Complex Octahedral Tilt Phases in the Ferroelectric Perovskite System Li$_x$Na$_{1-x}$NbO$_3$

TL;DR

This study reveals unprecedented complex octahedral tilt phases in the ferroelectric perovskite system LixNa1-xNbO3, showing novel superlattice modulations and structural complexity not seen in similar materials.

Contribution

It uncovers new complex tilt phases and superlattice structures in LixNa1-xNbO3, expanding understanding of perovskite structural chemistry.

Findings

Distinct complex tilt phases observed for different compositions

Novel 4-fold superlattice in Li0.12Na0.88NbO3

Higher-order modulations in Li0.03Na0.97NbO3 and Li0.08Na0.92NbO3

Abstract

High-temperature phase behavior in the system LixNa1-xNbO3 has been studied by using high-resolution powder neutron diffraction. Each of the three compositions studied in the Na-rich part of the phase diagram (x = 0.03, 0.08 and 0.12) shows evidence for distinct and complex structural modulations based on different tilting schemes of NbO6 octahedral units. Whilst octahedral tilting is prevalent in the structural chemistry of perovskites the nature of complexity of the phases observed here is unprecedented. Neither of the long-range tilt phases observed in NaNbO3 itself occurs here; instead a novel phase with a well-defined 4-fold superlattice is observed for the composition Li0.12Na0.88NbO3, and yet more complex phases with modulations based on 20-fold and 30-fold repeats are observed for Li0.03Na0.97NbO3 and Li0.08Na0.92NbO3, respectively. This peculiar structural behavior makes the system LixNa1-xNbO3 the most structurally complex 'simple' perovskite known.