Materials Properties of Ultra-Incompressible Re₂P

In situ high-pressure X-ray powder diffraction measurements on Re₂P up to 37.0 GPa at ambient temperature in diamond-anvil cells were carried out at two different synchrotron facilities (ESRF and DESY). The compressibility of Re₂P (<i>Pnma</i>, no. 62, <i>a</i> = 5.5464(17), <i>b</i> = 2.9421(8), <i>c</i> = 10.0483(35) Å, <i>V</i> = 163.97(9) ų, <i>Z</i> = 4, <i>R</i>_p = 0.1008, <i>wR</i>_p = 0.1341 at ambient conditions) was investigated and resulted in a bulk modulus of <i>B</i>₀ = 320(10) GPa after fitting the experimental <i>p</i>–<i>V</i> data to a second- and third-order Birch–Murnaghan equation of state. In addition, the determined bulk modulus is compared to values obtained from an Eulerian strain versus normalized stress plot with values ranging form 315(7) to 321(15) GPa. These experimental findings are confirmed by density functional theory (DFT)-calculations ranking Re₂P among ultra-incompressible materials. However, the Vickers hardness of a high-pressure sintered Re₂P–Re_<i>x</i>C_<i>y</i> composite material in the asymptotic hardness region was found to be of only 13(2) GPa. Electrical conductivity measurements indicate that metallic Re₂P exhibits Pauli-paramagnetism. Analysis of temperature-dependent in situ X-ray diffractometry reveals an approximately isotropic expansion of the lattice parameters with a thermal expansion coefficient of (α­(<i>V</i>) = 28.5–32.8(2)·10^–6 K^–1)

High-Pressure Synthesis of β‑Ir₄B₅ and Determination of the Compressibility of Various Iridium Borides

A new iridium boride, β-Ir₄B₅, was synthesized under high-pressure/high-temperature conditions of 10.5 GPa and 1500 °C in a multianvil press with a Walker-type module. The new modification β-Ir₄B₅ crystallizes in a new structure type in the orthorhombic space group <i>Pnma</i> (no. 62) with the lattice parameters <i>a</i> = 10.772(2) Å, <i>b</i> = 2.844(1) Å, and <i>c</i> = 6.052(2) Å with <i>R</i>1 = 0.0286, <i>wR</i>2 = 0.0642 (all data), and <i>Z</i> = 2. The structure was determined by single-crystal X-ray and neutron powder diffraction on samples enriched in ¹¹B. The compound is built up by an alternating stacking of boron and iridium layers with the sequence ABA′B′. Additionally, microcalorimetry, hardness, and compressibility measurements of the binary iridium borides α-Ir₄B₅, β-Ir₄B₅, Ir₅B₄, hexagonal Ir₄B_3–<i>x</i> and orthorhombic Ir₄B_3–<i>x</i> were carried out and theoretical investigations based on density function theory (DFT) were employed to complement a comprehensive evaluation of structure–property relations. The incorporation of boron into the structures does not enhance the compressibility but leads to a significant reduction of the bulk moduli and elastic constants in comparison to elemental iridium

Determination of the Crystal Structure of Hexaphenyldisilane from Powder Diffraction Data and Its Thermodynamic Properties

The crystal structure of hexaphenyldisilane, Si₂(C₆H₅)₆, was determined from synchrotron powder diffraction data. The compound crystallizes in orthorhombic space group <i>P</i>2₁2₁2₁ with the following unit cell dimensions: <i>a</i> = 20.2889(8) Å, <i>b</i> = 16.9602(7) Å, and <i>c</i> = 8.5506(4) Å. Second-harmonic generation measurements as well as density functional theory calculations were used to confirm the structure determination. The combination of experimental and theoretical studies yields a Si–Si distance [<i>d</i>(Si–Si)] of 2.38 Å. The phenyl rings of a molecule are staggered and slightly distorted, so that the molecule is acentric. Thermodynamic measurements showed no phase transition in the temperature range of 2–400 K. The molar heat capacity (<i>C_p</i>) at 298.15 K of 604(6) J mol^–1 K^–1 was established experimentally and by lattice dynamic calculations. The molar entropy (<i>S</i>°) and the molar enthalpy (Δ<i>H</i>) in the temperature range of 0–298.15 K are 674(7) J mol^–1 K^–1 and 97(6) kJ mol^–1 respectiveley. The Debye temperature (θ_D) is 207(5) K. The thermal expansion of Si₂(C₆H₅)₆ is strongly anisotropic, and negative in two directions as determined via temperature-dependent X-ray powder diffraction experiments. The linear thermal expansion coefficients at 298.15 K are as follows: α_<i>a</i> = −4(2) × 10^–6 K^–1, α_<i>b</i> = −4(2) × 10^–6 K^–1, and α_<i>c</i> = 2.21(4) × 10^–4 K^–1. The volumetric thermal expansion coefficient (α_<i>V</i>) at 298.15 K is 2.13(5) × 10^–4 K^–1