Matej Bobnar

Weak Interactions under Pressure: hp-CuBi and Its Analogues

The metastable binary compound hp-CuBi was obtained from the direct chemical reaction between copper and bismuth at a pressure of 5 GPa and a temperature of 720 K. The atomic arrangement comprises slabs of puckered Cu layers sandwiched between Bi planes. The QTAIM charges calculated for compounds of bismuth with transition metals reveal negligible charge transfer for hp-CuBi. Analysis of the chemical bonding with position-space techniques discloses multicenter interactions within the Bi-Cu-Bi slabs and lone-pair interactions of the van der Waals type between these entities. hp-CuBi exhibits metal-type electrical conductivity with superconductivity below Tc =1.3 K.

Distribution of Al atoms in the clathrate-I phase Ba8AlxSi46−x at x = 6.9

The clathrate-I phase Ba8AlxSi46-x has been structurally characterized at the composition x = 6.9 (space group Pm3[combining macron]n, no. 223, a = 10.4645(2) Å). A crystal structure model comprising the distribution of aluminium and silicon atoms in the clathrate framework was established: 5.7 Al atoms and 0.3 Si atoms occupy the crystallographic site 6c, while 1.2 Al atoms and 22.8 Si atoms occupy site 24k. The atomic distribution was established based on a combination of (27)Al and (29)Si NMR experiments, X-ray single-crystal diffraction and wavelength-dispersive X-ray spectroscopy.

Cluster Formation in the Superconducting Complex Intermetallic Compound Be21Pt5

Materials with the crystal structure of γ-brass type (Cu5Zn8 type) are typical representatives of intermetallic compounds. From the electronic point of view, they are often interpreted using the valence electron concentration approach of Hume-Rothery, developed previously for transition metals. The γ-brass-type phases of the main-group elements are rather rare. The intermetallic compound Be21Pt5, a new member of this family, was synthesized, and its crystal structure, chemical bonding, and physical properties were characterized. Be21Pt5 crystallizes in the cubic space group F4̅3m with lattice parameter a = 15.90417(3) Å and 416 atoms per unit cell. From the crystallographic point of view, the binary substance represents a special family of intermetallic compounds called complex metallic alloys (CMA). The crystal structure was solved by a combination of synchrotron and neutron powder diffraction data. Besides the large difference in the scattering power of the components, the structure solution was hampered by the systematic presence of very weak reflections mimicking wrong symmetry. The structural motif of Be21Pt5 is described as a 2 × 2 × 2 superstructure of the γ-brass structure (Cu5Zn8 type) or 6 × 6 × 6 superstructure of the simple bcc structural pattern with distinct distribution of defects. The main building elements of the crystal structure are four types of nested polyhedral units (clusters) with the compositions Be22Pt4 and Be20Pt6. Each cluster contains four shells (4 + 4 + 6 + 12 atoms). Clusters with different compositions reveal various occupation of the shells by platinum and beryllium. Polyhedral nested units with the same composition differ by the distance of the shell atoms to the cluster center. Analysis of chemical bonding was made applying the electron localizability approach, a quantum chemical technique operating in real space that is proven to be especially efficient for intermetallic compounds. Evaluations of the calculated electron density and electron localizability indicator (ELI-D) revealed multicenter bonding, being in accordance with the low valence electron count per atom in Be21Pt5. A new type of atomic interactions in intermetallic compounds, cluster bonds involving 8 or even 14 atoms, is found in the clusters with shorter distances between the shell atoms and the cluster centers. In the remaining clusters, four- and five-center bonds characterize the atomic interactions. Multicluster interactions within the polyhedral nested units and three-center polar intercluster bonds result in a three-dimensional framework resembling the structural pattern of NaCl. Be21Pt5 is a diamagnetic metal and one of rather rare CMA compounds revealing superconductivity (Tc = 2.06 K).

AlPd15B7: a new superconducting cage-compound with an anti-Yb3Rh4Sn13-type of structure

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.

Lithium alkaline earth tetrelides of the type Li2AeTt (Ae=Ca, Ba, Tt=Si, Ge, Sn, Pb): synthesis, crystal structures and physical properties

Abstract Single crystals of the compounds Li2AeTt (Ae=Ca, Ba, Tt=Si, Ge, Sn, Pb) were grown in reactive lithium melts in sealed tantalum ampoules from an equimolar ratio of the alkaline earth metal and the respective group 4 element. All compounds, with the exception of Li2CaSn and Li2CaPb, are isotypic and crystallize in an orthorhombic unit cell (space group Pmmn, no. 59). The crystal structure can be characterized as superimposed corrugated networks of Li2Tt connected by calcium or barium atoms within the third dimension. Li2CaSn and Li2CaPb crystallize in the cubic space group Fm3̅m (no. 225) in a Heusler-type (MnCu2Al) structure. According to magnetic susceptibility and electric resistivity measurements, the compounds Li2BaGe, Li2BaSn, and Li2BaPb represent diamagnetic activated semiconductors.

A type-II clathrate with a Li-Ge framework

Abstract Na16Cs8LixGe136−x (x≈2.8, space group Fd3̅m) is the first intermetallic type-II clathrate with a lithium-substituted framework. The phase was obtained from the elements in sealed Ta ampoules by annealing at 650°C for 5 days. Samples were investigated by synchrotron X-ray powder diffraction, solid-state NMR, microstructure and chemical analysis. The substitution of Ge by Li atoms causes a marked shrinking of the lattice parameter. Studies by 7Li NMR confirmed the presence of Li in the clathrate phase and the 23Na and 133Cs NMR spectra consistently showed distinct changes as compared to the ternary Na16Cs8Ge136. The SEDOR technique revealed a distance between Li and Cs atoms in agreement with the result of crystal structure refinement, indicating Li substitution at site 96g. The distinct Knight shift of all NMR signals implies metallic behaviour of the clathrate phase, measurements of the magnetic susceptibility indicate diamagnetic behaviour.

Thermodynamic characterization of lithium monosilicide (LiSi) by means of calorimetry and DFT-calculations

Abstract In this work we summarize a symbiotic approach to combine experimental and theoretical investigations for the derivation of high quality thermodynamic data for the description of potential lithium ion battery materials. The methodology of this concept was demonstrated in detail by exploring and describing the properties of the lithium monosilicide phase LiSi. The procedures were also applied in a series of investigations to all major LixSiy-phases which will be reviewed briefly. Regarding the LiSi phase, the measured and calculated isobaric heat capacity, which may enable further thermodynamic investigations (e. g. with CALPHAD method) of the phase diagram of the Li–Si-system is presented. The heat capacity of the stable phase LiSi was measured as a function of temperature in a range from (2 to 673) K and compared with corresponding ab-initio and molecular dynamic calculations resulting in values for absolute entropies. The heat of formation of the system was determined in an unconventional manner via hydrogenation experiments.

Trigermanides AEGe3 (AE = Ca, Sr, Ba): chemical bonding and superconductivity

Abstract The crystal structures of the trigermanides AEGe3(tI32) (AE = Ca, Sr, Ba; space group I4/mmm, for SrGe3: a = 7.7873(1), c = 12.0622(3) Å) comprise Ge2 dumbbells forming layered Ge substructures which enclose embedded AE atoms. The chemical bonding analysis by application of the electron localizability approach reveals a substantial charge transfer from the AE atoms to the germanium substructure. The bonding within the dumbbells is of the covalent two-center type. A detailed analysis of SrGe3 reveals that the interaction on the bond-opposite side of the Ge2 groups is not lone pair-like – as it would be expected from the Zintl-like interpretation of the crystal structure with anionic Ge layers separated by alkaline-earth cations – but multi-center strongly polar between the Ge2 dumbbells and the adjacent metal atoms. Similar atomic interactions are present in CaGe3 and BaGe3. The variation of the alkaline-earth metal has a merely insignificant influence on the superconducting transition temperatures in the s,p-electron compounds AEGe3.

Solid solution Pb1-xEuxTe: constitution and thermoelectric behavior

Polycrystalline samples of the solid solution Pb1−xEuxTe were prepared by the spark-plasma technique. In contrast to the literature data, the homogeneity range of the solid solution amounts only to 0 ≤ x ≤ 0.02 under the selected preparation conditions. The implementation of Eu into the PbTe lattice was monitored by refinement of the lattice parameters. The thermoelectric properties of the prepared materials were investigated above room temperature. In samples with compositions x ≤ 0.04, the solid solution Pb1−xEuxTe reveals a metal–semiconductor transition around 500 K going in parallel to the p–n transition in the conductivity. No significant influence of the europium substitution on the thermoelectric figure-of-merit was observed in stoichiometric bulk materials. Introduction to the international collaboration Collaboration between Prof. Yuri Grins group at Max-Planck Institute of Chemical Physics of Solids and Prof. Jing-Tai Zhaos group at Shanghai University (at Shanghai Institute of Ceramics, Chinese Academy of Sciences before 2014) in the field of Solid State Chemistry and Physics of intermetallic compounds started from 1999 followed by an establishment of a Partner Group between the Max-Planck Society and Chinese Academy of Sciences. The collaboration is highly recognized, in particular the studies on exploration of novel high-performance thermoelectric materials – so-called “122” Zintl phases – in pnictide systems and germanium clathrates. The long-term collaboration generated more than 30 joint publications in international journals and many presentations at national and international conferences, more than 10 co-supervised students and postdocs, and a series of workshops on X-ray Crystallography in Shanghai and Dresden.

Zintl-Phase Sr3LiAs2H: Crystal Structure and Chemical Bonding Analysis by the Electron Localizability Approach

The compound Sr3 LiAs2 H was synthesized by reaction of elemental strontium, lithium, and arsenic, as well as LiH as hydrogen source. The crystal structure was determined by single-crystal X-ray diffraction: space group Pnma; Pearson symbol oP28; a = 12.0340(7), b = 4.4698(2), c = 12.5907(5) Å; V = 677.2(1) Å(3) ; RF  = 0.047 for 1021 reflections and with 36 parameters refined. The positions of the hydrogen atoms were first revealed by the electron localizability indicator and subsequently confirmed by crystal structure refinement. In the crystal structure of Sr3 LiAs2 H the metal atoms are arranged in a Gd3 NiSi2 -type motif, whereas the hydrogen atoms are arranged in a distorted tetrahedral environment formed by strontium. The calculated band structure revealed that Sr3 LiAs2 H is a semiconductor, which is in agreement with its diamagnetic behavior. Thus, Sr3 LiAs2 H is considered as a (charge-balanced) Zintl phase.