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Abstract

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 T-0=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 T-0 in CeVO3 is due to a G-
type orbital ordering which lowers the structure symmetry from
orthorhombic Pbnm to monoclinic P2(1)/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 Neel temperature T-N increases while the orbital
ordering onset temperature decreases in these orthovanadates.