Structural and magnetic phase transitions of the orthovanadates RVO3 (R= Dy, Ho, Er) as seen via neutron diffraction

The structural and magnetic phase behavior of RVO₃ with R=v Dy, Ho, and Er was studied by single-crystal neutron diffraction. Upon cooling, all three compounds show structural transitions from orthorhombic (space group Pbnm) to monoclinic (p2₁/b) symmetry due to the onset of orbital order at T= 188-200 K, followed by Néel transitions at T= 110-113 K due to the onset of antiferromagnetic (C-type) order of the vanadium moments. Upon further cooling, additional structural phase transitions occur for DyVO₃ and ErVO₃ at 60 and 56 K, respectively, where the monoclinic structure changes to an orthorhombic structure with the space group Pbnm, and the magnetic order of the V sublattice changes to a G-type structure. These transition temperatures are reduced compared to the ones previously observed for nonmagnetic R³⁺ ions due to exchange interactions between the V³⁺ and R³⁺ ions. For ErVO₃, R-R exchange interactions drive a transition to collinear magnetic order at T= 2.5 K. For HoVO₃, the onset of noncollinear, weakly ferromagnetic order of the Ho moments nearly coincides with the structural phase transition from the monoclinic to the low-temperature orthorhombic structure. This transition is characterized by an extended hysteresis between 24 and 36 K. The Dy moments in DyVO₃ also exhibit noncollinear, weakly ferromagnetic order upon cooling below 13 K. With increasing temperature, the monoclinic structure of DyVO₃ reappears in the temperature range between 13 and 23 K. This reentrant structural transition is associated with a rearrangement of the Dy moments. A group theoretical analysis showed that the observed magnetic states of the R³⁺ ions are compatible with the lattice structure. The results are discussed in the light of recent data on the magnetic field dependence of the lattice structure and magnetization of DyVO₃ and HoVO₃.