Study of phase transitions and of the possible existence of a fifth phase in NH4Br by Raman scattering under high pressure

Abstract

We have investigated phases II, III, IV, and the previously suggested phase V in NH₄Br under high pressure by Raman scattering experiments. The pressure and temperature dependences of band intensities and frequencies at the transitions are discussed. The possible existence of a fifth phase is examined by comparison with calculated Raman spectra for different types of ordering. These calculations show that the spectra in the range of phase V reported in the present and in all former Raman investigations are due to the same type of ordering as in phase IV. As there is no indication of a transition from phase IV, we conclude that phase IV is present where phase V was suggested. The same conclusion can be inferred from all other high pressure Raman scattering studies so far reported. The fifth phase, originally observed by ultrasonic measurements, might have been a property of the particular sample.