Phase equilibria in the system NiO-CaO-SiO₂ and gibbs energy of formation of CaNiSi₂O₆

Abstract

Phase relations in the pseudoternary system NiO-CaO-SiO₂ at 1373 K are established. The coexisting phases are identified by X-ray diffraction and energy-dispersive X-ray analysis of equilibrated samples. There is only one quaternary oxide CaNiSi2O6 with clinopyroxene structure. The Gibbs energy of formation of CaNiSi₂O₆ is measured using a solid state galvanic cell incorporating stabilized zirconia as the solid electrolyte in the temperature range of 1000 to 1400 K: Pt,Ni+SiO₂+CaSiO₃+CaNiSi₂O₆|Ni+NiO,Pt From the electromotive force (emf) of the cell, the Gibbs energy of formation of CaNiSi₂O₆ from NiO, SiO₂, and CaSiO₃ is obtained. To derive the Gibbs energy of formation of the quaternary oxide from component binary oxides, the free energy of formation of CaSiO₃ is determined separately using a solid state cell based on single crystal CaF₂ as the electrolyte: Pt,O{in2}+CaF₂|CaF₂|CaSiO₃+SiO₂+CaFa₂,O₂Pt The results can be expressed by the following equations: NiO(r.s)+CaO(r.s)+2SiO₂(qz)→CaNiSi₂O₆(pyr)ΔG°=−115,700+10.63T(±100)Jmol⁻¹CaO(r.s)+SiO₂(qz)→CaSiO₃(wol)ΔG°=−90,030−0.61T(±60)Jmol⁻¹