The elastic properties of lithium oxide and their variation with temperature

The energies of the acoustic phonon modes in a single crystal of 7Li2O have been measured in the temperature range 293-1603 K using the technique of inelastic neutron scattering. The slopes of the phonon energy dispersion curves as they approach the Brillouin zone centre give values for the cubic elastic stiffness constants, Cij. C11 is found to undergo a sharp decrease above ≅ 1350 K similar to that observed in structurally related compounds, such as CaF2 and SrCl2, as they undergo a transition to a fast-ion phase. The Reuss and Voigt averaging methods have been used to calculate the temperature dependence of the adiabatic Young's modulus, shear modulus, bulk modulus and Poisson's ratio of polycrystalline Li2O. Estimates of the corresponding isothermal values are obtained using an expression for the linear thermal expansion coefficient of Li2O obtained in this work, together with thermodynamic data available from specific heat measurements. These results represent the first experimental data describing the elastic properties of Li2O at elevated temperatures and are important in predicting the behaviour of this material in its potential role as a tritium breeding blanket material for future fusion reactors. (orig.)