Ab initiophase diagram of iridium

The phase diagram of iridium is investigated using the Z methodology. The Z methodology is a technique for phase diagram studies that combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. In the direct Z method, the solid phases along the melting curve are determined by comparing the solid-liquid equilibrium boundaries of candidate crystal structures. The inverse Z method involves quenching the liquid into the most stable solid phase at various temperatures and pressures to locate a solid-solid boundary. Although excellent agreement with the available experimental data (to 65 GPa) is found for the equation of state (EOS) of Ir, it is the third-order Birch-Murnaghan EOS with B₀^'=5 rather than the more widely accepted B₀^'=4 that describes our ab initio data to higher pressure $(P)$. Our results suggest the existence of a random-stacking hexagonal close-packed structure of iridium at high $P.$ We offer an explanation for the 14-layer hexagonal structure observed in experiments by Cerenius and Dubrovinsky.