Orbital-ordering-induced phase transition inLaVO3andCeVO3

Abstract

The structural phase transition in the orthovanadates ${\mathrm{LaVO}}_{3}$ and ${\mathrm{CeVO}}_{3}$ has been studied with high energy synchrotron x-ray diffraction. ${\mathrm{LaVO}}_{3}$ undergoes a second order phase transition at ${T}_{N}=143\mathrm{K}$ and a first order transition at ${T}_{t}=141\mathrm{K},$ while in CeVO3 there are phase transitions occurring at ${T}_{0}=154\mathrm{K}$ of second order and at ${T}_{N}=134\mathrm{K}$ of first order. These phase transitions are confirmed by specific heat measurements. The phase transition at ${T}_{t}$ in ${\mathrm{LaVO}}_{3}$ or ${T}_{0}$ in ${\mathrm{CeVO}}_{3}$ is due to a G-type orbital ordering which lowers the structure symmetry from orthorhombic $\mathrm{Pbnm}$ to monoclinic ${P2}_{1}/b11.$ The structure change at ${T}_{N}$ in ${\mathrm{CeVO}}_{3}$ is ascribed to an orbital ordering enhanced magnetostrictive distortion, while that at ${T}_{N}$ in ${\mathrm{LaVO}}_{3}$ is most probably due to an ordered occupation of the vanadium $3d$ ${t}_{2g}$ orbitals associated with an antiferromagnetic ordering. We propose that the first order phase transition at ${T}_{t}$ in ${\mathrm{LaVO}}_{3}$ should be associated with a sudden change of both spin and orbital configurations, similar to the phase transition at ${T}_{s}=77\mathrm{K}$ in ${\mathrm{YVO}}_{3}$ [Ren et al., Nature (London) 396, 441 (1998)], causing a reversal of the net magnetization. However, the ordered state above ${T}_{t}$ in ${\mathrm{LaVO}}_{3}$ is identical to that below ${T}_{s}$ in ${\mathrm{YVO}}_{3}.$ It is found that, with increasing lanthanide ionic radius, the N\'eel temperature ${T}_{N}$ increases while the orbital ordering onset temperature decreases in these orthovanadates.