Kenzo Kitayama

Thermodynamic properties of Fe-lathanoid-O compounds at high temperatures

Abstract After establishing the phase equilibria in the systems FeFe2O3Ho2O3 and FeFe2O3Tm2O3 at 1150, 1200, and 1250°C, the standard Gibbs free energies of formation of HoFe2O4, Ho3Fe5O12, HoFeO3, TmFe2O4, Tm3Fe5O12, TmFeO3 have been determined on the basis of reactions among metallic iron, oxygen, and the respective lanthanoid sesquioxide. A new compound Tm2Fe3O7 was found at 1380°C. The standard Gibbs free energies of formation of Gd3Fe5O12, Tb3Fe5O12, and Dy3Fe5O12 were also determined. On the basis of these new data together with our previous studies on phase equilibria in the FeFe2O3lanthanoid sesquioxide systems, we have been able to establish a phase diagram at high temperatures from 900 to 1400°C. The thermochemical relative stability in each type of compound, that is to say, LnFeO3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), Ln3Fe5O12 (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), LnFe2O4 (Ln = Ho, Er, Tm, Yb, Lu, and Y), and Ln2Fe3O7 (Ln = Yb and Lu) was investigated by using the following standard Gibbs free energy changes of oxide-oxide reactions, 1 2 Fe 2 O 3 + 1 2 Ln 2 O 3 = Ln FeO 3 , 3Ln FeO 3 + Fe 2 O 3 = Ln 3 Fe 5 O 12 , FeO + Ln FeO 3 = Ln Fe 2 O 4 , and LnFe2O4 + LnFeO3 = Ln2Fe3O7. The results thus obtaine were compared with those of the structural relative stability.

Phase equilibria in the CeO and CeFeO systems

Phase equilibria in the Ce-O system at 1000, 1100, 1153, 1200, 1249, 1310, and 1330°C and those in the Ce-Fe-O system at 1000, 1100, and 1200°C were established. The former system has CeO2 with a fluorite type, Ce3O5 with a C-type rare-earth oxide, and Ce2O3 with a A-type rare-earth oxide. CeO2 and Ce3O5 have nonstoichiometric compositions, but the stoichiometry of Ce2O3 was confirmed at 1200, 1249, and 1310°C. In the latter system, only the one ternary compound CeFeO3 was confirmed under present experimental conditions. On the basis of the established phase diagrams, the standard Gibbs energies of the reactions: (1) 3/2 Ce2O3 + 1/4 O2 = Ce3O5, (2) 1/3 Ce3O5 + 1/6 O2 = CeO2, (3) 1/3 Fe3O4 + CeO2 = CeFeO3 + 1/6 O2, and (4) Fe + CeO2 + 1/2 O2 = CeFeO3, were determined at various temperatures. Also relationships between ΔG° and the temperatures of the same reactions were determined within the experimental temperature range.

Thermogravimetric study of the Ln2O3CoCo2O3 system: I. Ln = La

Abstract Phase equilibria in the system LaCoO at 1200°C are established by changing the oxygen partial pressure from 0 to 12.50 in − log ( P O 2 atm ) and are represented in the La2O3CoCo2O3 system. Under the present experimental conditions, La2O3, CoO, Co, LaCoO3, La2CoO4, and La4Co3O10 phases are stable. Co3O4 and Co2O3 are not found. Nonstoichiometry of the compounds is discussed with respect to the oxygen partial pressure, and activities of the components in the solid solutions are obtained by using the composition-oxygen partial pressure relations. The standard Gibbs energies of six reactions appearing in the phase diagram are calculated and compared with previous values.

Thermodynamic study of the Ln2O3CoCo2O3 system II. Ln = Nd and Gd

Phase equilibria in the system Ln/sub 2/O/sub 3/-Co-Co/sub 2/O/sub 3/ (Ln = Nd and Gd) at 1200/degrees/C were studied at oxygen partial pressures ranging from 1 to 10/sup /minus/12/ atm O/sub 2/. NdCoO/sub 3/ and Nd/sub 4/Co/sub 3/O/sub 10/ are stable ternary compounds in the Nd/sub 2/O/sub 3/-Co-Co/sub 2/O/sub 3/ system, while only GdCoO/sub 3/ is stable in the Gd/sub 2/O/sub 3/-Co-Co/sub 23/ system. Nd/sub 4/Co/sub 3/O/sub 10/ was found to be a new compound; its lattice constants have been determined from data on La/sub 4/Co/sub 3/O/sub 10/. The standard Gibbs energies (..delta..G/degrees/) of the reactions appearing in the systems were also determined from the oxygen partial pressures in equilibrium with three solids. The variation of ..delta..G/degrees/ with the ionic radius of Ln with 12 coordination is linear.

Thermogravimetric study of the Ln2O3CoCO2O3 system: III. Ln = Pr, Sm, Eu, and Tb

Phase equilibria in the system Ln2O3CoCo2O3 (Ln = Pr, Sm, Eu, and Tb) at 1200°C were studied at oxygen partial pressure ranging from 1 to 10−12 atm O2. “Pr2CoO4,” PrCoO3, and Pr4Co3O10, are stable ternary compounds in the Pr2O3CoCo2O3 system, while only LnCoO3-type compound is stable in the other systems. The standard Gibbs energies (ΔG°) of reactions appearing in the systems were also determined from the oxygen partial pressures in equilibrium with three solid phases. ΔG° values for a reaction 12Ln2O3 + CoO + 14O2 = LnCoO3 fit well with the linear ΔG° vs ionic radius relation reported previously.

Phase equilibria in the FeNbO system at 1200°C

Abstract Phase equilibria in the system FeNbO at 1200°C are established by changing the oxygen partial pressure from 0 to −15.00 in log ( P O 2 atm ) and are represented in the Nb2O5FeFe2O3 system. In this system, FeNbO4, FeNb2O6, Fe4Nb2O9, niobium-iron spinel, and NbO2 phases appear and have non-stoichiometric compositions. Lattice constants of the compounds and the solid solutions are determined and discussed with respect to the oxygen partial pressure dependency and to the coexistence of the other compounds. The lattice constant of the spinel solid solution increases with the increasing of the content of “Fe7Nb2O12.” The standard Gibbs energies of the reactions appearing in the phase diagram are calculated.

Thermogravimetric study of the MNiO system. II. M = La at 1200°C

Abstract Phase equilibria in the LaNiO system at 1200°C were established at oxygen partial pressures ranging from 1 to 10 −12.00 atm. In this system, La 2 NiO 4 and La 6 Ni 5 O 15 are stable as the ternary oxide. The latter is probably a new compound and its spacings and relative intensities are presented. La 3 Ni 2 O 7 and La 4 Ni 3 O 10 are not stable. La 2 NiO 4 has nonstoichiometric compositions, as is well known, and the relationship between the composition and the oxygen partial pressure is presented. The standard Gibbs energies of the reactions which appear in the system are obtained.

Thermogravimetric study of the MNiO system. I. M = Ta and Nb

Abstract Phase equilibria in the TaNiO and NbNiO systems were studied at 1200°C at oxygen partial pressures ranging from 1 to 10 −12.00 atm in the TaNiO system and from 1 to 10 −14.50 atm in the NbNiO system. The NiO system was reinvestigated to confirm the previous data and to check the experimental method. In both systems Ta 2 NiO 6 and Nb 2 NiO 6 are stable ternary compounds under the present experimental conditions; the former has a small compositional width and the latter is almost stoichiometric. Nb 2 Ni 3 O 8 and Nb 2 Ni 4 O 9 are not stable. Lattice constants of the ternary compounds were obtained for the quenched samples. The standard Gibbs energies of the reaction M 2 O 5 + Ni + 1 2 O 2 = M 2 NiO 6 were also determined from the oxygen partial pressures in equilibrium with three solids.

Formation of manganese- and manganese,zinc-bearing ferrites by oxidation of aqueous suspensions and analysis of their cation distributions

Abstract Oxidized manganese-bearing and manganese,zinc-bearing ferrites have been prepared by air oxidation of iron(II) hydroxide suspensions at initial Mn:Fe tot mol ratios ( r Mn ) of 0.20:2.80 to 1.40:1.60 and initial (Mn + Zn):Fe tot mol ratios ( r Mn+Zn ) of 0.20:2.80 to 1.00:2.00, respectively, at pH 10.0 and 65°C. Mossbauer spectra at room temperature and chemical analysis of the manganese-bearing ferrites suggest that Mn 2+ ions are incorporated into the tetrahedral sites in the ferrites with manganese compositions of less than 0.74 and into both the tetrahedral and the octahedral sites at compositions of 0.74 to 1.06. In addition, manganese ions in a higher valence state than 3+ are incorporated into the octahedral sites at manganese compositions above 0.74. Mossbauer spectra at room temperature of the manganese,zinc-bearing ferrites prepared at r Mn+Zn = 1.00:2.00 indicate that Mn 2+ ions are incorporated on the lattice points in the ferrites and in both the tetrahedral and the octahedral sites at manganese compositions of 0.33 to 0.51. Mossbauer spectra and chemical analysis also suggest the incorporation of manganese ions in valence states higher than 3+. Mossbauer spectra and the lattice constants imply that the octahedral lattice sites in the ferrites with manganese compositions equal to or greater than 0.51 are distorted. Mossbauer spectra at room temperature of the manganese,zinc-bearing ferrites prepared at r Mn+Zn = 0.20:2.80 to 0.80:2.20 suggest that Mn 2+ ions are incorporated into both the tetrahedral and the octahedral sites in the ferrites, and that the amounts of Mn 2+ ions on the octahedral sites increase with increases in the manganese composition of the ferrites prepared at each r Mn+Zn value.

Experimental study of equilibria in the Ta2O5FeFe2O3 system at 1200°C

Abstract Phase equilibria in the system Ta2O5FeFe2O3 were established at 1200°C by changing the oxygen partial pressure from 0 to 16.60 in − log ( P O 2 atm ) . In this system FeTaO4, FeTa2O6, and Fe4Ta2O9 are stable in addition to the well known two compounds in the FeFe2O3 system. These compounds have nonstoichiometric compositions. In particular, FeTaO4, FeTa2O6, and a spinel solid solution, which have an end member Fe3O4, have a considerable wide range of nonstoichiometry. Lattice constants of the compounds and the solid solutions which appeared in the system were also determined and discussed with respect to their dependence on the atmosphere in which the samples were made. By calculating the activities of each component in the above solid solutions, the standard Gibbs energies of reactions, (1) Fe + Ta 2 O 5 + 1 2 O 2 = FeTa 2 O 6 , (2) 2 FeTa 2 O 6 + 1 2 O 2 = 2FeTaO 4 + Ta 2 O 5 , (3) FeTa 2 O 6 + 3Fe + 3 2 O 2 = Fe 4 Ta 2 O 9 , and (4) Fe 4 Ta 2 O 9 + 1 2 O 2 = FeTa 2 O 6 + Fe 3 O 4 , were calculated as −221.4, −6.8, −522.4, and −133.7 kJ, respectively.