Low temperature, glass-like properties of crystalline beta-alumina

The low temperature properties measured for single crystal Na, Ag, K, and Li B-alumina are characteristic of those found in amorphous materials. The specific heat is dominated below 1 K by a term approximately linear in temperature instead of the cubic dependence normal for crystals. All except K B-alumina have an approximately quadratically temperature dependent thermal conductivity below 1 K and a nearly temperature independent thermal conductivity near 10 K. Both features are associated with glassy materials, while pure cryst~lline dielectrics have thermal conductivities cubic in temperature. The dielectric susceptibilities of the B-aluminas vary a few parts in 102 over three orders of magnitude in temperature, with the dependences again of the same form as those found in glasses. Unlike glasses, however, the variations of susceptibility for all but Li J3-alumina are found to be extremely anisotropic. The magnitudes of the measured properties are dependent on the species of metal ion present. Beta-alumina is a layered crystal composed of slabs of alumina separated by non-stoichiometrically occupied planes containing the monovalent cations in a disordered environment. The glass-like properties measured indicate that there is present in the Beta-aluminas a broad band of low energy excitations associated with the cations in the planes. The tunneling-states model, which explains glassy properties with excitations arising from quantum mechanical tunneling of units in double-well potentials, was used to analyze the data. An energy dependent density of tunneling states with a spectrum of relaxation times for each tunneling-state energy was found to be necessary to explain the properties of glasses and the B-aluminas.U of I OnlyThesi