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The structure of the tetragonal crystal was determined. The space group is with cell NaBi(MoO 4 ) 2 I4 6 = S 4 2 dimensions a = 5.267, c = 11.565 and Z = 2. The molybdenum atoms occupy two di †erent sites of symmetry.A 4 6 The Na' and Bi3' cations are situated on two positions with di †erent occupancy factors and are short-range ordered. The IR and Raman polarized spectra are discussed on the basis of the factor group analysis and phonon calculations. The modiÐed Urey-Bradley force Ðeld and potential energy distribution were applied for three molecular models of local disorder. Polarization properties are explained and related to the crystal structure.1997

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