A high-pressure optical absorption study and Raman and infrared spectroscopy are carried out on hafnium tungstate (HfW 2 O 8 ) at room temperature. The band gap decreases rapidly with pressure until ∼9 GPa is reached, increases between 9 and 16 GPa, and slowly decreases with pressure from 16 to 47 GPa. The changes under pressure of the vibrational modes have been studied, and an estimate of the thermal expansion coefficient has been calculated, and found to be in reasonable agreement with the

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... asured value. The α-γ phase transition can be observed via Raman and infrared spectroscopy. Around 2 GPa, HfW 2 O 8 undergoes an irreversible transition from the γ -phase to an amorphous structure.