Analysis of the Raman frequencies of a lattice mode in NH4Cl close to the tricritical and second order phase transitions

We analyze here our observed Raman frequencies of the ν5 (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/ν (∂ν/∂T)P for this optical mode is described by a power-law with the critical exponent a. Our values of a = 0.01 (T < TC) and a = 0.11 (T > TC) for P = 1.6 kbar in NH4Cl are close to the values of 0.066 (=1/16) below TC and 0.125 (=1/8) above TC for the specific heat CP due to a three-dimensional Ising model. Our value of a = 0.18 (T < TC) for P = 2.8 kbar in NH4Cl is also close to the Ising value (1/16), whereas our value of a = 0.64 (T > TC) is far from the Ising value (1/8), which can be reasonably compared with those obtained by previous workers, for the thermal expansivity αP and the specific heat CP in this crystalline system.