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 (dTc/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).