Synthesis and Crystal Structure of M11X10 Compounds, M=Sr, Ba and X=Bi, Sb. Electronic Requirements and Chemical Bonding

The intermetallic compounds Sr₁₁Bi₁₀, Ba₁₁Bi₁₀, and (Sr₅Ba₆)Sb₁₀ have been obtained from melts of mixtures of the elements. They crystallize in the tetragonal system, space group I4/mmm, Ho₁₁Ge₁₀ structure type, tI84 Pearson symbol, Z=4, with cell parameters a=12.765(3), 13.230(3), 12.748(2) Å and c=18.407(3), 19.365(3), 18.761(2) Å, respectively. The structures were solved from single-crystal X-ray data and refined by full-matrix least-squares to R1=6.71, 5.44, and 5.73%. The structure of M₁₁X₁₀ contains three discrete anionic moieties: square rings X⁴⁻₄, dumbbells X⁴⁻₂, and isolated X³⁻. Using formal charges the unit cell of M₁₁X₁₀ may be described as containing 44 M²⁺, 2X⁴⁻₄, 8X⁴⁻₂, and 16X³⁻ ions. This structure is discussed in comparison with other Bi or Sb pnictide compounds. Bonding is analyzed therein using molecular orbital (EHMO) calculations for the anions (dumbbell and square units) and also the periodic tight-binding method. Lone pair repulsions inside and between the anionic units are evidenced; they are compensated by strong bonding cation-to-anion interactions. Interatomic distances along the series appear to be more dependent on packing than on electronic effects.     

Ternary silicides M2Cr4Si5 (M=Ti, Zr, Hf): filled variants of the Ta4SiTe4 structure type

The isostructural ternary silicides M₂Cr₄Si₅ (M=Ti, Zr, Hf) were prepared by arc-melting of the elemental components. The single-crystal structure of Zr₂Cr₄Si₅ was determined by X-ray diffraction (Pearson symbol oI44, orthorhombic, space group Ibam, Z=4, a=7.6354(12) Å, b=16.125(3) Å, c=5.0008(8) Å). Zr₂Cr₄Si₅ adopts the Nb₂Cr₄Si₅-type structure, an ordered variant of the V₆Si₅-type structure. It consists of square antiprisms that have Zr and Cr atoms at the corners and Si atoms at the centers; they share opposite faces to form one-dimensional chains _∞¹[Zr_4/2Cr_4/2Si] surrounded by additional Si atoms and extending along the c direction. In a new interpretation of the structure, additional Cr atoms occupy interstitial octahedral sites between these chains, clarifying the relation between this structure and that of Ta₄SiTe₄. The formation of short Si-Si bonds in Zr₂Cr₄Si₅ is contrasted with the absence of Te-Te bonds in Ta₄SiTe₄. The compounds M₂Cr₄Si₅ (M=Ti, Zr, Hf) exhibit metallic behavior and essentially temperature-independent paramagnetism. Bonding interactions were analyzed by band structure calculations, which confirm the importance of Si-Si bonding in these metal-rich compounds.     

Synthesis and crystal structure of Ln2MGe4O12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln₂M²⁺Ge₄O₁₂, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y_2−xEr_xCaGe₄O₁₂ (x=0-2), Y_2−2xCe_xCa_1+xGe₄O₁₂ (x=0-1), Y₂Ca_1−xMn_xGe₄O₁₂ (x=0-1) and Y_2−xPr_xMnGe₄O₁₂ (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu₂MnGe₄O₁₂ has been found.     

Relations structurales entre U3O8 et quelques fluorures mixtes de formules M2M′3F11, MM′2F7 et MM′3F10

U₃O₈ oxide, as well as M₂M′₃F₁₁, MM′₂F₇ and MM′₃F₁₀ fluorides, with M = Rb, Tl, Cs, NH₄ and M′ = In, Lu, Yb, Tm, is described as the regular repetition according to the … A-A-A … sequence of identical and parallel sheets of edge-and corner-sharing M′F₇ or UO₇ pentagonal bipyramids and M′F₆ octahedra. M′ and U atoms are systematically located at the lattice points of a pseudohexagonal network, but in the fluorides some of these lattice points are vacant, producing hexagonal tunnels in which M atoms are located. It is shown that in the two kinds of compounds the same linear chains and M′₃X₁₇ groups of pentagonal bipyramids are present, and that the transformation of the U₃O₈ structure into the fluorides can be achieved by an ordered substitution of some linear … UOUO … chains by … M-M-M … chains. All these structures can be described with the same structural model based on the chemical twinning principle.     

Electrical Behavior of New Orthophosphates Na2M3(PO4)3 (M3=GaMn2, GaCd2, InMn2 and FeMnCd) with Alluaudite-Like Structure

New sodium orthophosphates of general formula Na₂M₃(PO₄)₃ belonging to the alluaudite-type structure have been synthesized and characterized by neutron and X-ray powder diffraction. The nature of the M₃ elements (M₃=GaMn₂, GaCd₂, InMn₂ and FeMnCd) was chosen in order to analyze their influence on electrical and magnetic properties. The conductivity of these materials was measured by the complex impedance method and the transport mechanism was studied from complex permittivities and modulus formalism. Electrical results including charge/discharge experiments showed two main behaviors: GaCd₂ and FeMnCd behave as purely ionic conductors whereas GaMn₂ and InMn₂ are mixed ionic-electronic conductors. The magnetic susceptibility data reveal the antiferromagnetic behavior of FeMnCd, InMn₂ and GaMn₂, with a weak ferromagnetic transition at low temperatures.     

The crystal chemistry of the M+VO3 (M+ = Li,Na, K,NH4, Tl,Rb, and Cs) pyroxenes

The crystal structures of M⁺VO₃(M⁺ = K, NH₄, and Cs) have been refined using three-dimensional counter-diffractometer X-ray data and full-matrix least-squares methods. The structure of these compounds is characterized by a (V⁵⁺O^2?₃)^?_∞ chain extending along the c-axis (Pbcm orientation), with adjacent chains linked by the alkali metal cation. The structure may be considered as a variant of the pyroxene structure, and standard atom nomenclature is proposed in order to facilitate comparison with silicate pyroxenes. Structural variation across this series is discussed in detail and is compared with the analogous M⁺M³⁺Si₂O₆ (M⁺ = Li, Na; M³⁺ = Al, Cr, Fe, Sc, In) series.     

Synthesis and structure of In(IO3)3 and vibrational spectroscopy of M(IO3)3 (M=Al, Ga, In)

The reaction of Al, Ga, or In metals and H₅IO₆ in aqueous media at 180 °C leads to the formation of Al(IO₃)₃, Ga(IO₃)_3, or In(IO₃)₃, respectively. Single-crystal X-ray diffraction experiments have shown In(IO₃)₃ contains the Te₄O₉-type structure, while both Al(IO₃)₃ and Ga(IO₃)₃ are known to exhibit the polar Fe(IO₃)₃-type structure. Crystallographic data for In(IO₃)₃, trigonal, space group , a=9.7482(4) Å, c=14.1374(6) Å, V=1163.45(8) Z=6, R(F)=1.38% for 41 parameters with 644 reflections with I>2σ(I). All three iodate structures contain group 13 metal cations in a distorted octahedral coordination environment. M(IO₃)₃ (M=Al, Ga) contain a three-dimensional network formed by the bridging of Al³⁺ or Ga³⁺ cations by iodate anions. With In(IO₃)₃, iodate anions bridge In³⁺ cations in two-dimensional layers. Both materials contain distorted octahedral holes in their structures formed by terminal oxygen atoms from the iodate anions. The Raman spectra have been collected for these metal iodates; In(IO₃)₃ was found to display a distinctively different vibrational profile than Al(IO₃)₃ or Ga(IO₃)₃. Hence, the Raman profile can be used as a rapid diagnostic tool to discern between the different structural motifs.     

Crystal Structures and Magnetic Properties of the Two Misfit Layer Compounds: [SrGd0.5S1.5]1.16 NbS2 and [Sr(Fe,Nb)0.5S1.5]1.13 NbS2

Crystal structures of two new misfit compounds, [SrGd_0.5S_1.5]_1.16NbS₂ and [Sr(Fe,Nb)_0.5S_1.5]_1.13NbS₂, were determined through the composite approach, i.e., by refining each subpart (Q, H-parts, and the common part) of these composite materials, separately. The Q-part is a three-atom-thick layer, with the NaCl-type structure, where external SrS planes enclose the inner GdS or (Fe,Nb)S plane; the structural difference between these two compounds lies in the central layer within the Q-part: Gd and S atoms are in special positions (octahedral coordination), while Fe and S atoms are statistically distributed on split (×4) positions (tetrahedral coordination) around a central unique site (=special position occupied by Nb). The H-part is a sandwich of sulfur planes enclosing the inner Nb plane as observed for the structure of the binary compound NbS₂ itself. The Sr-Gd derivative shows a paramagnetic behavior in the whole studied temperature range (2-300 K). On the other hand, antiferromagnetic interactions occur in the Sr-Fe derivative; the complex magnetic behavior of this compound is related to the statistical distribution of Fe atoms which leads to frustration of the magnetic interactions. At room temperature, experimental values obtained from Mössbauer spectrum correspond to Fe³⁺ in tetrahedral sulfur environment: isomer shift δ=0.32 mm s⁻¹, and quadrupole splitting ΔE=0.48 mm s⁻¹.