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

TL;DR

This study demonstrates that the ferroelectric to paraelectric phase transition in a specific bismuth-based compound occurs directly and is first order, resolving previous controversies about intermediate phases.

Contribution

It provides experimental evidence that the transition from ferroelectric to cubic paraelectric phase in 0.8BiFeO3-0.2Pb(Fe1/2Nb1/2)O3 is direct and first order, without intermediate phases.

Findings

Direct transition from R3c to cubic phase confirmed.

Transition is of first order with phase coexistence.

No intermediate 'beta' phase observed.

Abstract

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.