Deep‐red moisture and air sensitive single crystals of K4Ge9 were obtained by reacting GeO2 and elemental Ge with metallic W and K at high temperature in a niobium ampoule. The crystal structure of the compound was determined by single crystal X‐ray diffraction experiments. K4Ge9 crystallizes in a polar space group R3c (No. 161), Z = 4 with a = 21.208(1) and c = 25.096(2) Å. The compound contains discrete Ge94− Wade's nido‐clusters which are packed according to a hierarchical atom‐to‐cluster replacement of the Cr3Si prototype and are separated by K+ cations. Two independent [Ge9]4− clusters A (at Cr positions) and B (at Si positions) are found with a ratio A:B = 3:1. The B‐type cluster satisfactorily represents orientational disorder around the three‐fold axis.

A new state-of-the-art thermoelectric material, TlAgTe, which possesses an extremely low thermal conductivity of about 0.25 W m K and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its subcell is a variant of the ZrFeP type, but ultimately its structure was refined as a composite structure of a TlAgTe framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P6(00γ) s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain substructure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ) s. The modulation leads to the formation of Te and Te fragments in this chain and a refined formula of TlAgTe. The structure consists of a complex network of three-dimensionally connected AgTe tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, TlAgTe, TlAgTe, and 2× TlAgTe, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.

The ternary Zintl phase Ca 3 Ag 1+x Ge 3-x (x = 1/3) was synthesized by the high-temperature solid-state technique and its crystal structure was refined from single-crystal diffraction data. The compound Ca 3 Ag 1.32 Ge 2.68(1) adopts the Sc 3 NiSi 3 type structure, crystal data: space group C2/m, a = 10.813(1) Å, b = 4.5346(4) Å, c = 14.3391(7) Å, β = 110.05(1)°and V = 660.48(10) Å 3 for Z = 4. Its structure can be interpreted as an intergrowth of fragments cut from the CaGe (CrB-type) and the CaAg 1+x Ge 1-x (TiNiSi-type) structures, and it therefore represents an alkaline-earth member of * Dr. S. Ponou Fax: +46-46-222-4012 E-Mail: Simeon.Ponou@polymat.lth.se [a] 35 the structure series with the general formula R 2+n T 2 X 2+n with n = 4. Unlike the rare-earth homologues that are fully ordered phases, one seventh of the atomic sites in the unit cell of the title compound are mixed occupied (roughly 2/3Ge and 1/3Ag), and this can be explained by the Zintl concept. The alloying of this phase using aluminum metal yielded the isotypic solid solution Ca 3 (Ag/Al) 1+x Ge 3-x , in which the aluminum for silver substitution is strictly localized in the TiNiSi substructure, revealing the very different functionality of the two building blocks.

A series of four new analogue phases Ca 2 M 2 X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Δχ) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca 2 Pd 2 Ge (I), is an unprecedented ternary ordered variant of the Zr 2 Al 3 -type (orthorhombic, Fdd2). The three other phases, Ca 2 Pt 2 Ge (II), Ca 2 Pd 2 Al (III) and Ca 2 Pt 2 Al (IV), adopt the Gd 2 Ge 2 Al-type structure (monoclinic, C2/c). All title structures feature linear chains of the noble metals (Pd or Pt). The Pd linear chains in I are undistorted with equidistant Pd···Pd atoms, whereas the metal chains in II-IV are pairwise distorted, resulting in short connected {Pd 2 } or {Pt 2 } dumbbells that are separated by longer M···M contacts. The occurrence and magnitude of the pairing distortion in these chains are controlled by the vec and the Δχ between the constituent elements, a result which is supported by analysis of the calculated Bader effective charges. The metal chains act as charge modulation units, critical for the stability and the electronic flexibility of the structures by an adequate adjustment of the metal-metal bond order to both the vec and the degree of charge transfer. Thus, Ca 2 Pd 2 Ge (28 ve/f.u) is a Zintl-like, charge optimized phase with formally zerovalent Pd atoms forming the undistorted metal chains; semimetallic properties are predicted by TB-LMTO calculations. In contrast, the isoelectronic Ca 2 Pt 2 Ge is predicted to be a good metal with the Fermi level located at a local maximum of the DOS, a fingerprint of potential electronic instability. This is due to greater charge transfer to the more electronegative Pt atoms forming the metal chains and probably to packing frustration in the well packed structure that may prevent a larger distortion of the Pt chains. However, the instability is suppressed in the aliovalent but isostructural phases Ca 2 M 2 Al (27 ve/f.u) with an enhancement of the pairing distortion within the metal chains but lower M-M bond order. Further reduction of the vec as in Ca 2 M 2 Cd (26 ve/f.u) may induce a transition toward the more geometrically flexible W 2 CoB 2 -type with a low dimensional structure, to create more room for a larger distortion of the metal chain as dictated by the shortage of valence electrons. ABSTRACT: A series of four new analogue phases Ca 2 M 2 X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Δχ) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two ...