New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO3

The crystal structure of NaNbO3 has been studied in detail in the temperature regime 360 T T Pmmn. This unusual and unique structure features a novel example of a compound octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 T < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted.