Analysis of vibrational spectra in relation to the thermodynamic quantities close to the phase transitions in ammonium halides (NH4F and NH4I)

Tari O., YURTSEVEN H. H.

OPTIK, vol.135, pp.244-251, 2017 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 135
  • Publication Date: 2017
  • Doi Number: 10.1016/j.ijleo.2017.01.107
  • Journal Name: OPTIK
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.244-251
  • Middle East Technical University Affiliated: Yes

Abstract

The pressure dependences of the vibrational frequencies of the TO and L modes are analyzed from the literature data and the thermodynamic quantities such as isothermal compressibility K gamma, thermal expansion alpha p and the specific heat C-p - C-v are calculated through the mode Grilneisen parameters of those modes in ammonium halides (NH4F and NH4I). For this calculation, the observed frequencies of the TO mode (238 cm(-1) at 1 bar) and the L mode (74.7 meV at 1.9 GPa) are used for the phases of I, II and III in NH4F, whereas for the phases of V and VI in NH4F, the observed frequencies of the 238 cm(-1) are used as a function of pressure at T=293 K. We also analyze the observed frequencies for the TO (17.1 meV) and L (34.1 meV) modes (P=0, T=-25 degrees C) in the phase II of NH4I and the thermodynamic quantities (K-T, alpha(p) and C-p - C-v) are calculated as a function of pressure (T=-25 degrees C) through the mode Gruneisen parameters of the TO and L modes in phase II of NH(4)l.