Orbital-ordering-induced phase transition in (formula presented) and (formula presented)

The structural phase transition in the orthovanadates (formula presented) and (formula presented) has been studied with high energy synchrotron x-ray diffraction. (formula presented) undergoes a second order phase transition at (formula presented) and a first order transition at (formula presented) while in CeVO3 there are phase transitions occurring at (formula presented) of second order and at (formula presented) of first order. These phase transitions are confirmed by specific heat measurements. The phase transition at (formula presented) in (formula presented) or (formula presented) in (formula presented) is due to a G-type orbital ordering which lowers the structure symmetry from orthorhombic (formula presented) to monoclinic (formula presented) The structure change at (formula presented) in (formula presented) is ascribed to an orbital ordering enhanced magnetostrictive distortion, while that at (formula presented) in (formula presented) is most probably due to an ordered occupation of the vanadium (formula presented) (formula presented) orbitals associated with an antiferromagnetic ordering. We propose that the first order phase transition at (formula presented) in (formula presented) should be associated with a sudden change of both spin and orbital configurations, similar to the phase transition at (formula presented) in (formula presented) [Ren et al., Nature (London) 396, 441 (1998)], causing a reversal of the net magnetization. However, the ordered state above (formula presented) in (formula presented) is identical to that below (formula presented) in (formula presented) It is found that, with increasing lanthanide ionic radius, the Néel temperature (formula presented) increases while the orbital ordering onset temperature decreases in these orthovanadates. © 2003 The American Physical Society.