The crystal structure of lithium tungstate

Abstract

Li2WO 4 has the phenacite type of structure, space group R3, with six molecules in a rhombohedral cell with a =8.888 A, :x = 107.78 °. The positions of all atoms have been found with considerable accuracy. The tungstate group is tetrahe(tral within experimental error with ~V-O = 1.79 z~. The configuration about lithium is also tetrahedral with an average bond length of Li-O = 1.96 A. Many years ago one of us reported (Zachariasen, 1926) that the compounds Be2Si04 (mineral phenacite), Zn2SiO4 (willemite), Li2BeF4, Li2MoOa and Li2WO4 were isostructural. Shortly afterwards Bragg (1927) described the structure of phenacite, and Bragg & Zachariasen (1930) the structure of willemite. Recently Prof. g. ,I. Moon of this University asked the writers to examine inch long single crystals of lithium tungstate. The earlier statement that Li2W04 has the phenacite type of structure was confirmed. Since no reliable results are availaMe in the literature for the bond lengths of the tungstate group, a deter-ruination of all atomic coordinates in the lithium tungstate structure was carried out. The dimensions of the hexagonal cell with 18 molecules and of the rhoml)ohedral cell with 6 mole-cllles are : Hexagonal cell" a = 14.361 +_ 0-003, c=9.602 + 0.002 ~. Rhombohedral cell: a = 8.888 + 0.002, x = 11)7-78 + 0.03 ~. The calculated density is 4.560 g.cm. :~, whereas 14. J. Moon reported to us a directly measured value of 4-535 g.cm.-3. The space grou I) is R3, and all atoms are in general positions. Referred to hexagonal axes (in the following hexagonal axes are used unless otherwise specified) the equivalent coordinates are: (o, o, o)(~, '~, ~) (~, ~, ~)+ + (x, y, z) (-y, x-y, z) (y-x,-.r, z). _ * Work done under the auspices of the Atomic Energy Commission. Determination of the atomic positions A single crystal fragment was ground into a nearly perfect sphere of radius 0-0215 cm. Precise intensities were measured on a General Electric XRD-3 spectrometer modified for single crystal work, using filtered Cu K~-radiation and a proportional counter. Complete data were taken about the [010] axis for the layers K=O, l, 2 and about the [001] axis for the layers L=O, 1, 2, 3. In this manner a total of about 750 distinct reflections were recorded. The final parameter values were deduced with the aid of a least-squares refinement carried out on an IBM 7(.14 machine using the Busing-Levi (1959) program. The atomic coordinates reported for the phenacite structure were used as a starting point and isotropie temperature factors were assumed. Because of the small effect of lithium no attempt was made t() vary the temperature factor for these atoms, an(t a wflue of B=2.5 .&,2 was presupposed. The subsequent results for the temperature factors of tungsten and oxygen indicate that the assumed value for lithium is somewhat too high. The f-curves of Berghuis et al.