Low-temperature specific heat of orientational glasses

This review summarizes specific heat data measured at low temperatures (T<1 K) on orientational glasses. Three species of mixed molecular crystals exhibiting orientational disorder are considered, namely (KBr)_1–x(KCN)_x, (NaCN)_1–x(KCN)_x (Rb)_1–x(NH₄)_x H₂PO₄. For intermediate concentrations of the anisotropic components, glass-like excitations have been observed. It is demonstrated that with respect to thermal properties, orientational disorder leads to the same universal behaviours than for structural disorder, i.e. a specific heat which varies below 1 K and for times 10^–4 s–10 s as:C_p(T,t)T¹×ln(t). The variation of the glass-like anomaly with compositional disorder is also discussed. It is concluded that the low-energy excitations are related to orientational degrees of freedom which are frozen at low temperatures but still perform reorientations via quantum tunneling motions.