  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.
