Structural evolution of bismuth sodium titanate induced by A-site non-stoichiometry: Neutron powder diffraction studies

TL;DR

This study investigates how non-stoichiometry affects the crystal structure of bismuth sodium titanate using neutron powder diffraction, revealing local structural stability but changes in long-range order and octahedral tilting.

Contribution

It provides detailed insights into the structural evolution caused by A-site non-stoichiometry in bismuth sodium titanate, highlighting opposite effects of sodium and bismuth deficiencies.

Findings

Local structure remains stable despite sodium deficiency.

Sodium deficiency weakens long-range atomic correlations.

Sodium and bismuth non-stoichiometry have opposite effects on octahedral tilting.

Abstract

We performed neutron powder diffraction measurements on (Bi$_{0.5}$Na$_{0.5+x}$)TiO$_3$ and (Bi$_{0.5+y}$Na$_{0.5}$)TiO$_3$ to study structural evolution induced by the non-stoichiometry. Despite the non-stoichiometry, the local structure ($r$$\leq$ 3.5 {\AA}) from the pair distribution function analysis is barely affected by the sodium deficit of up to -5 mol%. With increasing pair distance, however, the atomic pair correlations weaken due to the disorder caused by the sodium deficiency. Although the sodium and the bismuth share the same crystallographic site, their non-stoichiometry have rather opposite effects as revealed from a distinctive distortion of the Bragg peaks. In addition, Rietveld refinement demonstrates that the octahedral tilting is continually suppressed by the sodium deficit of up to -5 mol%. This is contrary to the effect of the bismuth deficiency, which enhances the octahedral tilting.