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