Orbital-ordering-induced phase transition in LaVO₃ and CeVO₃

The structural phase transition in the orthovanadates LaVO₃ and CeVO₃ has been studied with high energy synchrotron x-ray diffraction. LaVO₃ undergoes a second order phase transition at T_N = 143 K and a first order transition at T_t = 141 K, while in CeVO3 there are phase transitions occurring at T₀ = 154 K of second order and at T_N = 134 K of first order. These phase transitions are confirmed by specific heat measurements. The phase transition at T_t in LaVO₃ or T₀ in CeVO₃ is due to a G-type orbital ordering which lowers the structure symmetry from orthorhombic Pbnm to monoclinic P2_t/b11. The structure change at T_N in CeVO₃ is ascribed to an orbital ordering enhanced magnetostrictive distortion, while that at T_N in LaVO₃ 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 LaVO₃ should be associated with a sudden change of both spin and orbital configurations, similar to the phase transition at T_s = 77 K in YVO₃ [Ren et al., Nature (London) 396, 441 (1998)], causing a reversal of the net magnetization. However, the ordered state above T_t in LaVO₃ is identical to that below T_s in YVO₃. It is found that, with increasing lanthanide ionic radius, the Néel temperature T_N increases while the orbital ordering onset temperature decreases in these orthovanadates.