Phase transitions and phase diagram of the ferroelectric perovskite NBT-BT by anelastic and dielectric measurements

TL;DR

This study investigates the phase transitions and phase diagram of the ferroelectric perovskite NBT-BT using anelastic and dielectric measurements, revealing sharp and diffuse transitions and suggesting local monoclinic symmetry near the phase boundary.

Contribution

It provides detailed elastic and dielectric data across compositions, clarifies the nature of phase transitions, and proposes a link to local monoclinic symmetry in NBT-BT.

Findings

Sharp rhombohedral/tetragonal transition peaks up to x=0.06

Diffuse and relaxor-like dielectric behavior at higher x

Evidence of local monoclinic symmetry near the phase boundary

Abstract

The complex elastic compliance and dielectric susceptibility of (Na_{0.5}Bi_{0.5})_{1-x}Ba_{x}TiO_{3} (NBT-BT) have been measured in the composition range between pure NBT and the morphotropic phase boundary included, 0 <= x <= 0.08. The compliance of NBT presents sharp peaks at the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the tetragonal region of the phase diagram, up to now impossible due to the strong lattice disorder and small distortions and polarizations involved. In spite of ample evidence of disorder and structural heterogeneity, the R-T transition remains sharp up to x = 0.06, whereas the T-C transition merges into the diffuse and relaxor-like transition associated with broad maxima of the dielectric and elastic susceptibilities. An attempt is made at relating the different features in the anelastic and dielectric curves to different modes of octahedral rotations and polar cation shifts. The possibility is also considered that the cation displacements locally have monoclinic symmetry, as for PZT near the morphotropic phase boundary.