Low-energy excitations in yttria-stabilized zirconia

Measurements of the specific heat, thermal conductivity, dielectric constant and thermal expansion of the single-crystal fast-ion conductor ${\left(\mathrm{Zr}{\mathrm{O}}_2\right)}_{1-x}{\left({\mathrm{Y}}_2{\mathrm{O}}_3\right)}_x$ with $x=0.10,0.16, \mathrm{and} 0.18$ exhibit behavior similar to that observed in amorphous solids and attributed to two-level states. The data can be fitted by the phenomenological tunneling states model, but at the expense of a more complex spectrum of two-level-state equilibration times than has generally been used for amorphous solids. The possible relationship of the two-level states with known defects is explored and found to be unsatisfactory.