Calculation of the ν4 (NH4+) IR Mode Frequency and the Damping Constant (FWHM) close to the Phase Transitions in NH4ZN(HCOO)3 and ND4ZN(DCOO)3

Abstract

Temperature dependence of the IR frequency and the damping constant (FWHM) of the ν 4 (1440 cm -1 ) NH 4 + and ν 4 (1084 cm -1 ) ND 4 + modes, are calculated for the metal formate frameworks (MOFs) of NH 4 Zn(HCOO) 3 and ND 4 Zn(DCOO) 3 , respectively, by using the experimental data from the literature. By assuming the IR frequency of the ν 4 mode as an order parameter, its temperature dependence is calculated close to the phase transition (T C =191 K) in the NH 4 Zn(HCOO) 3 and ND 4 Zn(DCOO) 3 by the molecular field theory. The temperature dependence of the damping constant of the ν 4 (NH 4 + ) IR mode is also calculated by means of the pseudospin-phonon coupled (PS) and the energy fluctuation (EF) models for these metal formate frameworks. The damping constant due to both models (PS and EF) is fitted to the observed FWHM data of the ν 4 (NH 4 + ) IR mode from the literature at various temperatures close to T C in the MOFs studied.Our results show that the molecular field theory is adequate for the temperature dependence of the IR frequency and also the PS model explains the observed behaviour of the FWHM for the ν 4 (NH 4 + ) mode in NH 4 Zn(HCOO) 3 and ND 4 Zn(DCOO) 3 .