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 = ½ ½ ½). Ab-initio calculation of the enthalpy in the various phases of NaNbO3 is able to predict the phase transition pressure well.
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