Dielectric properties and phase transitions of Cd2Nb2O7: Effects of pressure and temperature

Abstract

The effects of hydrostatic pressure on the dielectric response of single crystal Cd2Nb2O7 (CNO) were investigated over the temperature range of 80–450K. The main pressure effects are (1) large suppression of the magnitude of the dielectric constant ε′ over most of the temperature range and (2) very weak pressure dependence of the ferroelastic and ferroelectric (FE) transition temperatures and of the relaxational response near 200K, as reported earlier for ceramic CNO. In view of result (2) the emphasis of the present paper is on the dielectric response above the transition region, i.e., in the high temperature paraelectric phase. The ε′(T) response in this phase can be explained by the T dependence of the uncoupled soft FE mode frequency associated with the NbO6 octahedra, and the large decrease in ε′ with pressure follows from the expected increase of this frequency which can be estimated from the dielectric data. In this phase ε′ is very well represented by a Curie-Weiss law ε′=C∕(T−T0), where C=1.16×105K and T0=174K at 1bar. Remarkably, the logarithmic pressure derivatives of C and T0 are found to be essentially identical to those of BaTiO3 implicating the crucial role of the BO6 octahedra in the soft mode character of the pyrochlore (CNO) structure, as in the case of the ABO3 perovskite structure. Pressure had a relatively weak influence on the various overlapping dielectric relaxations below 200K, but the results revealed a dipolar relaxation in the 350–400K region. It is suggested that this relaxation is associated with the motion of a defect complex involving the oxygen vacancy, a common feature in the related ABO3 oxides.