The Polar Phase of NaNbO_3: A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations

摘要

A polar phase of NaNbO_3 has been successfully synthesized using sol-gel techniques. Detailed characterization of this phase has been undertaken using high-resolution powder diffraction (X-ray and neutron) and ~(23)Na multiple-quantum (MQ) MAS NMR, supported by second harmonic generation measurements and density functional theory calculations. Samples of NaNbO_3 were also synthesized using conventional solid-state methods and were observed to routinely comprise of a mixture of two different polymorphs of NaNbO_3, namely, the well-known orthorhombic phase (space group Pbcm) and the current polar phase, the relative quantities of which vary considerably depending upon precise reaction conditions. Our studies show that each of these two polymorphs of NaNbO_3 contains two crystallographically distinct Na sites. This is consistent with assignment of the polar phase to the orthorhombic space group P2 ma, although peak broadenings in the diffraction data suggest a subtle monoclinic distortion. Using carefully monitored molten salt techniques, it was possible to eradicate the polar polymorph and synthesize the pure Pbcm phase.