H. Yurtseven et Wf. Sherman, SPECTROSCOPIC MODIFICATIONS OF PIPPARD RELATIONS FOR AMMONIUM HALIDES, Journal of molecular structure, 435(2), 1997, pp. 143-150
Spectroscopic modifications of the Pippard relations have been establi
shed for ammonium halides near their X-phase transitions. A linear rel
ationship has been obtained between the specific heat and the frequenc
y shifts using our observed Raman frequencies of the nu(5)TO(134 cm(-1
)) and nu(5)LO(177 cm(-1)) lattice modes of NH4Br near its first-order
phase transition (P = 0). Similar linear relationships have also been
obtained using our observed Raman frequencies of the nu(5)TO(174 cm(-
1)) lattice mode and the nu(2)(1708 cm(-1)) internal mode of NH4Cl nea
r the tricritical (P similar or equal to 1.6 kbar) and the second-orde
r (P similar or equal to 2.8 kbar) phase transitions in this crystal s
ystem. Our values of the slope (dP/dT)(lambda) deduced from the spectr
oscopic modifications of the Pippard relations are comparable with the
literature value for the NH4Br crystal near its first-order phase tra
nsition. We also give in this study our (dP/dT)(lambda) values which a
re the first reported in the literature for the tricritical and the se
cond-order phase transitions in NH4Cl. (C) 1997 Elsevier Science B.V.