Glassy anomalies in the heat capacity of an ordered 2-bromobenzophenone single crystal

The heat capacity $C(T)$ of a 2-bromobenzophenone (2-BrBP) single crystal measured at temperatures from 0.4 to 30 K demonstrates anomalies inherent in disordered solids: Instead of the Debye law ${C}_{\mathrm{D}}\ensuremath{\propto}{T}^{3}, C(T)$ shows a linear temperature dependence and a boson peak, i.e., peculiarities typical of solids with disorder. Computations for a pair of interacting 2-BrBP molecules revealed a few low-frequency states which are expected to actively couple to long-wave phonons. Raman scattering spectra demonstrate two strong spikes at energies close to the boson peak center at $26\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. We relate the Raman spikes to low-energy intramolecular modes, which are the rotational oscillations of the substituted phenyl ring around the C-C link to the ketone. Thus, we have found that a completely ordered crystal made up of molecules with low-energy intramolecular modes can show low-$T$ properties, which are inherent in irregular solids such as various glasses, etc.