Glassy behavior of crystalline solids at low temperatures

Measurements of low-temperature specific heat $C$, thermal conductivity $\kappa$, and dielectric dispersion have been made on crystalline samples of the superionic conductor ${\mathrm{Li}}_3$N, the oxygen conductor ${\mathrm{Y}}_2$ ${\mathrm{O}}_3$-stabilized Zr${\mathrm{O}}_2$, and the relaxation ferroelectric ${\mathrm{Pb}}_3$ (Mg${\mathrm{Nb}}_2$)${\mathrm{O}}_9$. In each case, the behavior is similar to that found in amorphous materials. Data on other crystalline materials are reviewed to emphasize that the low-temperature phenomena found in glasses can occur in crystals. It is shown that a peak in $\frac{C}{T^3}$ appears to be correlated, in both amorphous and crystalline materials, with a nearly temperature-independent regime (plateau) in $\kappa$.