Abstract
Polarized Raman scattering from single domain NH4Br has been measured in the frequency region of the librational mode at temperatures near but below the order–disorder phase transition temperature Tλ=235°K. The libronic excitation frequency decreases as Tλ is approached from below and the linewidth exhibits an Arrehenius-type temperature dependence with an activation energy of 1.07 kcal/mole. We analyze the libronic frequency shift by including a contribution to the ammonium ion torsion constant due to the disorder dependent NH+4–NH+4 interaction. Also, the polarized measurements indicate a strong temperature dependent, broad (∼70 cm−1) band at 279 cm−1 and at low temperatures, possibly due to disorder induced, first order Raman scattering at the sites of the disordered ammonium ions and suggest a larger bandwidth for the librational mode dispersion curve than has been measured for the low temperature ordered phase of ND4Cl.
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