Pressure Effects on the Optical Properties of NdVO4

We report on optical spectroscopic measurements in pure NdVO4 crystals at pressures up to 12 GPa. The influence of pressure on the fundamental absorption band gap and Nd3+ absorption bands has been correlated with structural changes in the crystal. The experiments indicate that a phase transition takes place between 4.7 and 5.4 GPa. We have also determined the pressure dependence of the band-gap and discussed the behavior of the Nd3+ absorption lines under compression. Important changes in the optical properties of NdVO4 occur at the phase transition, which, according to Raman measurements, corresponds to a zircon to monazite phase change. In particular, in these conditions a collapse of the band gap occurs, changing the color of the crystal. The changes are not reversible. The results are analyzed in comparison with those deriving from previous studies on NdVO4 and related vanadates.