An intermediate antipolar phase in NaNbO₃ under compression

Abstract

The crystal structure and vibration spectra of sodium niobate NaNbO3 have been studied by means of neutron diffraction, X-ray diffraction and Raman spectroscopy at high pressures. An isostructural phase transformation from the initial antiferroelectric phase to an intermediate antiferroelectric phase HP-I have been observed under 2 GPa. This transformation is caused by complex reorientations of NbO6 octahedra. Subsequent structural phase transition from the phase HP-I to an orthorhombic phase HP-III have been detected at pressures above 10 GPa. This transition leads to suppression of an antiferroelectric state of NaNbO3. The observed phase transitions are accompanied by anomalies in lattice parameters compression.