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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 C-P to the frequency shift (1/v)(partial derivative v/partial derivative T)(P) for the disorder-allowed Raman modes of v(7) (93 cm(-1)) and v(5) (144 cm(-1)) in NH4Cl close to the tricritical point (P = 1.6 kbar, T-C = 257 K), we show that the first Pippard relation is valid for this crystalline system. (c) 2006 Elsevier Ltd. All rights reserved.