Raman Scattering Study of the Disorder—Order Phase Transition in NH4Cl

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Abstract
The Raman scattering of the 93 cm−1 anomalous mode in NH4Cl above and below the λ‐transition temperature (Tλ) was studied in detail. The peak and integrated intensities of this mode decrease rapidly to zero below Tλ. In contrast to the temperature behavior of a similar mode in NH4Br, the intensity data does not follow the anomalous specific heat accompanying the λ transition, suggesting that below Tλ the short‐range order decays much more rapidly in NH4Cl than in NH4Br. A theory which considers the first order disordering effect (neglecting short‐range order) on the Raman scattering was worked out. The result predicts that the temperature reduced integrated intensity has the same temperature dependence as Hc (the additional configurational enthalpy due to the λ transition) provided that the short‐range order can be neglected. The result of experimental tests of the theory indicates that while for temperatures below 230°K the agreement is excellent, significant deviations occur between 230 and 250°K, therefore indicating that short‐range order must also be included in the quantitative interpretation of the Raman scattering from the anomalous mode.