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 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 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