We establish here a linear variation of the specific heat C-p with the frequency shifts (l/v)(partial derivative v/partial derivative T)(p) close to the X-phase transition in NH4Br (T-lambda=234 K). This linear relationship is based on our spectroscopically modified Pippard relations. For this verification of our relations, we use our observed Raman frequencies of the lattice mode of v(7) (56 cm(-1)) and the internal mode of v(2) (1684 cm(-1)) in the NH4Br crystalline system. Our study given here indicates that the thermodynamic data can be obtained from the measured frequencies. (C) 2007 Elsevier B.V. All rights reserved.