Orbital-ordering-induced phase transition inLaVO3andCeVO3

Abstract

The structural phase transition in the orthovanadates LaVO 3 and CeVO 3 has been studied with high energy synchrotron x-ray diffraction. LaVO 3 undergoes a second order phase transition at T N ϭ143 K and a first order transition at T t ϭ141 K, while in CeVO3 there are phase transitions occurring at T 0 ϭ154 K of second order and at T N ϭ134 K of first order. These phase transitions are confirmed by specific heat measurements. The phase transition at T t in LaVO 3 or T 0 in CeVO 3 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 CeVO 3 is ascribed to an orbital ordering enhanced magnetostrictive distortion, while that at T N in 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 LaVO 3 should be associated with a sudden change of both spin and orbital configurations, similar to the phase transition at T s ϭ77 K in YVO 3 ͓Ren et al., Nature ͑London͒ 396, 441 ͑1998͔͒, causing a reversal of the net magnetization. However, the ordered state above T t in LaVO 3 is identical to that below T s in YVO 3 . 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.

Crystal and magnetic structure of CeVO3

“…The orbital ordering pattern, in turn, is established by a cooperative Jahn-Teller distortion at a higher temperature. The sequence of transitions we find also agrees with reference [12], but the transition temperatures given there are about 20 K higher than the ones we observe. Our transition temperatures are in good agreement with those given in references [4] and [13], but the sequence of transitions reported there is opposite to the ones we find.…”

“…This stringently implies that the space group of the monoclinic phase of CeVO 3 is P 2 1 /b (or P 2 1 /a as a subgroup of Pnma). The same space group was already suggested by Ren et al [12], but the authors of reference 13 reported the space group P 2 1 /n. For this space group the ac-plane (n-glide plane) is the monoclinic plane, and the monoclinic angle is β, so that one would expect a splitting of the reflection 202 instead of the 022, contrary to our observation.…”

“…A recent study including neutron diffraction, specific heat, and magnetization measurements on polycrystalline CeVO 3 has addressed this issue [13]. However, the results of this study show many inconsistencies with data reported in the prior literature [12,15]. In the present work we have therefore carried out a comprehensive reinvestigation of the structural and magnetic structure of CeVO 3 by synchrotron X-ray powder diffraction and single-crystal neutron diffraction.…”

“…However, both the transition temperatures and the sequence in which orbital and magnetic order is formed have been disputed. While Ren et al [12] reported structural and magnetic transitions at T S = 154 K and T N = 134 K, respectively, the opposite sequence was reported by Miyasaka et al [4] (T N = 133 K, T S = 122 K) and Muñoz et al [13] (T N = 136 K, T S = 124 K). Moreover, both transition temperatures are anomalously low and the unit cell volume is anomalously small compared to those of other members of the series [4,7,14].…”

Doping variation of anisotropic charge and orbital dynamics inY1−xCaxVO3: Comparison withFujioka1, Miyasaka2, Tokura3 2008Phys. Rev. B

“…1. 13,14,15,16 In the C-type spin and G-type orbital ordered phase, the orbital exchange interaction is quasi-one-dimensional due to the interference among the exchange processes, leading to the anisotropic feature of the optical Mott-gap excitation. 10,17,18 By partially replacing the nominally trivalent La ion with the divalent Sr one, i.e.…”