Phase equilibria,thermal analysis,and reactivity of tin tungsten bronzes and related phases

Crystal chemistry and phase relations of the bronze forming region of the SnWO system have been investigated. Above 780°C the tin bronzes Sn_xWO₃ are shown to be thermally unstable and an equilibrium diagram is established at 700°C which shows that the composition limits of the tetragonal phase are 0.21 ? x ? 0.29. Below x = 0.21 a series of single and two phase regions containing orthorhombic bronzes exists for which the composition limits have been established. In the range 0.29 ? x ? 0.76 the system comprises the tetragonal bronze, Sn₂W₃O₈ and SnWO₄, while above 0.76 there is no bronze, only Sn₂W₃O₈, SnWO₄ and free Sn. The phase Sn₂W₃O₈ has been isolated and shown to have a hexagonal unit cell, a = 7.696 Å, c = 18.654 Å. The evidence of differential thermal analysis and X-ray studies suggests that this hexagonal phase arises from the decomposition of the tungsten bronze phase and is itself decomposed to cubic SnWO₄ above 700°C. Small thermal effects observed in the DTA scans of tin-containing tetragonal bronzes are interpreted in terms of an order-disorder phenomenon arising from asymmetric tunnel occupancy by Sn²⁺ ions caused by the presence of the lone pair of electrons. Hydrogen reduction of Sn_xWO₃ has been shown to result in complete removal of oxygen, producing Sn + α-W in the range 600–850°C. Some activation energy data are given for the reduction process.