Mode-Grüneisen parameters in disordered molecular crystals: Raman spectra of NH4Br and NH4I under hydrostatic pressure

The Raman spectrum of NH₄I has been measured under hydrostatic pressure. The high pressure phase of NH₄I was identified with the disordered CsCl-type phase by comparing the spectrum with that of NH₄ Br. The mode-Grünesen parameters of NH₄Br and NH₄I were determined from the frequency shift of phonon bands under hydrostatic pressure. These values were compared with the thermodynamic value and that obtained from the Slater relation. Contributions to the thermodynamic Grüneisen constant are discussed from free energy terms of NH⁺₄-orientation and -interactions with lattice modes.