Analysis of the frequency shifts close to the second order phase transition in NH4Cl

In this study we analyzed observed Raman data for the frequencies of the di sorder-induced modes of v(7) (93 cm(-1)) and vs (144 cm(-1)) in the second order phase region of NH4Cl (P = 2.8 kbar). By means of a power-law formula describing the critical behaviour of the frequency shifts 1/v(partial deri vativev/partial derivativeT)p for those phonon modes studied, we extract th e critical exponent value of a = 0.25, which can also describe the critical behaviour of the specific hear Cp close to the second order phase transiti on in NH4Cl. Within this content, our spectroscopically modified Pippard re lations are introduced here.