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RAMAN-STUDY OF AMMONIUM HALIDES TREATED AS AN ISING PSEUDOSPIN-PHONON COUPLED SYSTEM

Raman spectroscopy has been used to study the λ-transition regions of NH4Cl and NH4Br. An Ising pseudospin-phonon coupled model has been fitted to the observed Raman data. In particular we report here the results of fitting the model to the frequencies and damping constants of the non-soft TO component of the lattice modes of NH4Cl and NH4Br. Both crystals have the orientations of their NH+4 ions disordered at temperatures above their λ transitions, and have (orientation-ignored) Oh structures (β) containing one formula unit per unit cell. At temperature just below the λ transitions, NH4Cl shows a ferro-ordered Td structure (gS) with one formula unit per unit cell whereas NH4Br shows an antiferro-ordered D4h structure (γ) with two formula units per unit cell. The disorder at high temperature allows all modes to appear in the Raman spectrum, but the spectra are dominated by Γ and M point phonons. The bands studied here are due to the TO component of the Flu β-phase lattice mode. For NH4Cl the Γ point phonons are studied, whereas for NH4Br M point phonons of the β-phase which become Γ point phonons of the γ phase are used. Close fitting of the model to the experimental results is obtained for the ordered (γ and Σ) phases and meaningful critical exponents are obtained.