Infrared reflection spectra of (Rb)1−x(H4)xH2AsO4 mixed crystal

The internal vibrational modes of (Rb) 1−x (NH 4 ) x H 2 AsO 4 mixed crystals have been investigated using infrared spectroscopy techniques. The reflectivities are measured in the frequency region of 200 - 2000 cm −1 between 100 and 300 K. Comparing the observed number of internal vibrational modes with the group analysis predictions, we found that the site symmetry of AsO 4 in NH 4 H 2 AsO 4 is to be S 4 in the paraelectric phase and C 1 in the antiferroelectric phase. It is also found that the site symmetry of AsO 4 in RbH 2 AsO 4 is to be S 4 in the paraelectric phase and C 2 in the ferroelectric phase. For the mixed crystals (x = 0.75, 0.70), the site symmetry of AsO 4 in the low temperature phase is shown to be C 1 in agreement with an earlier predictions that these samples should show antiferroelectricity.