93Nb NMR and DFT investigation of the polymorphs of NaNbO3

Sodium niobate (NaNbO3) has a particularly complex phase diagram, with a series of phase transitions as a function of temperature and pressure, and even at room temperature a number of different structural variations have been suggested. Recent work has demonstrated that bulk powders of NaNbO3, prepared using a variety of synthetic approaches, contain a mixture of perovskite phases; the commonly reported Pbcm phase and a second, polar phase tentatively identified as belonging to space group P2(1)ma. The two phases exhibit very similar Na-23 MAS NMR spectra, although high-resolution MQMAS spectra were able to distinguish between them. Here, we investigate whether different perovskite polymorphs can be distinguished and/or identified using a variety of Nb-93 NMR methods, including MAS, MQMAS and wideline experiments. We compare the experimental results obtained for these more common perovskite materials to those for the metastable ilmenite polymorph of NaNbO3. Our experimental results are supported by first-principles calculations of NMR parameters using a planewave pseudopotential approach. The calculated NMR parameters appear very different for each of the phases investigated, but high forces on the atoms indicate many of the structural models derived from diffraction require optimisation of the atomic coordinates. After geometry optimisation, most of these perovskite phases exhibit very similar NMR parameters, in contrast to recent work where it was suggested that Nb-93 provides a useful tool for distinguishing NaNbO3 polymorphs. Finally, we consider the origin of the quadrupolar coupling in these materials, and its dependence on the deviation from ideality of the NbO6 octahedra.