Analysis of the Raman frequencies of a lattice mode in NH 4Cl close to the tricritical and second order phase transitions

Abstract

We analyze here our observed Raman frequencies of the ν 5 (174 cm −1) mode in NH 4Cl 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 < T C) and a = 0.11 ( T > T C) for P = 1.6 kbar in NH 4Cl 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 NH 4Cl 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 α P and the specific heat C P in this crystalline system.