Equations of state of GdFeO3 and GdAlO3 perovskites

Abstract

The structures of orthorhombicGdAlO3 and GdFeO3 perovskites have been refined at room temperature and pressure using single-crystal x-raydiffraction and their equations of state have been measured to pressures of 7.95 and 7.58 GPa,respectively. Both structures are distorted through the tilting and distortion of the octahedra.GdAlO3 isless distortedthan GdFeO3 with an averageAl–O–Al tilt angle of 156.42(16)° compared to an average Fe–O–Fe tilt angle of147.10(10)° inGdFeO3. Both theFeO6 octahedraand GdO12 sites inGdFeO3 are more distortedthan those in GdAlO3. Neither perovskite exhibits any phase transitions throughout the pressurerange studied. A fit of a third-order Birch–Murnaghan equation of state to theP–V data yieldsvalues of KT0 = 191(1) GPa and for GdAlO3 and KT0 = 182(1) GPa and for GdFeO3.Analysis of the unit cell parameter data shows that [100] is least compressible in both compounds and thatGdFeO3 compresses moreisotropically than GdAlO3. The compressional moduli for the unit cell parameters ofGdFeO3 areKa0 = 188(3) GPa,Kb0 = 181(1) GPa and Kc0 = 177(2) GPa, with , and , compared with Ka0 = 234(5) GPa, Kb0 = 151(2) GPa and Kc0 = 205(1) GPa, with , and for GdAlO3. There is no significant change in the pseudocubic unit cell parameters with pressure inGdFeO3 whereas theyconverge in GdAlO3. Inparticular, the ac and bc are predicted to merge by 12 GPa, signifying a possible transition from orthorhombic totetragonal symmetry.