Low-temperature heat capacity of some alkali metal tungstates

Heat capacity measurements have been made on the two triclinic tungstates, Li_0.2WO₃ and Na_0.33WO₃ from 1 to 60 K. In addition to the normal Debye term the data show a large contribution which can be fit to a single Einstein mode associated with the oscillation of the alkali ions in the holes formed by the corner bonding of six WO₆ octahedra. The Einstein characteristic temperatures obtained are 71 ± 2 and 78 ± 2 K for Li_0.2WO₃ and Na_0.33WO₃, respectively. The results are compared with those reported earlier for the hexagonal tungsten bronzes.