Interpretation of electric susceptibility and order-parameter measurements near the first- and second-order Jahn-Teller phase transitions in DyAsO4and DyVO4using correlated-effective-field theory

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Abstract
Precise measurements of the order parameter and the c axis electric susceptibility have been made near the Jahn-Teller phase transitions in DyAsO4 and DyVO4. Both compounds show large departures from simple mean-field (MF) theory. The phase transition in DyAsO4 is of first order while that of DyVO4 is continuous. The data are analysed using a correlated-effective-field (CEF) model in which a simple approximation, consistent with classical critical behaviour, is made for the dependence on q of the effective exchange constant J(q). The CEF model gives an excellent description of the data over a wide range of temperatures and magnetic fields, both below and above TD. This agreement establishes that the competing effects of short- and long-range interactions are responsible for the deviations from MF behaviour; the model accounts for the first- and second-order nature of the phase transitions in these compounds. The large c axis dielectric anomalies imply the existence of appreciable electric dipole moments and are interpreted in terms of coupling to odd-symmetry q=0 modes. From the measurements they find that the odd-symmetry (electric dipolar) couplings are in order of magnitude larger in DyVO4 than in DyAsO4. These interactions of relatively long range appear to be strong enough in DyVO4 to ensure a continuous phase transition, but are too weak to do so in DyAsO4.

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