The origins of the dielectric anomalies associated with the Jahn-Teller phase transitions in DyV0, and DyAsO, are examined in detail. The lowest, nearly degenerate, Dy electronic levels couple strongly to distortions that carry an electric dipole moment parallel to the tetragonal axis. The electric susceptibility as a function of temperature, xc, accordingly shows Curie-like behaviour above TD and a prominent cusp at the phase transition. In the perpendicular direction the electric dipole moments are negligible, and xo, xb have a temperature dependence similar to that of the order parameter. In sensitive experiments x. and x b are observed to differ in magnitude and temperature dependence, suggesting a non-linear dependence on order parameter. Analysis shows that the principal contribution to xa, x b comes from matrix elements of the electric dipole operator between the ground doublets and excited crystal-field levels. These Van Vleck contributions lead to a temperature dependence both linear and quadratic in the order parameter in good agreement with experimental results.
IntroductionD y V 0 4 and DyAsO, undergo cooperative Jahn-Teller phase transitions near 14 K from tetragonal (Dii) to orthorhombic (Dii) symmetry (Elliott et af 1972, Gehring and Gehring 1975, Kasten 1980, Page et af 1984. The order parameter is a B,, bulk strain, and has been measured directly by x-ray diffraction (Gobel and Will 1972a, b).The lowest Dy electronic levels are two Kramers doublets which are nearly degenerate above TD. These levels have finite couplings to distortions transforming according to the B1, B2 and E representations of the D2,, Dy site symmetry. It is of interest to note that the distortion which actually occurs, BZ, transforms like z , and that the E distortion transforms like x , y . This implies that the Dy electronic levels couple to electric fields E, and E,, E,. and hence that electric susceptibility measurements parallel and perpendicular to the tetragonal axis ( z I/ c) can provide information on the interactions and the phase transition.Measurements of xc on D y V 0 4 and DyAs04 (Unoki and Sakudo 1977, Harley and Taylor 1978, Page et a1 1979, 1984 showed a prominent dielectric anomaly indicating antiferroelectric (zone-centre) ordering at TD. On the other hand, electric susceptibility measurements perpendicular to the tetragonal axis showed negligible basal plane electric dipole moments. xu and X b were found to change substantially below T,, with Ax,, and AXb apparently proportional to the order parameter. In a recent detailed investigation (Page ef a1 1984, hereafter referred to as PTS), it was found that AX,, and AXb differed 0022-3719/85/173285 + 12 $02.25