ANOMALOUS TEMPERATURE-DEPENDENCE OF THE LATTICE-PARAMETERS IN HOPO4 AND HOVO4 - RARE-EARTH QUADRUPOLAR EFFECTS

The temperature dependencies of the lattice parameters in tetragonal H oPO4 and HoVO4 were measured using neutron powder diffraction techniqu es. Below about 100 K, the lattice parameter a of HoPO4 increases with decreasing temperature while c decreases. In HoVO4, the above behavio ur is reversed, that is a decreases with decreasing temperature while c increases. Similar measurements on non-magnetic LuPO4 and LuVO4 do n ot show any anomaly. This observation indicates that the unusual tempe rature dependence of the lattice constants is magnetic in origin. The observed effect can be explained in terms of an Ho3+ quadrupole intera ction with the crystalline lattice. In particular, the calculated elec tronically generated quadrupole moment of Ho3+ in HoPO4 and HoVO4 exhi bits a temperature dependence similar to that observed for the lattice parameters.