Terahertz time-domain spectroscopy and optical properties of AM2O8 (A=Zr, Hf and M=W, Mo)

Terahertz time domain spectroscopy and optical properties of cubic ZrW2O8 and HfW2O8, and trigonal ZrMo2O8 and HfMo2O8 have been studied for the first time. It is shown that α-ZrW2O8 and α-HfW2O8 exhibit absorption peaks around 1.23 THz (41 cm−1) and 1.18 THz (39.5 cm−1), respectively, whereas no corresponding absorptions are observed for trigonal ZrMo2O8 and HfMo2O8. These external modes are suggested to originate from the librational and/or translational motions of the linked ZrO6/HfO6 and WO4 polyhedra and to contribute to the negative thermal expansion of α-ZrW2O8 and α-HfW2O8. The absorption coefficients and refractive indices of these materials are derived from the transmission amplitudes of the THz field passing through the samples. The results suggest that cubic ZrW2O8 and HfW2O8 have the potential for THz filters or attenuators.