It is known that high ionic conductivity in fluorite-structured materials is associated with thermally created Frenkel-type disorder at temperatures well-below the melting point. Using crystallineLi2Oas a model system, we studied the mechanism of formation of Frenkel pairs at temperatures close to the transition to the fast-ionic phase, and ionic motion at temperatures above the transition point. The dynamical behaviour of lithium oxide has been studied using both lattice dynamics within the quasi-harmonic (QH) approximation and molecular dynamics (MD).