Suppression of Antiferroelectric State in NaNbO3 at High Pressure from In Situ Neutron Diffraction

TL;DR

This study provides direct experimental evidence of a pressure-induced antiferroelectric to paraelectric phase transition in NaNbO3, revealing structural changes and anisotropic lattice responses using neutron diffraction and ab-initio calculations.

Contribution

It offers the first direct neutron diffraction evidence of the phase transition in NaNbO3 under pressure and clarifies the structural behavior around 8 GPa, contrasting previous Raman-based suggestions.

Findings

Paraelectric phase stabilizes above 8 GPa with orthorhombic Pbnm structure.

No structural transition observed around 2 GPa, despite Raman evidence.

Lattice parameters respond anisotropically to pressure, with maximum contraction along <100>.

Abstract

We report direct experimental evidence of antiferroelectric to paraelectric phase transition under pressure in NaNbO3 using neutron diffraction at room temperature. The paraelectric phase is found to stabilize above 8 GPa and its crystal structure has been determined in orthorhombic symmetry with space group Pbnm. The variation of the structural parameters of the both orthorhombic phases as a function of pressure was determined. We have not found evidence for structural phase transition around 2 GPa as previously suggested in the literature based on Raman scattering experiments, however, significant change in Nb-O-Nb bond angles are found around this pressure. The response of the lattice parameters to pressure is strongly anisotropic with a largest contraction along <100>. The structural phase transition around ~ 8 GPa is followed by an anomalous increase in the orthorhombic strain and tilt angle associated with the R point (q= 1/2 1/2 1/2). Ab-initio calculation of the enthalpy in the various phases of NaNbO3 is able to predict the phase transition pressure well.