Structural analysis of molten Na₂W₂O₇

The structure of molten Na₂W₂O₇ has been investigated by the radial distribution function based on X-ray scattered intensity data collected at ≈810°C and has been analysed by the correlation method. The coexistence of WO₄ tetrahedra and WO ₆ octahedra in molten Na₂W₂O₇ is considered to be plausible. The mean W-O distance in the molten state is found to be 1.80 Å. Four models, i.e., three crystalline modification models: (1) Na₂W₂O₇; (2) Li₂W ₂O₇; (4) K₂Mo₂O₇; and (3) W₂O₇ tetrahedral model, are examined by the correlation method. The possible model which gives the best agreement with the observed intensity curve of molten Na₂W₂O₇ is concluded to be the Na₂W₂O₇ crystalline model. The structure of the molten state almost preserves that of the crystalline state within the range r < 4.5 Å. The long-range order within one [(W ₂O₇)^2-]_∞ anion chain is suggested to be almost lost and and neighbouring chains may be almost randomly orientated with respect to one another in the molten state.