Structural Studies and Order–Disorder Phenomenon in a Series of New Quaternary Tellurates of the Type A2+M4+Te6+O6 and A1+2M4+Te6+O6

Five new compounds in the AxMTeO6 family were prepared and structurally characterized: Li2GeTeO6, Na2TiTeO6, Na2SnTeO6, and two forms of Na2GeTeO6. All compounds are layered structures based on various stacking arrangements of MTeO2-6 layers. The structures of BaGeTeO6 and SrGeTeO6 were also determined. The former compound was found to contain Ba2+ in trigonal prismatic coordination, in agreement with previous literature reports, while SrGeTeO6 contains Sr2+ in octahedral coordination and is not isostructural with BaGeTeO6 as was previously reported. Structural characterizations were carried out using a variety of tools, including Rietveld refinements of X-ray and neutron powder diffraction data, solid-state MAS–NMR, and Raman and infrared spectroscopy. With the exception of M=Ti4+, the MTeO2-6 layers show a high degree of order between M4+ and Te6+ cations within the layers, but the stacking faults are generally present, which results in a significant decrease in the long-range ordering of the M4+ and Te6+ cations in the third dimension. The compound K2xNaxTiTeO6 was prepared by reacting Na2TiTeO6 and KNO3 under hydrothermal conditions at 200°C in a Teflon-lined Parr bomb. This compound adopts a pyrochlore structure with cell edge 10.18 ?. NMR measurements indicate a disordered Ti/Te distribution.