title: Temperature dependence of the damping constant and the order parameter close to the lambda phase transitions in ammonium halides.
abstract: This study gives our calculation for the damping constant, using the expressions derived for an Ising pseudospin-phonon coupled system in the ammonium halides (NH(4)Cl and NH(4)Br). For this calculation of the damping constant, we use the temperature dependence of the order parameter calculated from the molecular field theory. We predict here the damping constants for the v(5) (174 cm(-1)) and v(5) (177 cm(-1)) Raman modes of NH(4)Cl and NH(4)Br, respectively, below T(lambda) and compare them with our observed bandwidths measured as a function of temperature for those phonon modes at zero pressure. Our predictions agree well with the observed bandwidths below T(lambda) for those modes in both ammonium halide structures, in particular for NH(4)Br. Some discrepancy that occurs below T(lambda) for the v(5) (174 cm(-1)) mode of NH(4)Cl, is explained in terms of the model studied here.
authors.affiliation: Department of Physics, Middle East Technical University, 06531 Ankara, Turkey.
authors.forename: H
authors.initials: H
authors.lastname: Yurtseven
authors.forename: H
authors.initials: H
authors.lastname: Karacali
mesh_terms: D000465:Algorithms; D000643:Ammonium Chloride / Q000737:chemistry*; D064751:Ammonium Compounds; D001965:Bromides / Q000737:chemistry*; D005459:Fluorides; D008433:Mathematics; D008956:Models, Chemical; D044367:Phase Transition; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D013696:Temperature
chemical_list: D064751:Ammonium Compounds; D001965:Bromides; D000644:Quaternary Ammonium Compounds; D000643:Ammonium Chloride; C024822:ammonium fluoride; D005459:Fluorides; C051470:ammonium bromide