Ordering by cation replacement in the system Na2-xLixGa7.

Samples of the pseudo-binary system Na2-xLixGa7 (x ≤ 1) were synthesized from the elements at 300 °C in sealed Ta ampoules or by the reaction of Na2Ga7 with LiCl. The peritectic formation temperature decreases with increasing Li content from 501(2) °C (x = 0) to 489(2) °C (x = 1). The boundary compositions Na2Ga7 and Na1Li1Ga7 crystallize with different structure types related by a group-subgroup relation. While the Na-rich compositions (x ≤ 0.5) represent a substitutional solid solution (space group Pnma), the Li-rich compositions feature an unconventional replacement mechanism in which Li atoms occupying interstitial positions induce vancancies at the Na positions (space group Cmce). The crystal structure of Na1Li1Ga7 (a = 8.562(1) Å, b = 14.822(2) Å, c = 11.454(2) Å; Z = 8) was determined from X-ray single-crystal diffraction data, and reveals an anionic framework comprising 12-bonded Ga12 icosahedra and 4-bonded Ga atoms, with alkali-metal atoms occupying channels and cavities. The arrangement of cations makes NaLiGa7 a new structure type within the MgB12Si2 structure family. Band structure calculations for the composition NaLiGa7 predict semiconducting behavior consistent with the balance [Na+]2[Li+]2[(Ga12)2-][Ga-]2, considering closo Wade clusters [(12b)Ga12]2- and Zintl anions [(4b)Ga]-. Susceptibility measurements indicate temperature-independent diamagnetic behavior.

Na2Ga7: A Zintl-Wade Phase Related to "α-Tetragonal Boron".

Na2Ga7 crystallizes with the orthorhombic space group Pnma (no. 62; a = 14.8580(6) Å, b = 8.6766(6) Å, and c = 11.6105(5) Å; Z = 8) and constitutes a filled variant of the Li2B12Si2 structure type. The crystal structure consists of a network of icosahedral Ga12 units with 12 exohedral bonds and four-bonded Ga atoms in which the Na atoms occupy the channels and cavities. The atomic arrangement is consistent with the Zintl [(4b)Ga]- and Wade [(12b)Ga12]2- electron counting approach. The compound forms peritectically from Na7Ga13 and the melt at 501 °C and does not show a homogeneity range. The band structure calculations predict semiconducting behavior consistent with the electron balance [Na+]4[(Ga12)2-][Ga-]2. Magnetic susceptibility measurements show that Na2Ga7 is diamagnetic.

Superconductivity of structurally disordered Y5Ir6Sn18.

The structural and physical properties of Y5Ir6Sn18 grown from Sn-flux as large single crystals are studied. Y5Ir6Sn18 crystallizes with a unique structure [space group Fm3̄m, a = 13.7706(1) Å], which is characterized by a strong disorder. A transmission electron microscopy (TEM) study indicated that the structural model of Y5Ir6Sn18 obtained from X-ray diffraction methods is an average description of a complex intergrowth of domains with different structural arrangements. The studied stannide is a type-II superconductor with a critical temperature Tc = 2.1 K, a rather weak electron-phonon coupling and conventional s-wave BCS-like mechanisms. Performed theoretical electronic band structure calculations indicated the inconsistency of an idealized structural model earlier reported for Y5Ir6Sn18.

Valence fluctuations in the 3D + 3 modulated Yb3Co4Ge13 Remeika phase.

Yb3Co4Ge13 is the first example of a Remeika phase with a 3D + 3 [space group P4̄3n(α,0,0)000(0,α,0)000(0,0,α)000; a = 8.72328(1) Å, α = 0.4974(2)] modulated crystal structure. A slight shift of the composition towards higher Yb-content (i.e. Yb3.2Co4Ge12.8) leads to the disappearance of the satellite reflections and stabilization of the disordered primitive cubic [space group Pm3̄n, a = 8.74072(2) Å] Remeika prototype structure. The stoichiometric structurally modulated germanide is a metal with hole-like charge carriers, where Yb-ions are in a temperature-dependent intermediate valence state varying from +2.60 to +2.66 for the temperature range 85-293 K. The valence fluctuations have been investigated by means of temperature dependent X-ray absorption spectroscopy, magnetic susceptibility and thermopower measurements.

Crystal structure, phase transition and properties of indium(III) sulfide.

Poly- and single-crystalline samples of In0.67□0.33In2S4 thiospinel were obtained by various powder metallurgical and chemical vapor transport methods, respectively. All synthesized samples contained ß-In0.67□0.33In2S4 modification only, independent of the synthesis procedure. High-resolution powder X-ray diffraction (PXRD) experiments at 80 K enabled the observation of split tetragonal reflections (completely overlapped at room temperature), which prove the correctness of the crystal structure model accepted for the ß-polymorph. Combining single-crystal XRD, transmission electron microscopy and selected-area electron diffraction studies, the presence of three twin domains in the as-grown crystals was confirmed. A high temperature PXRD study revealed both abrupt (in full widths at half maxima of main reflections and in unit-cell volume) and gradual (in intensity of satellites and c/a ratio) changes in the vicinity of the α-ß phase transition. These observations, together with a clear endothermic peak in the heat capacity, the magnitude of enthalpy/entropy change and the temperature dependence of electrical resistivity (associated with hysteresis), hinted towards the 1st order type of transition. Three scenarios, based on Rietveld refinement analysis, were considered for the description of the crystal structure evolution from ß- to α-modification, including the (3+3)D-modulated cubic structure at 693 K as an intermediate state during the ß-α transformation. The Seebeck coefficient, electrical resistivity and thermal conductivity were not only influenced by phase transition, but also by annealing conditions (S-poor or S-rich atmosphere). Density functional theory calculations predicted semiconducting behavior of In0.67□0.33In2S4, as well as instability of the fictitious InIn2S4 thiospinel.

Crystal Structure and Physical Properties of the Cage Compound Hf2B2-2δIr5+δ.

Hf2B2-2δIr5+δ crystallizes with a new type of structure: space group Pbam, a = 5.6300(3) Å, b = 11.2599(5) Å, and c = 3.8328(2) Å. Nearly 5% of the boron pairs are randomly replaced by single iridium atoms (Ir5+δB2-2δ). From an analysis of the chemical bonding, the crystal structure can be understood as a three-dimensional framework stabilized by covalent two-atom B-B and Ir-Ir as well as three-atom Ir-Ir-B and Ir-Ir-Ir interactions. The hafnium atoms center 14-atom cavities and transfer a significant amount of charge to the polyanionic boron-iridium framework. This refractory boride displays moderate hardness and is a Pauli paramagnet with metallic electrical resistivity, Seebeck coefficient, and thermal conductivity. The metallic character of this system is also confirmed by electronic structure calculations revealing 5.8 states eV-1 fu-1 at the Fermi level. Zr2B2-2δIr5+δ is found to be isotypic with Hf2B2-2δIr5+δ, and both form a continuous solid solution.

Crystal structure, chemical bonding, and electrical and thermal transport in Sc5Rh6Sn18.

Single crystals of Sc5Rh6Sn18 were grown from Sn-flux. The crystal structure (SG: I41/acd, a = 13.5529(2) Å, c = 27.0976(7) Å) was studied by high-resolution X-ray diffraction on powder and single crystal material as well as by TEM. All methods confirm it to crystallize with a Sc5Ir6Sn18 (space group I41/acd) type structure. The performed structural studies also suggest the presence of local domains with a broken average translational symmetry. An analysis of the chemical bonding situation reveals highly polar covalent Sc2-Sn1, Sn-Rh and Sc2-Rh bonds, two- and three-centre bonds involving Sn-atoms as well as the ionic nature of Sc1 bonding. The thermopower of Sc5Rh6Sn18 is isotropic, small and negative (i.e. dominance of electron-like charge carriers). Due to structural disorder, the thermal conductivity is lower in comparison with regular metallic systems.

Crystal structure and superconducting properties of Sc5Ir6Sn18.

Sc5Ir6Sn18 crystallizes with a split variant of the Tb5Rh6Sn18 structure type (space group I41/acd, [Formula: see text] [Formula: see text], [Formula: see text] [Formula: see text]). DFT calculations confirmed the instability of the structural arrangement with the fully occupied and unsplit crystallographic sites. High quality single crystals were grown from a Sn melt. Sc5Ir6Sn18 is a diamagnetic metal showing a superconducting transition at a critical temperature [Formula: see text] K. The relatively low critical magnetic field [Formula: see text] 3.2 T as well as the obtained values of the specific heat ratio [Formula: see text] and energy-gap ratio [Formula: see text] suggest this system to be a weakly coupled BCS-like superconductor.

Ferromagnetic ε-Fe2MnN: High-Pressure Synthesis, Hardness and Magnetic Properties.

The iron manganese nitride Fe2MnN was obtained by high-pressure-high-temperature synthesis from ζ-Fe2N and elemental Mn at 15(2) GPa and 1573(200) K. The phase crystallizes isostructural to binary ε-Fe3N. In comparison to the corresponding binary iron nitride, the microhardness of ε-Fe2MnN is reduced to 6.2(2) GPa. Above about 800 K the ternary compound decomposes exothermally under loss of nitrogen. ε-Fe2MnN is ferromagnetic with a Curie temperature of roughly 402 K.

High-Pressure Synthesis and Chemical Bonding of Barium Trisilicide BaSi3.

BaSi3 is obtained at pressures between 12(2) and 15(2) GPa and temperatures from 800(80) and 1050(105) K applied for one to five hours before quenching. The new trisilicide crystallizes in the space group I 4 ¯ 2m (no. 121) and adopts a unique atomic arrangement which is a distorted variant of the CaGe3 type. At ambient pressure and 570(5) K, the compound decomposes in an exothermal reaction into (hP3)BaSi2 and two amorphous silicon-rich phases. Chemical bonding analysis reveals covalent bonding in the silicon partial structure and polar multicenter interactions between the silicon layers and the barium atoms. The temperature dependence of electrical resistivity and magnetic susceptibility measurements indicate metallic behavior.

Distortions in the cubic primitive high-pressure phases of calcium.

The superconductivity in highly compressed calcium involves the occurrence of closely related low-symmetry structural patterns with an exceptionally low coordination number. Earlier theoretical and experimental results are controversial and some findings are inconsistent with our later observations in the pressure range up to 60 GPa. This situation motivated the present concerted computational and experimental re-investigation of the structural arrangement of calcium slightly above the high-pressure limit of the bcc arrangement at low-temperatures. We report here reproducible experimental evidence for a monoclinic distortion (mC4, space group C2/c) of the calcium polymorph previously assigned to the tetragonal ß-Sn structure type. In accordance, the enthalpies calculated by electronic band structure calculations show the mC4 phase to be more stable than the undistorted ß-Sn type by about 100 meV in the entire phase space. The other low-temperature phase of calcium adopts space group Cmcm (oC4) rather than the earlier assigned Cmmm symmetry. These structural alterations substantially effect the density of states at the Fermi level and, thus, the electronic properties.

Lutetium Trigermanide LuGe3: High-Pressure Synthesis, Superconductivity, and Chemical Bonding.

LuGe3 was obtained under high-pressure and high-temperature conditions at pressures between 8(1) and 14(2) GPa and at temperatures in the range from 1100(150) to 1500(150) K. The high-pressure phase is isotypic to DyGe3 and decomposes at ambient pressure and T = 690 K mainly into ( cF8)Ge and LuGe2- x. Chemical bonding analysis of LuGe3 reveals two-center electron-deficient Ge-Ge bonds, multicenter polar Lu-Ge interactions, and lone pairs on germanium. Magnetic susceptibility, specific heat, and electrical conductivity measurements indicate transition into a superconducting state below Tc = 3.3(3) K.

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).

Intermediate-Valence Ytterbium Compound Yb4Ga24Pt9: Synthesis, Crystal Structure, and Physical Properties.

The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 °C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 °C. The crystal structure of Yb4Ga24Pt9 was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group C2/m, Pearson symbol mS74, a = 7.4809(1) Å, b = 12.9546(2) Å, c = 13.2479(2) Å, ß = 100.879(1)°, V = 1260.82(6) Å3, RF = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an ABABB stacking of two slabs with trigonal symmetry and compositions Yb4Ga6 (A) and Ga12Pt6 (B). The hard X-ray photoelectron spectrum (HAXPES) of Yb4Ga24Pt9 shows both Yb2+ and Yb3+ contributions as evidence of an intermediate valence state of ytterbium. The evaluated Yb valence of ∼2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is a bad metallic conductor.

High-Pressure NiAs-Type Modification of FeN.

The combination of laser-heated diamond anvil cells and synchrotron Mössbauer source spectroscopy were used to investigate high-temperature high-pressure chemical reactions of iron and iron nitride Fe2 N with nitrogen. At pressures between 10 and 45 GPa, significant magnetic hyperfine splitting indicated compound formation after annealing at 1300 K. Subsequent in situ X-ray diffraction reveals a new modification of FeN with NiAs-type crystal structure, as also rationalized by first-principles total-energy and chemical-bonding studies.

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.

On the transferability of electron density in binary vanadium borides VB, V3B4 and VB2.

Binary vanadium borides are suitable model systems for a systematic analysis of the transferability concept in intermetallic compounds due to chemical intergrowth in their crystal structures. In order to underline this structural relationship, topological properties of the electron density in VB, V3B4 and VB2 reconstructed from high-resolution single-crystal X-ray diffraction data as well as derived from quantum chemical calculations, are analysed in terms of Bader's Quantum Theory of Atoms in Molecules [Bader (1990). Atoms in Molecules: A Quantum Theory, 1st ed. Oxford: Clarendon Press]. The compounds VB, V3B4 and VB2 are characterized by a charge transfer from the metal to boron together with two predominant atomic interactions, the shared covalent B-B interactions and the polar covalent B-M interactions. The resembling features of the crystal structures are well reflected by the respective B-B interatomic distances as well as by ρ(r) values at the B-B bond critical points. The latter decrease with an increase in the corresponding interatomic distances. The B-B bonds show transferable electron density properties at bond critical points depending on the respective bond distances.

BaGe6 and BaGe(6-x): incommensurately ordered vacancies as electron traps.

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe6 (P = 15 GPa, T = 1073 K) and BaGe(6-x) (P = 10 GPa, T = 1073 K) which are metastable at ambient conditions. In BaGe(6-x), partial fragmentation of the BaGe6 network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe(6-x) is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe(6-x) for x = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity.

LiZn(4†-†x) (x = 0.825) as a (3†+†1)-dimensional modulated derivative of hexagonal close packing.

The (3+1)-dimensional modulated structure of the LiZn(4 - x) (x = 0.825) binary compound has been determined in the superspace. The compound crystallizes in the orthorhombic superspace group Cmcm(α00)0s0 with a = 2.7680 (6), b = 4.7942 (6), c = 4.3864 (9) Å, modulation wavevector: q ≃ 4/7a*. The structure is a derivative from the hexagonal close packing. The cubo-octahedron as a coordination polyhedron (c.n. = 12) is typical for all atoms. Bonding between atoms is explored by means of the TB-LMTO-ASA program package. The absence of strong interatomic interactions in LiZn(4 - x) is the main reason for the possible structure transformations.

New monoclinic phase at the composition Cu2SnSe3 and its thermoelectric properties.

A new monoclinic phase (m2) of ternary diamond-like compound Cu2SnSe3 was synthesized by reaction of the elements at 850 K. The crystal structure of m2-Cu2SnSe3 was determined through electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data (space group Cc, a = 6.9714(2) Å, b = 12.0787(5) Å, c = 13.3935(5) Å, ß = 99.865(5)°, Z = 8). Thermal analysis and annealing experiments suggest that m2-Cu2SnSe3 is a low-temperature phase, while the high-temperature phase has a cubic crystal structure. According to quantum chemical calculations, m2-Cu2SnSe3 is a narrow-gap semiconductor. A study of the chemical bonding, applying the electron localizability approach, reveals covalent polar Cu-Se and Sn-Se interactions in the crystal structure. Thermoelectric properties were measured on a specimen consolidated using spark plasma sintering (SPS), confirming the semiconducting character. The thermoelectric figure of merit ZT reaches a maximum value of 0.33 at 650 K.