The crystal structure of NaNbO(3) 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.

The system Li x Na 1-x NbO 3 has been studied by using a combination of X-ray and neutron powder diffraction and 23 Na solid-state NMR spectroscopy. For x = 0.05 we confirm a single polar orthorhombic phase. For 0.08 ≤ x ≤ 0.20 phase mixtures of this orthorhombic phase, together with a rhombohedral phase, isostructural with the low-temperature ferroelectric polymorph of NaNbO 3 , are observed. The relative fractions of these two phases are shown to be critically dependent on synthetic conditions: the rhombohedral phase is favoured by higher annealing temperatures and rapid cooling. We also observe that the orthorhombic phase transforms slowly to the rhombohedral phase on standing in air at ambient temperature. For 0.25 ≤ x ≤ 0.90 two rhombohedral phases co-exist, one Na-rich and the other Li-rich. In this region the phase behavior is independent of reaction conditions. 2

Keywords: Perovskite; hydrothermal; XANES; NMR; neutron diffraction 2 Graphical AbstractA multi-element A-site perovskite crystallises directly from aqueous, basic solutions at 240 ºC; while the paramagnetic effect of Ce 3+ on the 23 Na NMR shows a homogeneous solidsolution, the incorporation of A-site water is also found from 2 H NMR and IR, with oxidation of some cerium to charge balance proved by XANES spectroscopy.3

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New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO3. -The crystal structure of NaNbO3 is determined in the temperature range between 360 and 520°C by high-resolution neutron and synchrotron X-ray powder diffraction, supported by DFT computations. New structural models are proposed for the elusive phases S (stable between 480 and 510°C) and R (370-470°C). A unique model for phase S is elucidated, having a 2 x 2 x 4 superlattice of the aristotype perovskite structure (space group Pmmn). This structure features a new example of an octahedral tilt system in a perovskite type compound. Two possible structural models for phase R are determined. -(PEEL, M. D.; THOMPSON, S. P.; DAOUD-ALADINE, A.; ASHBROOK, S. E.; LIGHTFOOT*, P.; Inorg. Chem. 51 (2012) 12, 6876-6889, http://dx.doi.org/10.1021/ic3006585 ; EaStChem, Sch. Chem.,