Calculation of the Raman frequencies using volume data close to the tricritical and second order phase transitions in NH4Cl

We calculate here the Raman frequencies of the v(5) TO(174 cm(-1)) and v(2) (1708 cm(-1)) modes as a function of temperature in the region of the tricritical (P = 1.6 kbar) and the second order (P = 2.8 kbar) phase transitions in NH4Cl. This calculation of the Raman frequencies is performed through the mode Gruneisen parameter by using the experimental length-change data obtained at zero pressure where the NH4Cl crystal exhibits a weakly first order phase transition (T-lambda = 242 K). The predicted Raman frequencies of the modes studied here, agree with our observed frequencies for the tricritical (T-c = 257 K) and second order (T-c = 268 K) transitions in NH4Cl. This indicates that the Raman frequencies can be predicted from the measurements of the crystal volume close to phase transitions by the method given here. (C) 2009 Elsevier B.V. All rights reserved.