We analyze here our observed Raman frequencies of the nu(5) (174 cm(-1)) mode in NH4Cl close to its tricritical (P = 1.6 kbar) and the second order (P = 2.8 kbar) phase transitions. The critical behavior of the frequency shifts 1/nu (partial derivative nu/partial derivative T)(P) for this optical mode is described by a power-law with the critical exponent a. Our values of a = 0.01 (T < T-C) and a = 0.11 (T > T-C) for P = 1.6 kbar in NH4Cl are close to the values of 0.066 (=1/16) below T-C and 0.125 (=1/8) above T-C for the specific heat C-P due to a three-dimensional Ising model. Our value of a = 0. 18 (T < T-C) for P = 2.8 kbar in NH4Cl is also close to the Ising value (1/16), whereas our value of a = 0.64 (T > T-C) is far from the Ising value (1/8), which can be reasonably compared with those obtained by previous workers, for the thermal expansivity alpha(P) and the specific heat C-P in this crystalline system. (c) 2005 Elsevier B.V. All rights reserved.