Competition between Jahn-Teller coupling and orbital fluctuations in HoVO3

We have carried out a detailed study of the structural properties of HoVO3 perovskite using a combination of single-crystal neutron diffraction and synchrotron x-ray and neutron powder diffraction. We focus on the competition between one-dimensional fluctuations of the occupied vanadium d orbitals and coherent Jahn-Teller distortion due to long-range orbital ordering. At room temperature orbital fluctuations are dominant. Below 188 K a structural phase transition from orthorhombic Pbnm to monoclinic Pb11 symmetry takes place, corresponding to a state where strong orbital fluctuations are superimposed on an underlying orbitally ordered state. However, the fluctuations are not strong enough to give rise to a long-range orbitally dimerized state as theoretically predicted. Ordering of the vanadium spins at 114 K has little effect on the orbital fluctuations, but the orbital ordering becomes coherent below a first-order transition to an orthorhombic Pbnm phase at 40 K.