Phase transition and freezing of ionic disorder in CsNO 2 and TINO 2 crystals†

Abstract

The heat capacities of CsNO 2 and TlNO 2 have been measured in the temperature region between 13 and 350 K. The phase transitions of CsNO 2 and TlNO 2 were found at (209.16 ± 0.10)K and (282.4 ± 0.1)K. The enthalpy and entropy of the phase transition were (3.45 ± 0.20) kJ mol −1 and (17.2 ± 1.0) JK −1 mol −1 for the former, and (6.44 ± 0.31) kJ mol −1 and (23.8 ± 1.1) JK −1 mol −1 for the latter. The glass transitions were found around 42 K in CsNO 2 and around 60 K for TlNO 2, respectively. The corresponding dielectric relaxations were observed between 58 and 130 K for CsNO 2 in the frequency range between 10 2 and 10 5 Hz and between 80 and 180 K for TlNO 2 in the frequency range between 2 × 10 2 and 10 5 Hz. The calorimetric and dielectric relaxation times yielded a straight line in the Arrhenius plot over a wide time scale ranging from 10 −6 to 10 5 sec. The slope gave the activation enthalpy of 13.8 kJ mol −1 and 19.5 kJ mol −1 for CsNO 2 and TlNO 2, respectively. The transition entropy supplemented by a residual entropy R ln 3 for CsNO 2 and R ln 2 for TlNO 2 gave (26.3 ± 1.0) JK −1 mol −1 and (29.6 ± 1.1) JK −1 mol −1 for the orientational entropy of the NO 2 − ion in the high-temperature phase. Based on the packing and symmetry considerations, these entropies were interpreted by the model which included two different sets of orientations of the NO 2 − ions parallel to [110] and [111] in the CsCl type unit cell. The existence of the different sets of orientation was proved by the doublet (Δ v ~ 10 cm −1) of the Raman spectrum of the bending mode of the NO 2 − ion in the cubic phase of the CsNO 2 crystal. The band narrowed to an ordinary singlet with increasing temperature. This observation was accounted for as the motional narrowing in which the NO 2 − ion felt an averaged field of the two different sets owing to the increased rate of jumping as the temperature increased.