Knowledge Bank

We have studied here the critical behaviour of the Raman frequencies for the $\nu 5LO(177cm-1)$ phonon mode of $NH4Br$. 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 ($T\lambda =235K.P=0$). This exponent value may be considered as the renormalized Fisher exponent $aR\prime =-a\prime /(T-a\prime )$, 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 $T1$, our exponent value can describe the critical behaviour of the specific heat in $NH4Br$.