THE TEMPERATURE DEPENDENCE OF THE RAMAN BANDWIDTHS FOR THE HARD MODES OF THE AMMONIUM HALIDES CLOSE TO PHASE TRANSITIONS

Author Institution: Department of Physics, Middle East Technical UniversityThis study gives the temperature dependence of the Raman bandwidths for the $\nu_{5} (174 cm^{-1})$ mode of $NH_{4}Cl$ and for the $\nu_{5} (177 cm^{-1})$ mode of $NH_{4}Br$. We analyze our observed Raman bandwidths of those phonon modes according to the soft-hard mode coupled model close to the phase transitions in these crystals. Our analyses give that the values of the critical exponent for the Raman bandwidths are $\beta = 0.13$ for the $\nu_{5} (174 cm^{-1})$ mode of $NH_{4}Cl (T_{c} = 241.3K)$ and for the $\nu_{5} = (177 cm^{-1})$ mode of $NH_{4}Br (T_{c} = 234 K)$ in the first order phase region of these crystalline systems