Phase relations, dopant effects, structure, and high electrical conductivity in the Na2WO4---Na2MoO4 system

Abstract

The x, T-phase diagram of the binary system Na2WO4---Na2MoO4 has been redetermined at ambient pressure, taking into account the influence of hysteresis effects. Thermodynamic calculations, based upon transition entropies as determined by precision DSC (differential scanning calorimetry), indicate that the system is almost ideal with respect to the high-temperature phases.

As anion dopes, Na2SO4 and Na2CrO4 give a metastable extension of the β-phase of Na2WO4 at decreasing temperature, involving some 40°C at 0.01 mole fraction of dopant. Cation dopes like Li2WO4 and K2WO4 behave quite differently.

The electrical conductivity through the phase diagram is high in the α-phase (σ 10−2 mho cm−1) almost regardless of composition. The anomalous high conductivity of the β-phase decreases with increasing molybdate content. In pure Na2MoO4 an anomaly occurs at the α-α2 transition, resembling the behavior of Na2WO4 at the β-α transition. The (highest) α2-phase is hexagonal, (P63/mmc, showing large anisotropic thermal vibrations. The α-phase is orthorhombic (Fddd) as is the β-phase (probably Pbn21).