The ferroelectric properties of Cd2Nb2O7: a Monte Carlo simulation study

Abstract

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 forkTp/J= 0.3891 (kis the Boltzmann constant,Tpis the model phase transition temperature andJis the interaction energy). To further adapt the model to the properties of CNO, coupling of local Nb displacements to theT2usoft mode is simulatedviathe addition of an appropriate field term in the model Hamiltonian. The critical temperatureTcof the soft mode is scaled tokTc/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 thatTpcan be associated with the transition temperatureT1= 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 ∼100 K.