High-pressure and high-temperature X-ray diffraction studies of scheelite BaWO4

Abstract

We carried out high-pressure (HP) and high-temperature (HT) in situ ADXRD synchrotron measurements in barium tungstate (BaWO4) up to 7.5 GPa and 800 K. Coexistence of the scheelite and fergusonite structures was found beyond 7 GPa, both at room temperature and HT, suggesting a polymorphism zone in the P–T phase diagram. The experiments are complemented by thermodynamic calculations within the quasi-harmonic approximation. At ambient pressure, a volume thermal expansivity of 9.5×10−6 K−1 was obtained for scheelite BaWO4. At HP, the thermal expansivity of the fergusonite doubles that of scheelite. Theoretical equation of state curves at HP and HT are also presented.