Neutron diffraction, x-ray diffraction, and specific heat studies of orbital ordering inYVO3

Neutron diffraction, synchrotron x-ray diffraction, and specific heat studies have been carried out to investigate the nature of the ordered occupation of the vanadium d orbitals in perovskite ${\mathrm{YVO}}_3.$ Evidence has been found for a change in the type of orbital ordering at the 77-K phase transition in this material, manifested by a change in the type of Jahn-Teller distortion. This transition between orbital orderings is caused by an increase in octahedral tilting with decreasing temperature. The orbital ordering above 77 K is not destroyed at the magnetic ordering temperature of 116 K, but is present as far as a second structural phase transition at 200 K. The entropy changes at the onset of both spin and orbital ordering are much lower than the smallest semiclassical value of $R\ln 2$ J/(mole K).