Thermodynamics of double oxides. II. Study of the CoO-TiO2 system by the emf method

Role of entropy in the stability of cobalt titanates

“…Similar results were obtained for samples equilibrated in high purity argon gas (CoTiO 3.01 ± 0.02 ) and gas mixture containing CO 2 and CO in the ratio 12:1 (CoTiO 2.98 ± 0.02 ). It was therefore concluded that the compound is essentially stoichiometric, contrary to the large nonstoichiometry (CoTiO 2.85 ) suggested by Golubenko et al [5], and subsequently endorsed by Popov and Levitskii [8]. The essentially stoichiometric character of the compound is consistent with its use as a gate dielectric [3].…”

“…This is clearly illustrated in the computed phase diagram presented in Section 4.4. Figure 2 compares the Gibbs free energy of formation of Co 2 TiO 4 obtained in the present study with values reported in the literature [4,[6][7][8][9]. In this study Gibbs free energy of formation has been measured over a more extended temperature range with greater accuracy.…”

“…Temperature dependence of the corresponding Gibbs free energy of formation reported by different investigators [4,6,8,9] suggests significantly more negative enthalpies of formation. The discrepancy may be caused by the limited range of temperature covered in Gibbs free energy measurements.…”

“…The standard Gibbs free energy of formation of the three crystalline phases from their oxide components at T = (1373 and 1573) K were determined by studying equilibria between pairs of oxide phases, metallic cobalt and a CO + CO 2 gas phase of known composition [6]. By analysis of ternary phase diagrams and tie-lines Navrotsky and Muan [7] obtained values for the Gibbs free energy of formation of CoTiO 3 and Co 2 TiO 4 at T = 1323 K. Popov and Levitskii [8] used the solid-state EMF method with Fe + Fe 0.95 O as the reference electrode to determine the Gibbs free energy of formation of CoTiO 3 and Co 2 TiO 4 in the temperature range (1160 to 1420) K. Using electrochemical reduction, they confirmed that CoTiO 3 is nonstoichiometric as suggested by Golubenko et al [5]. Other investigators [4,6,7] considered CoTiO 3 as a stoichiometric.…”

“…In view of the significant difference in the Gibbs free energies of formation of the cobalt titanates reported in the literature [4][5][6][7][8][9] and the controversy regarding oxygen nonstoichiometry of CoTiO 3 and its consequences for the use of CoTiO 3 as a gate dielectric, new measurements were undertaken. Gibbs free energy of formation of the three titanates were investigated using a solid-state cell over a large temperature range (900 to 1675) K with Co + CoO as the reference electrode so that accurate values of enthalpy and entropy of formation can be derived.…”