Effect of pressure on octahedral distortions in RCrO₃ (R= Lu, Tb, Gd, Eu, Sm): the role of R-ion size and its implications

Abstract

The effect of rare-earth ion size on the octahedral distortions in rare-earth chromites (RCrO₃, R = Lu, Tb, Gd, Eu, Sm) crystallizing in the orthorhombic structure has been studied using Raman scattering and synchrotron powder X-ray diffraction up to 20 GPa. From our studies on RCrO₃ we found that the octahedral tilts (distortions) increase with pressure. This is contrary to the earlier report which suggests that in LaCrO₃, the distortions decrease with pressure leading to a more ordered phase at high pressure. Here, we observe that the rate of increase in distortion decreases with the increase in R-ion radii. This occurs due to the reduction in the compression of RO₁₂ polyhedra with a corresponding increase in the compression of the CrO6 octahedra with increasing R-ion radii. From the Raman studies, we predict a critical R-ion radii, above which we expect the distortions in RCrO₃ to reduce with increasing pressure leading to what is observed in the case of LaCrO₃. These Raman results are consistent with our pressure dependent structural studies on RCrO₃ (R = Gd, Eu, Sm). Also, our results suggest that the pressure dependence of Neel temperature, T_N^Cr, (where the Cr³⁺ spin orders) in RCrO₃ is mostly affected by the compressions of Cr-O bonds rather than the alteration of octahedral tilts.