Pressure-Induced Phase Transitions in Micro-, Submicro-, and Nanocrystalline NaNbO3

Pressure-induced successive phase transitions of micro-, submicro-, and nanocrystalline NaNbO3 particles were investigated by high-pressure Raman spectroscopy. Microcrystalline NaNbO3, which stabilizes in an orthorhombic Pbcm structure at ambient pressure, showed bulk-like successive transitions at around 2, 6, and 9 GPa. The transitions were essentially reversible but some specific bands became sharper after pressure release, indicating that the positions of the ions in the perovskite lattice were rearranged. Submicrocrystalline NaNbO3 (orthorhombic Pmc21 at ambient pressure) also showed phase transitions at around 2, 6, and 10 GPa on pressing. The pressure characteristic above 6 GPa was similar to that for microcrystalline NaNbO3. The transition was almost reversible on pressing up to 2 GPa. Pressing above 2 GPa and subsequent pressure release resulted in a bulk-like spectral profile for specific sites of the powder. Nanocrystalline NaNbO3 (orthorhombic Pmma at ambient pressure) showed a diffused and completely reversible transition behavior. Above 6 GPa, all powders with different crystallite sizes showed a similar pressure-evolution of the spectra. The lattice distortion induced by further pressing after touching of Na+ and NbO6− ions is comparable for all powders. On the other hand, a major difference of the pressure characteristics among the three types of powders was revealed for pressures below 6 GPa. Tilted NbO6 octahedra start to reorient at around 2 GPa with alteration of Na+−NbO6− interactions. Remarkable spectral changes at ca. 2 GPa were observed for submicrocrystalline powders, which have the lowest crystal symmetry among possible polymorphs. The phase stability of the Pmma structure in the nanocrystallline NaNbO3 could not be explained by internal pressure in fine particles.