Raman spectrum of NH 4 ReO 4 at high pressure

The pressure dependences at room temperature of the translational lattice modes and of the ν 2 and ν 4 internal bending modes of the ReO   4 −ion in solid NH 4 ReO 4 are reported. There is no evidence of any phase transition up to a pressure of 15 kilobar. The pressure coefficients of the translational modes are 2.0 cm −1 GPa −1 and 3.0 cm −1 GPa −1 for the B g and E g modes respectively. The pressure coefficients of the bending modes are 1.0 cm −1 GPa −1 and 5.4 cm −1 GPa −1 for the B g and A g components of ν 2 , and 4.1 cm −1 for the overlapped B g and E g components of ν 4 . The symmetry assignments of the low frequency lines in the spectrum were checked by means of polarized Raman spectra of a single crystal. The previous assignments are correct.