Local structure of NaNbO3: A neutron scattering study

We report the results of a neutron diffraction study of structural evolution in sodium niobate, NaNbO3, which is the parent compound for lead-free ferroelectric materials, as a function of temperature from 15 to 930 K over six phases. The Rietveld analysis of the high-resolution powder neutron diffraction data shows the variation in the structure from cubic to rhombohedral ferroelectric structures. However, the refinements on local structure by the pair distribution function (PDF) method indicates that there are only three basic patterns of the local structure, and the ground states of NaNbO3 in the low-temperature antiferroelectric and ferroelectric phases have the R3c symmetry, even though in the long range the system shows the Pbcm symmetry or the coexistence of two phases. The origin of the complex phase behavior and its implications on the performance as lead-free ferroelectrics are discussed.