A crystallographic study of group I niobate perovskites

Abstract

In this work, X-ray and neutron powder diffraction experiments and complementary solid-state NMR spectroscopy are used to characterise NaNbO₃-based perovskite phases. Samples of NaNbO₃, KₓNa₁₋ₓNbO₃ and LiₓNa₁₋ₓNbO₃ are synthesised using a variety of techniques and subsequently characterised. For NaNbO₃, it is observed that at least two room temperature perovskite phases can co-exist, P and Q, and that each phase can be formed exclusively by manipulating the synthetic approach utilised. Phase Q can also be formed by the substitution of a small amount of K⁺ or Li⁺ for Na⁺. The room temperature phases of these materials are also analysed using NMR spectroscopy and X-ray diffraction. It is found that, for KₓNa₁₋ₓNbO₃, preferential A-site substitution of K⁺ for Na⁺ may occur, and this observation is supported using a range of NMR techniques and density functional theory calculations. The high-temperature phase behaviour of NaNbO₃ and KₓNa₁₋ₓNbO₃ (x = 0.03 to 0.08) is analysed using high-resolution neutron and X-ray powder diffraction to determine when phase changes occur and to characterise each phase. Characterisation of these materials is supported used complementary symmetry mode analysis. For the LiₓNa₁₋ₓNbO₃ perovskite system, complex phase behaviour is observed at room temperature. High-resolution neutron powder diffraction data shows that, over the range 0.08 < x < 0.20, phase Q may co-exist with a rhombohedral phase, with the proportions of the two highly dependent upon the synthetic conditions used. Furthermore, using X-ray diffraction and NMR spectroscopy, phase Q is shown to undergo a crystal-to-crystal transition to the rhombohedral phase. For higher values of x, two compositionally-distinct rhombohedral phases are formed, termed Na-R3c and Li-R3c, as determined from neutron powder diffraction data.