Triangular Ge 3 Units in a New Modification of EuGe 3: Triangular Ge 3 Units in a New Modification of EuGe 3

Thermodynamics of phase transitions in Zintl clusters from density functional theory: making and breaking of bonds in Ba3Ge4

Density functional theory, in conjunction with the quasi-harmonic approximation, has been used to study the equilibrium between the orthorhombic and tetragonal phases of Ba3Ge4. A transition from the high-temperature tetragonal phase containing isolated Ge46- units to the low-temperature orthorhombic phase, where precisely half of the Ge46- units are polymerised along one axis, is predicted at 930 K, somewhat higher than the experimental value of 630 K. An analysis of the phonon density of states shows that the lower entropy of the orthorhombic phase is not associated directly with the polymerisation of the Ge46- units, but rather with the contraction of the unit cell, which raises the frequencies of ion-ion modes involving the relative motions of the Ba2+ and Ge46- units. Calculations also predict that a third, as yet unobserved, p-tetragonal phase, where all of the Ge46- units are polymerised to form two separate chains running in orthogonal directions, might be accessible at pressures close to 1 GPa.

Emergent Transitions: Discord between Electronic and Chemical Pressure Effects in the REAl3 (RE = Sc, Y, Lanthanides) Series

Atomic packing and electronic structure are key factors underlying the crystal structures adopted by solid-state compounds. In cases where these factors conflict, structural complexity often arises. Such is born in the series of REAl₃ (RE = Sc, Y, lanthanides), which adopt structures with varied stacking patterns of face-centered cubic close packed (FCC, AuCu₃ type) and hexagonal close packed (HCP, Ni₃Sn type) layers. The percentage of the hexagonal stacking in the structures is correlated with the size of the rare earth atom, but the mechanism by which changes in atomic size drive these large-scale shifts is unclear. In this Article, we reveal this mechanism through DFT-Chemical Pressure (CP) and reversed approximation Molecular Orbital (raMO) analyses. CP analysis illustrates that the Ni₃Sn structure type is preferable from the viewpoint of atomic packing as it offers relief to packing issues in the AuCu₃ type by consolidating Al octahedra into columns, which shortens Al-Al contacts while simultaneously expanding the RE atom's coordination environment. On the other hand, the AuCu₃ type offers more electronic stability with an 18-n closed-shell configuration that is not available in the Ni₃Sn type (due to electron transfer from the RE d_z² atomic orbitals into Al-based states). Based on these results, we then turn to a schematic analysis of how the energetic contributions from atomic packing and the electronic structure vary as a function of the ratio of FCC and HCP stacking configurations within the structure and the RE atomic radius. The minima on the atomic packing and electronic surfaces are non-overlapping, creating frustration. However, when their contributions are added, new minima can emerge from their combination for specific RE radii representing intergrowth structures in the REAl₃ series. Based on this picture, we propose the concept of emergent transitions, within the framework of the Frustrated and Allowed Structural Transitions principle, for tracing the connection between competing energetic factors and complexity in intermetallic structures.

High-pressure synthesis of SmGe3

The new samarium germanide SmGe3 is obtained by high-pressure high-temperature synthesis of pre-reacted mixtures of samarium and germanium at a pressure of 9.5 GPa and temperatures between 1073 and 1273 K. SmGe3 decomposes at 470(5) K into SmGe2, α-Sm3Ge5 and a hitherto unknown phase. SmGe3 exhibits a superstructure of the cubic Cu3Au-type. Transmission electron microscopy measurements of crystalline particles and prepared lamellae indicate a high density of defects on the nanoscale. Selected area electron diffraction and elaborate X-ray powder diffraction measurements consistently indicate a 2a 0 × 2a 0 × 2a 0 superstructure adopting space group F m 3 ¯ m $Fm\overline{3}m$ with a = 8.6719(2) Å.

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 I4¯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.

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 LuGe_{2- 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 T_c = 3.3(3) K.

Germanium Dumbbells in a New Superconducting Modification of BaGe3

We report the high-pressure high-temperature synthesis (P = 15 GPa, T = 1300 K) of BaGe3(tI32) adopting a CaGe3-type crystal structure. Bonding analysis reveals layers of covalently bonded germanium dumbbells being involved in multicenter Ba-Ge interactions. Physical measurements evidence metal-type electrical conductivity and a transition to a superconducting state at 6.5 K. Chemical bonding and physical properties of the new modification are discussed in comparison to the earlier described hexagonal form BaGe3(hP8) with a columnar arrangement of Ge3 triangles.

High-Pressure Synthesis and Electronic Structure of a New Superconducting Strontium Germanide (SrGe3) Containing Ge2 Dumbbells

We obtained a new strontium germanide (SrGe3) by high-pressure and high-temperature synthesis. It was prepared at 13 GPa and 1100 °C. The space group and cell constants are I4/mmm (No. 139), a = 7.7800(8) Å, c = 12.0561(13) Å, and V = 729.74(17) Å(3). SrGe3 crystallizes in the CaSi3 structure composed of Ge-Ge dumbbells and Sr(2+) ions. SrGe3 is a type II superconductor with a transition temperature of 6.0 K.