Evidence for First Order Cubic Paraelectric to Rhombohedral Ferroelectric Phase Transition in 0.8BiFeO3-0.2Pb(Fe1/2Nb1/2)O3

The current controversies about the existence of an intermediate 'beta' phase of BiFeO_3 and the high temperature paraelectric 'gamma' phase are resolved by studying the sequence of ferroic transitions in 0.8BiFeO_3-0.2Pb(Fe_1/2Nb_1/2)O_3 (BF-0.2PFN) with lowered transition temperature. It is shown that the room temperature ferroelectric phase of 0.8BF-0.2PFN in the R3c space group transforms to the paraelectric/paraelastic cubic (Pm-3m) phase directly without any intermediate 'beta' phase reported in the literature. This transition is of first order type as confirmed by the coexistence of R3c and Pm-3m phases over a 100K range and a discontinuous change in the unit cell volume.