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 α-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 ∼62 K, where its Cp, the expansivity α, and the T derivative of the mean-square atomic displacement decrease in a manner similar to that on the dynamic freezing of the α 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 Cp and α. When the vacancy diffusion does not occur during the measurement period, the anharmonic contribution vanishes and Cp and α decrease.