Specific Heat at Low Temperatures in Quasiplanar Molecular Crystals. Origin of Glassy Anomalies in Minimally Disordered Crystals

We present low-temperature specific heat (𝐶p) measurements of a monoclinic 𝑃⁢21/𝑐 crystal formed by quasiplanar molecules of tetrachloro-𝑚-xylene. The dynamic disorder frozen at low temperature of the asymmetric unit (formed by a half molecule) consists of reorientation around a threefold-like axis perpendicular to the benzene ring. Such a minimal disorder gives rise to typical glassy anomalies, as a linear contribution in 𝐶p ascribed to two-level systems and a broad maximum around 6.6 K in 𝐶p/𝑇3 (the boson peak). We discuss these results in the framework of other quasiplanar molecular crystals with different accountable numbers of in-plane molecular orientations. We find that the density of two-level systems does not correlate with the degree of orientational disorder. Rather, it is the molecular asymmetry that seems to play a relevant role in the thermal anomalies. Furthermore, we discuss the suggested correlation between the boson peak (𝑇BP) and Debye (Θ𝐷) temperatures. We find that a linear correlation between 𝑇BP and Θ𝐷 holds for many—but not all—structural glasses and strikingly holds even better for some disordered crystals, including our studied quasiplanar molecular crystals.