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Abstract: Polar crystals provide an interesting avenue for materials research both in the structures they form and the properties they possess. This work describes the hydrothermal synthesis and structural characterization of three novel silicate fluorides. Compound (1), LiY3(SiO4)2F2 crystallizes in space group C2/c, with a=17.651(4)Å, b=4.8868(10)Å, c=11.625(2)Å and β=131.13(3)°. BaY2(Si2O7)F2 (2) crystallizes in space group P−1, with a=5.1576(10)Å, b=6.8389(14)Å, c=11.786(2)Å, α=93.02(3)°, β=102.05(3)° and γ=111.55(3)°. Finally, the structure of Ba2Y3(SiO4)2F5 (3) was determined in the polar orthorhombic space group Pba2, having unit cell parameters a=8.8864(18)Å, b=12.764(3)Å and c=5.0843(10)Å. The structures are compared based on their building blocks and long range polarities. Aligned silicate tetrahedra segregated into a single layer in (3) impart the observed polarity. [Copyright &y& Elsevier]

Abstract: Rare earth silicates with the structure of apatite are attracting considerable interest since they show oxygen ion conductivities higher than that of yttria-stabilized zirconia (YSZ) at moderate temperature. Based on the hydrothermal synthesis we presented a simple one step process for the direct preparation of the pure and the high crystalline nanosized rare earth silicates with the structure of apatite under a mild condition (230°C). Since the preparation of the high crystalline silicon based rare earth apatites is performed at high temperature previously and accompanied by subsequent process of grinding, results of this work provide a promising alternative of the existing methodology. Furthermore, due to the relatively low temperature of the preparation of these materials, high doping of monovalent cation can be done, which was not achieved before. [ABSTRACT FROM AUTHOR]