STRUCTURE AND VIBRATIONAL DYNAMICS OF TETRAGONAL NABI(WO4)(2) SCHEELITE CRYSTAL

The structure of tetragonal NaBi(WO4)(2) crystal is determined. The sp ace group is <I(4)over bar> = S-4(2) with cell dimensions a = 5.275, c = 11.493 Angstrom and two molecules per unit cell. The tungsten atoms occupy two different sites of 4 symmetry. Also Bi and Na atoms statis tically occupy two positions with different occupancy factors. The IR and polarized Raman spectra are discussed on the basis of factor group analysis and normal coordinate calculations. Urey-Bradley force field and potential energy distribution were used in description of the nor mal modes. The polarization behavior of vibrational spectra is explain ed and related to the crystal structure.