HIGH TEMPERATURE X-RAY DIFFRACTION STUDIES ON HYDROGEN TUNGSTEN BRONZE

B.S. Acharya, Rajeev and L.D. Pradhan

Regional Research Laboratory Bhubaneswar-751013, India
E-mail : root@csrrlbhu.ren.nic.in

Keywords : Phase transitions, Phase Analysis, X-ray diffraction, Hydrotungstite, Reactian kinetics, Power law.

Hydrogen tungsten bronzes have been the subject of investigations in recent years because of their electrochromic properties and mixed valence behaviour. Detailed structural information are not available on crystalline bronzes also. Hydrotungstite or H₂WO₄ powder which is easily available has been taken up as a candidate material for in-situ high temperature diffraction study. H₂WO₄ which crystallises in monoclinic structure changes to cubic when heated at 210^oC and transforms into H_0.5WO₃ phase. The growth of this phase at 210^oC with time takes place through two stage; first a linear rise and then a slow rise showing a tendency for saturation. A semilog plot of intensity of (020} peak has been found to be nonlinear and parabolic in nature. It has been argued that for a bulk samples if the reaction is controlled by processes occuring near the interface, the reaction will show linear growth rate and if controlled by interdiffusion it will show parabolic growth. However, in both the cases the driving forces are highly complex in nature. Isothermal decay of H₂WO₄ phase at 190^oC and growth of H_0.5WO₃ phase at 210^oC have indicated the reaction to follow a power law type growth and decay respectively. Further these have been found to follow Johnson-Mehl-Avrami power law. The growth of H_0.5WO₃ and transformation of H₂WO₄ to WO₃ via H_0.5WO₃ phase has been discussed in the light of interdiffusion and other complex processes taking place in this system.