Critical behaviour of the specific heat calculated using the Raman frequencies of the lattice and internal modes near the lambda-phase transition in NH4Br

Sen S., Yurtseven H.

JOURNAL OF MOLECULAR STRUCTURE, vol.834, pp.561-566, 2007 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 834
  • Publication Date: 2007
  • Doi Number: 10.1016/j.molstruc.2006.10.063
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.561-566
  • Middle East Technical University Affiliated: Yes


We calculate here the specific heat of NH4Br using our Raman frequency shifts for the lattice mode Of nu(7) TA (56 cm(-1)) and the internal mode Of nu(2) (1684 cm(-1)) near its lambda-phase transition (T-lambda = 234 K, P = 0). By analyzing our Raman frequency shifts, values of alpha = 0. 19 (T < T-lambda, and T > T-lambda) for the lattice mode, and alpha = 0.45 (T < T-lambda) and alpha = 0.57 (T > T-lambda) for the internal mode, are used as the values of the critical exponent for the specific heat to predict its critical behaviour based on an Ising model near the lambda-phase transition in NH4Br. The specific heat which we calculate here using our frequencies of both Raman modes, agrees well with the observed specific heat Cp near the lambda-phase transition in NH4Br. Some discrepancy occurring in the specific heat above T-lambda, due to the lattice mode is explained within the framework of an Ising pseudospin-phonon coupled model for NH4Br. (c) 2007 Elsevier B.V. All rights reserved.