Neutron diffraction study of YVO3, NdVO3, and TbVO3

The structural and magnetic properties of YVO3, NdVO3 and TbVO3 were investigated by single-crystal and powder neutron diffraction. YVO3 shows a structural phase transition at 200 K from an orthorhombic structure with the space group Pbnm to a monoclinic one with the space group P21/b. But supplementary high-resolution synchrotron diffraction experiments showed that the monoclinic distortion is extremely small. A group theoretical analysis shows that this magnetic state in the monoclinic phase is incompatible with the lattice structure, unless terms of higher than bilinear order in the spin operators are incorporated in the spin Hamiltonian. This observation is discussed in the light of recent theories invoking unusual many-body correlations between the vanadium t2g orbitals. A structural phase transition back to the orthorhombic space group Pbnm is observed upon cooling below 77 K. This transition is accompanied by a rearrangement of the magnetic structure into a mode compatible with the lattice structure. The crystal structures of NdVO3 and TbVO3 are closely similar to that of YVO3. However, only a single magnetic phase transition was found in the vanadium sublattice down to 9.5 K. Below 60 K the magnetic moments of the Nd- and Tb-ions are gradually polarized by the ordered vanadium moments. Below 11 K, we found a noncollinear order of the terbium moments.