Spectroscopic modifications of Pippard relations: Tricritical phase transition in NH4Cl

We study here the Pippard relations modified spectroscopically for the NH4Cl crystal. By relating the specific heat CP to the frequency shift (1/ν)(∂ν/∂T)P for the disorder-allowed Raman modes of ν7 (93 cm−1) and ν5 (144 cm−1) in NH4Cl close to the tricritical point (P=1.6 kbar, TC=257 K), we show that the first Pippard relation is valid for this crystalline system.