Raman frequency shifts of an internal mode near the tricritical and second order phase transitions in NH4Cl

This study gives our analysis for the frequency shifts of the v2 (1708 cm−1) Raman mode in NH4Cl close to its tricritical (P = 1.6 kbar) and second order (P = 2.8 kbar) phase transitions. From our analysis, we extract the values of the critical exponent which describes the critical behavior of the Raman frequency shifts for this internal mode for the pressure conditions studied in NH4Cl. Our exponent value of α ≈ 0.2 for the tricritical phase transition is close to the values of 1/16 (T < Tc) and 1/8 (T > Tc) for the specific heat, predicted from a 3D Ising model. Our exponent values for the second order phase transition (P = 2.8 kbar) for T < Tc and T > Tc are comparable with those reported in earlier studies.