Polymorphism of Sr₂ZnIrO₆ Double Perovskite at High Pressure and Temperature

The structural behavior of Sr₂ZnIrO₆ double perovskite has been systematically studied under extreme conditions, both at high temperature and high pressure with synchrotron powder X-ray diffraction. At high temperatures, three structural phase transitions are detected at 473, 673, and 873 K, giving the sequence P2₁/n → I2/m → I4/m → Fm3̅m. At high pressure, a phase transition from monoclinic P2₁/n to tetragonal I4/m symmetry is observed at 7.5(1) GPa, as confirmed by ab initio calculations. The tetragonal high-pressure I4/m polymorph appears at both high and intermediate temperatures, highlighting a displacive transition with an estimated negative pressure dependence of the critical temperature (dT_c/dp = -51 K/GPa). The bulk modulus (∼150 GPa) and the volumetric thermal expansion (∼3.4 × 10^-5 K^-1) are determined for the low- and high-pressure phases (P2₁/n and I4/m).