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New clathrate type phases where Li atoms fill vacancies in the Ge-framework have been identified.

The clathrate-I borosilicide K8−xBySi46−y (0.8 ≤ x ≤ 1.2 and 6.4 ≤ y ≤ 7.2; space group Pmn) was prepared in sealed tantalum ampoules between 900 °C and 1000 °C. By high-pressure preparation at 8 GPa and 1000 °C, a higher boron content is achieved (x = 0.2, y = 7.8). Crystal structure and composition were established from X-ray diffraction data, chemical analysis, WDX spectroscopy, and confirmed by 11B and 29Si NMR, and magnetic susceptibility measurements. The compositions are electron-balanced according to the Zintl rule within one estimated standard deviation. The lattice parameter varies with composition from a = 9.905 A for K7.85(2)B7.8(1)Si38.2(1) to a = 9.968(1) A for K6.80(2)B6.4(5)Si39.6(5).

The new ternary compound Y4Be33Pt16 was prepared from elements by arc melting, and its crystal structure was determined from single-crystal X-ray diffraction data (space group I4[combining macron]3d, a = 13.4849(3) A). The material is the first representative of a new structure type of complex intermetallic compounds and reveals a cage-like crystal structure. Analysis of chemical bonding by means of the electron localizabilty approach indicates ionic interaction of yttrium with the rest of the crystal structure, characteristic for cage compounds, in particular for clathrates. In contrast to the mostly two-centre bonding in the framework of clathrates, the new compound is characterized by a multi-centre interaction within the framework, caused by the demand of the valence electrons in the system. The non-centrosymmetric material enters the superconducting state at TC = 0.9 K.

A detailed study of polycrystalline indium-based In1-x□xIn2S4 (x = 0.16, 0.22, 0.28, and 0.33) thiospinel is presented (□- vacancy). Comprehensive investigation of synthesis conditions, phase composition and thermoelectric properties was performed by means of various diffraction, microscopic and spectroscopic methods. Single-phase α- and β-In1-x□xIn2S4 were found in samples with 0.16 ≤x≤ 0.22 and x = 0.33 (In2S3), respectively. In contrast, it is shown that In0.72□0.28In2S4 contains both α- and β-polymorphic modifications. Consequently, the thermoelectric characterization of well-defined α- and β-In1-x□xIn2S4 is conducted for the first time. α-In1-x□xIn2S4 (x = 0.16 and 0.22) revealed n-type semiconducting behavior, a large Seebeck coefficient (>|200|μV K-1) and moderate charge carrier mobility on the level of ∼20 cm2 V-1 s-1 at room temperature (RT). Decreases in charge carrier concentration (increase of electrical resistivity) and thermal conductivity (even below 0.6 W m-1 K-1 at 760 K) for larger In-content are observed. Although β-In0.67□0.33In2S4 (β-In2S3) is a distinct polymorphic modification, it followed the abovementioned trend in thermal conductivity and displayed significantly higher charge carrier mobility (∼104 cm2 V-1 s-1 at RT). These findings indicate that structural disorder in the α-modification affects both electronic and thermal properties in this thiospinel. The reduction of thermal conductivity counterbalances a lowered power factor and, thus, the thermoelectric figure of merit ZTmax = 0.2 at 760 K is nearly the same for both α- and β-In1-x□xIn2S4.

Phase relationship and structural behaviour in the substitutional series LaNi13-xGax and CeNi13-xGax have been studied by a combination of X-ray powder diffraction measurements, differential scanning calorimetry, electron diffraction tomography and metallographic analyses. The sequence of morphotropic phase transformations has been found in the series LaNi13-xGax resulting in five varieties of the NaZn13 structure: the cubic phase with aristotype structure at x = 2 (space group Fm3[combining macron]c, Pearson symbol cF112), two tetragonal phases at x = 2.5-4.25 (space group I4/mcm, Pearson symbol tI56-I) and 7-7.5 (space group I4/mcm, Pearson symbol tI56-II), both with an atomic arrangement of the CeNi8.5Si4.5 type and two orthorhombic phases at x = 4.5-5.75 (LaNi7In6 structure type, space group Ibam, Pearson symbol oI56) and x = 6.37-6.87 (a new derivative of the NaZn13, prototype structure, space group Fmmm, Pearson symbol oF112). The related series CeNi13-xGax shows similar behaviour. The corresponding tI56-I ↔oI56 ↔oF112 ↔tI56-II phases are formed at x = 4-4.25, 4.5-6, 6.37-6.87 and 7-7.37, respectively. In contrast to the lanthanum analogues, the phase with cubic symmetry was not found for this system. Complex twinned and multiple twinned (twinning of twins) domain structures which are revealed for the tetragonal and both orthorhombic phases clearly indicate temperature-induced polymorphic phase transitions during the formation of these phases. LaNi13-xGax samples show paramagnetic behavior, whereas the CeNi13-xGax series exhibits Curie-Weiss paramagnetism.

The compounds MNi21B20 (M = In, Sn) have been synthesized and their cubic crystal structure determined (space group Pm[3 with combining macron]m, lattice parameters a = 7.1730(1) Å and a = 7.1834(1) Å, respectively). The structure can be described as a hierarchical partitioning of space based on a reo-e net formed by Ni3 species with large cubical, cuboctahedral and rhombicuboctahedral voids being filled according to [Ni1@Ni38], [M@Ni312], and [Ni26@B20@Ni324], respectively. The [Ni6@B20] motif inside the rhombicuboctahedral voids features an empty [Ni6] octahedron surrounded by a [B20] cage recently described in E2Ni21B20 (E = Zn, Ga). Position-space bonding analysis using ELI-D and QTAIM space partitioning as well as 2- and 3-center delocalization indices gives strong support to an alternative chemical description of space partitioning based on face-condensed [B@Ni6] trigonal prisms as basic building blocks. The shortest B–B contacts display locally nested 3-center B–B–Ni bonding inside each trigonal prism. This clearly rules out the notion of [Ni6@B20] clusters and leads to the arrangement of 20 face-condensed [B@Ni23Ni33] trigonal prisms resulting in a triple-shell like situation Ni26@B20@Ni324(reo-e), where the shells display comparable intra- and inter-shell bonding. Both compounds are Pauli paramagnets displaying metallic conductivity.

A new intermetallic compound AlPd15B7 was synthesized by arc-melting the stoichiometric mixture of the elements. Single crystal X-ray diffraction data of ternary metal-rich boride reveal a new type of structure with the space group Ia3d and the lattice parameter a = 16.4466(3) Å. It adopts a filled anti-Yb3Rh4Sn13-type structure, where the positions corresponding to 3Yb, 4Rh and 13Sn atoms are occupied by 3Pd, 4B, and 1Al + 12 Pd, respectively and 3B additionally at interstitial sites. Magnetic susceptibility, electrical resistivity, and specific heat measurements reveal bulk superconductivity with a critical temperature Tc ≈ 2.9 K. Electronic structure calculations show that Pd 4d and B 2p states dominate the density of states (DOS) at the Fermi level EF.

New germanides with composition RE3Pt4Ge13 (RE = Y, Pr, Sm, Gd, Tb, Tm) have been prepared by high-pressure, high-temperature synthesis. Their crystal structures have been refined, and the relationship of this new rhombohedral and monoclinic structure types with the primitive cubic Yb3Rh4Sn13 prototype is discussed. Band structure calculations within density functional theory confirm the distorted rhombohedral and monoclinic structural arrangements to be energetically more favorable than the simple cubic one. X-ray absorption spectroscopy and magnetic susceptibility measurements indicate that the RE-atoms are in the +3 oxidation state in all studied compounds.

The new clathrate-I phase Ba(8)Rh(x)Si(46-x-y□y) (2.36x2.74; y = 0.65 for x = 2.74), Pearson symbol cP54, space group Pm3[combining macron]n, was prepared as a single phase and characterized. For these compositions, the lattice parameter does not change significantly and was found to be a = 10.347(1) Å. Phase relations were investigated by means of metallographic and thermal analyses. The incongruently melting phase Ba(8)Rh(x)Si(46-x-y□y) was obtained by annealing a solidified melt of the stoichiometric composition at 1000 °C for five days. Below 850 °C, the clathrate phase slowly decomposes to BaSi(2), Si and the new compound BaRh(2)Si(9). Crystal structure and vacancy concentration were investigated by means of single crystal and powder X-ray diffraction and wavelength dispersive X-ray spectroscopy analyses. Quantum chemical calculations show that the Rh 4d states hybridize with Si 3p states so that the DOS structure cannot be simply deduced in a rigid band approach from that of the binary Ba(8)Si(46). Analysis of the chemical bonding applying the ELI-D approach yielded covalent two-center two-electron bonds between Rh and Si atoms. For the composition Ba(8)Rh(2.74)Si(42.61□0.65), a temperature dependent diamagnetic susceptibility and an almost temperature independent electrical resistivity (ρ≈ 5 μΩm) were observed. The magnitude of the negative Seebeck coefficient is increasing linearly with temperature from 0 μV K(-1) at 2 K to -35 μV K(-1) at 850 K indicating n-type electrical conductivity.

The new phases Ca3Pt4+xGe13−y (x = 0.1; y = 0.4; space group I213; a = 18.0578(1) Å; RI = 0.063; RP = 0.083) and Yb3Pt4Ge13 (space group P42cm; a = 12.7479(1) Å; c = 9.0009(1) Å; RI = 0.061, RP = 0.117) are obtained by high-pressure, high-temperature synthesis and crystallize in new distortion variants of the Pr3Rh4Sn13 type. Yb3Pt4Ge13 features Yb in a temperature-independent non-magnetic 4f14 (Yb2+) configuration validated by X-ray absorption spectra and resonant inelastic X-ray scattering data. Ca3Pt4+xGe13−y is diamagnetic (χ0 = −5.05 × 10−6 emu mol−1). The Sommerfeld coefficient γ = 4.4 mJ mol−1 K−2 for Ca3Pt4+xGe13−y, indicates metallic properties with a low density of states at the Fermi level in good agreement with electronic structure calculation (N(EF) = 3.3 eV−1/f.u.)); the Debye temperature (θD) is 398 K. Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.