The ferroelectric properties of Cd2Nb2O7: a Monte Carlo simulation study

The order-disorder contributions to the ferroelectric properties of Cd2Nb2O7 (CNO) have been studied by Monte Carlo simulation of a 12-state modified Potts model on the pyrochlore lattice. Spin configurations obtained by these simulations are mapped to local Nb displacements. Secondary Cd displacements normal to the Nb displacement directions are considered as well. The model correctly reproduces diffuse scattering experimentally observed in CNO. A first-order phase transition is observed for kT(p)/J = 0.3891 (k is the Boltzmann constant, T-p is the model phase transition temperature and J is the interaction energy). To further adapt the model to the properties of CNO, coupling of local Nb displacements to the T-2u soft mode is simulated via the addition of an appropriate field term in the model Hamiltonian. The critical temperature T-c of the soft mode is scaled to kT(c)/J = 0.3704. Similarities to experimental observations, i.e. the occurrence of stable domains with {100} boundaries, as well as spontaneous polarization along the cubic < 100 > and < 110 > directions, indicate that T-p can be associated with the transition temperature T-1 = 205 K in CNO. Frequency dispersion of the dielectric permittivity of CNO can be attributed to the low-frequency switching of correlated chains of Nb displacement that remain partially disordered in the temperature range between 195 and similar to 100 K.