High-pressure structural behavior of GdAlO3 and GdFeO3 perovskites

The orthorhombic perovskites, GdAlO₃ and GdFeO₃, have been studied using single-crystal X-ray diffraction up to 8.52 and 8.13 GPa, respectively, in a diamond anvil cell at 298 K. The evolution of the structures of GdAlO₃ and GdFeO₃ involves compression of both the GdO₁₂ and the octahedral (AlO₆ and FeO₆) sites. The compression of the GdO₁₂ site is anisotropic in both perovskites, with the four longest Gd-O distances compressing more than the eight shorter Gd-O bond lengths, resulting in a decrease in the distortion of GdO₁₂ with pressure. In GdAlO₃, the GdO₁₂ site is less compressible than the AlO₆ site, resulting in an increase of both the interoctahedral Al-O1-Al and Al-O2-Al angles with increasing pressure. Thus GdAlO₃ perovskite becomes less distorted with increasing pressure. In GdFeO₃, the GdO₁₂ site displays a similar compressibility as the FeO₆ site, with little change in the Fe-O2-Fe angle with pressure but an increase of the Fe-O1-Fe tilting angle. Thus GdFeO₃ perovskite becomes less distorted with increasing pressure, but the change is not as pronounced as GdAlO₃. The high-pressure behavior of GdAlO₃ and GdFeO₃ is similar to orthorhombic YAlO₃ perovskite but contrasts with orthorhombic CaSnO₃, which becomes more distorted with increasing pressure.