Pressure effects in the Raman spectrum of WO3 microcrystals

The Raman scattering of mu c-WO3 was investigated for pressures ranging from atmospheric to 4 GPa. The material, originally a mixture of two crystalline structures with groups, P2(1)/n (monoclinic). and P (1) over bar (triclinic), transforms gradually to a new variety belonging to the group P2(1)/c (monoclinic). This gradual process starts at a very low pressure and ends up at a pressure P-t=1.4 GPa. This is the only structural transformation observed to occur for pressures up to P=4.0 GPa. Pressure coefficients are given for WO3 in the phases studied. Analysis of the data clearly demonstrates the existence of a unique phase (epsilon) for mu c-WO3 at high pressures. As the epsilon-phase Raman features were found to subsist on releasing the pressure, we believe the mu c-WO3 retain the corresponding structure. This structure relaxes partially to the triclinic form one day after releasing the pressure to P=0. No indication of size effects was found in this experiment.