Thermodynamic properties of strontium titanates: Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10, and SrTiO3

The chemical potentials of SrO in two-phase fields (TiO2 + SrTiO3), (SrTiO3 + Sr4Ti3O10), (Sr4Ti3O10 + Sr3Ti2O7), and (Sr3Ti2O7 + Sr2TiO4) of the pseudo-binary system (SrO + TiO2) have been measured in the temperature range (900 to 1250) K, relative to pure SrO as the reference state, using solid-state galvanic cells incorporating single crystal SrF2 as the electrolyte. The cells were operated under pure oxygen at ambient pressure. The standard Gibbs free energies of formation of strontium titanates, SrTiO3, Sr4Ti3O10, Sr3Ti2O7, and Sr2TiO4, from their component binary oxides were derived from the reversible electromotive force (EMF) of the cells. For the formation of the four compounds from their component oxides TiO2 with rutile structure and SrO, the standard Gibbs free energy changes are given by:ΔG(ox)∘(SrTiO3)±89/(J·mol-1)=-121878+3.881(T/K),ΔG(ox)∘(Sr4Ti3O10)±284/(J·mol-1)=-409197+14.749(T/K),ΔG(ox)∘(Sr3Ti2O7)±190/(J·mol-1)=-285827+10.022(T/K),ΔG(ox)∘(Sr2TiO4)±110/(J·mol-1)=-159385+3.770(T/K).The reference state for solid TiO2 is the rutile form. The results of this study are in good agreement with Gibbs free energy of formation data reported in the literature for SrTiO3, but differ significantly with data for Sr4Ti3O10. For Sr3Ti2O7 and Sr2TiO4 experimental measurements are not available in the literature for direct comparison with the results obtained in this study.