LOW-TEMPERATURE STRUCTURAL PHASE-TRANSITION IN A MONOCLINIC KDY(WO4)(2) CRYSTAL

The low-temperature thermal and magnetic-resonance properties of a mon oclinic KDy(WO4)(2) single crystal are investigated. It is established that a structural phase transition takes place at T-c=6.38 K. The fie ld dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial p art of the H-T phase diagram is plotted for H parallel to a. The promi nent features of the structural phase transition are typical of a seco nd-order Jahn-Teller transition, which is not accompanied by any chang e in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition go es to an antiferrodistortion phase. An anomalous increase in the relax ation time (by almost an order of magnitude) following a thermal pulse is observed at T>T-c(H), owing to the structural instability of the l attice. A theoretical model is proposed for the structural phase trans ition in a magnetic field, and the magnetic-field dependence of T, is investigated for various directions of the field. (C) 1998 American In stitute of Physics.