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

Abstract

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.