Effect of pressure on octahedral distortions in<i>R</i>CrO₃(<i>R</i>= Lu, Tb, Gd, Eu, Sm): the role of<i>R</i>-ion size and its implications

Abstract

The effect of rare-earth ion size on the octahedral distortions in rare-earth\nchromites (RCrO3, R = Lu, Tb, Gd, Eu, Sm) crystallizing in the orthorhombic\nstructure has been studied using Raman scattering and synchrotron powder x-ray\ndiffraction up to 20 GPa. From our studies on RCrO3 we found that the\noctahedral tilts (distortions) increase with pressure. This is contrary to the\nearlier report which suggests that in LaCrO3, the distortions decrease with\npressure leading to a more ordered phase at high pressure. Here we observe that\nthe rate of increase in distortion decreases with the increase in R-ion radii.\nThis occurs due to the reduction in the compression of RO12 polyhedra with a\ncorresponding increase in the compression of the CrO6 octahedra with increasing\nR-ion radii. From the Raman studies, we predict a critical R-ion radii, above\nwhich we expect the distortions in RCrO3 to reduce with increasing pressure\nleading to what is observed in the case of LaCrO3. These Raman results are\nconsistent with our pressure dependent structural studies on RCrO3 (R = Gd, Eu,\nSm). Also, our results suggest that the pressure dependence of N\\'eel\ntemperature, TNCr, (where the Cr3+ spin orders) in RCrO3 is mostly affected by\nthe compressions of Cr-O bonds rather than the alteration of octahedral tilts.\n