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 <
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 < T < 510 °C) is elucidated,
having a 2 × 2 × 4 superlattice of the aristotype perovskite
structure, space group 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.