Octahedral tilting, monoclinic phase and the phase diagram of PZT

TL;DR

This study uses spectroscopy to clarify the phase diagram of PZT near the MPB, revealing coexistence of monoclinic and rhombohedral phases, a new diffuse transition, and insights into octahedral tilting and polarization rotation.

Contribution

It provides new experimental evidence supporting phase coexistence, identifies a previously unreported diffuse transition, and offers a revised understanding of octahedral tilting and phase boundaries in PZT.

Findings

No clear boundary between monoclinic and rhombohedral phases.

Discovery of a diffuse transition near x ~ 0.1 at T_IT.

Evidence of octahedral tilting starting at T_IT, influencing phase behavior.

Abstract

Anelastic and dielectric spectroscopy measurements on PZT close to the morphotropic (MPB) and antiferroelectric boundaries provide new insight in some controversial aspects of its phase diagram. No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, in agreement with recent structural studies supporting a coexistence of the two phases over a broad composition range x < 0.5, with the fraction of M increasing toward the MPB. It is also discussed why the observed maximum of elastic compliance appears to be due to a rotational instability of the polarisation and therefore cannot be explained by extrinsic softening from finely twinned R phase alone, but indicates the presence also of M phase, not necessarily homogeneous. A new diffuse transition is found within the ferroelectric phase near x ~ 0.1, at a temperature T_IT higher than the well established boundary T_T to the phase with tilted octahedra. It is proposed that around T_IT the octahedra start rotating in a disordered manner and finally become ordered below T_T. In this interpretation, the onset temperature for octahedral tilting monotonically increases up to the antiferroelectric transition of PbZrO3, and the depression of T_T(x) below x = 0.18 would be a consequence of the partial relieve of the mismatch between the cation radii with the initial stage of tilting below T_IT.