Anharmonicity-induced glasslike transition in a plastic crystal without α relaxation

In the low-temperature crystal phase of TlNO2 studied by dielectric spectroscopy (10 mHz to 3 GHz), the alpha-relaxation process does not evolve, and the molecular dynamics is controlled only by one relaxation process with Arrhenius energy of 19.8 kJ mol(-1). Its characteristic time is comparable to the measurement time scale at similar to 62 K, where its C-p, the expansivity alpha, and the T derivative of the mean-square atomic displacement decrease in a manner similar to that on the dynamic freezing of the alpha process. However, neutron powder diffraction measurements show no evidence-for two-site occupancy or orientational disordering [Phys. Rev. B 57, 11 125 (1998)]. We propose a nonconfigurational mechanism for this dynamics, namely, that when a vacancy appears at a Tl site, the anharmonicity of librations of the NO2- ion increases, which changes the mean dipole vector and raises C-p and alpha. When the vacancy diffusion does not occur during the measurement period me anharmonic contribution vanishes and C-p and alpha decrease.