CRITICAL BEHAVIOUR OF THE $\nu_{5}$ LO($177 cm^{-1}$) RAMAN MODE OF $NH_{4}Br$

1.H. Yurtseven, J. Struc. 115. 173 1 (1984)Author Institution: Department of Physics, Istanbul Technical UniversityWe have studied here the critical behaviour of the Raman frequencies for the $\nu_{5}LO(177 cm^{-1})$ phonon mode of $NH_{4}Br$. We have analyzed our observed Raman frequencies for this mode which were obtained as a function of temperature near the $\lambda$ point for zero pressure. Our analysis gives the value of $a^{\prime}=-1/2$ for the critical exponent of the Raman frequency shits below the $\lambda$ transition temperature ($T_{\lambda}=235 K. P=0$). This exponent value may be considered as the renormalized Fisher exponent $a^{\prime}_{R}=-a^{\prime}/(T-a^{\prime})$, which gives $a^{\prime}=1/3$ below $T_{\lambda}$. Since a liner variation with temperature between the specific heat and the Raman frequency shifts can be established in the vicinity of ammonium halides below $T^{1}$, our exponent value can describe the critical behaviour of the specific heat in $NH_{4}Br$