Ion exchange reactions of NaSbO3 and morphotropic series MSbO3

By reactions of ilmenite-type NaSbO3 with molten nitrates at ca. 300 °C, ilmenite forms of AgSbO3 and TlSbO3 have been obtained and characterized by chemical analysis and X-ray diffraction (rhombohedral, a=5.3272(9)a=5.3272(9) and 5.3149(7) Å, c=16.6923(2)c=16.6923(2) and 21.5136(4) Å, respectively). Unexpectedly, a new form of LiSbO3, prepared in a similar way, is not rhombohedral. Its XRD pattern has been indexed on a monoclinic C-centred pseudo-orthorhombic cell with a=5.2948(7)a=5.2948(7), b=9.0082(7)b=9.0082(7), c=5.2923(6) Åc=5.2923(6) Å, β=113.790(5)β=113.790(5). This cell may be derived from the ilmenite structure by gliding SbO6/2 layers. Diffuse nature of the XRD pattern is due to stacking faults. TlSbO3 also contains faults, which have been modelled by ∼10% 2H stacking analogous to known P-31c polytype. On heating, layered AgSbO3 and LiSbO3 transform to stable phases (pyrochlore and Pncn, respectively), but TlSbO3 remains unchanged at least to 850 °C. Attempts to substitute Cs for Na have been unsuccessful. Previously reported X-ray data for ilmenite-type CsSbO3 are discredited. Volume relations in the MSbO3 series are discussed.

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