Structural studies and order-disorder phenomenon in a series of new quaternary tellurates of the type A(2+)M(4+)Te(6+)O(6) and A(2)(1+)M(4+)Te(6+)O(6)

Five new compounds in the A(x)MTeO(6) family were prepared and structurally characterized: Li2GeTeO6, Na2TiTeO6, Na2SnTeO6, and two forms of Na2GeTeO6 , All compounds are layered structures based on various stacking arrangemen ts of MTeO62- layers. The structures of BaGeTeO6 and SrGeTeO6 were also det ermined. The former compound was found to contain Ba2+ in trigonal prismati c coordination, in agreement with previous literature reports, while SrGeTe O6 contains Sr2+ in octahedral coordination and is not isostructural with B aGeTeO6 as was previously reported. Structural characterizations were carri ed out using a variety of tools, including Rietveld refinements of X-ray an d neutron powder diffraction data, solid-state MASNMR, and Raman and infrar ed spectroscopy. With the exception of M = Ti4+, the MTeO62- layers show a high degree of order between M4+ and Te6+ cations within the layers, but th e stacking faults are generally present, which results in a significant dec rease in the long-range ordering of the M4+ and Te6+ cations in the third d imension, The compound K2-xNaxTiTeO6 was prepared by reacting Na2TiTeO6 and KNO3 under hydrothermal conditions at 200 degrees C in a Teflon-lined Parr bomb. This compound adopts a pyrochlore structure with cell edge 10.18 Ang strom. NMR measurements indicate a disordered Ti/Te distribution. (C) 1999 Academic Press.