ZrW2O8 and HfW2O8: band gap shifts under pressure

High-pressure optical absorption measurements are carried out on zirconium tungstate and hafnium tungstate (ZrW2O8 and HfW2O8) up to 59 and 47 GPa, respectively, at room temperature. We observe a striking color changes in both tungstates with the application of pressure. Initially, yellowish ZrW2O8 becomes red around 28 GPa and opaque above 55 GPa and the initially transparent HfW2O8 becomes red at ∼38 GPa. Using both a linear extrapolation of the transmission and the Tauc model, the high-energy region of the absorption edge is fit to determine the pressure dependencies of the band gaps. Based on a linear extrapolation of the data, we estimate the metallization pressures of ZrW2O8 and HfW2O8 to be approximately 165 and 176 GPa, respectively.