Heterogeneous Phase Equilibria in Rare Earth–Mn–O Systems in Air

The phase relations in rare earth–Mn–O systems in air are considered. Most of the phase diagrams of these systems fall into two distinct groups: R"–Mn–O (R" = Y, Ho–Lu) and R"–Mn–O (R" = Pr, Nd, Sm–Dy). In addition, the Sc–Mn–O, La–Mn–O, and Ce–Mn–O systems have phase-diagram features of their own. The Ce–Mn–O system contains no ternary oxides or solid solutions: there are only mixtures of cerium and manganese oxides. The Sc–Mn–O system has phase-diagram features in common with both the R"–Mn–O and M–Mn–O (M = Mg, Al, 3d transition metal) systems. The La–Mn–O phase diagram can be thought of as a degenerate diagram of the R"–Mn–O group, since LaMn₂O₅ exists at oxygen pressures higher than atmospheric pressure. The R"–Mn–O and R"–Mn–O systems contain two chemical compounds, RMnO₃ and RMn₂O₅, but differ in the crystal structure of RMnO₃: hexagonal in the R" group and orthorhombic perovskite-like in the R" group. A key role in determining the structure of RMnO₃ is played by the size factor. In both groups, the RMn₂O₅ compounds dissociate in air by the reaction \({\text{RMn}}_{\text{2}} {\text{O}}_{\text{5}} {\text{ = RMnO}}_{\text{3}} + \frac{1}{3}{\text{RMn}}_{\text{3}} {\text{O}}_{\text{4}} + \frac{1}{3}{\text{O}}_{\text{2}} \). The dissociation temperature of RMn₂O₅ is shown to correlate with the atomic number of R, the total number of 4f electrons, the number of unpaired 4f electrons, and the ionic radius of R³⁺.     

Stable and Metastable Phase Equilibria in the Bi2O3–BiPO4 System

The stable and metastable phase equilibria in the Bi₂O₃–P₂O₅ system were studied in the range 0–50 mol % P₂O₅ by differential thermal analysis and x-ray diffraction. The results were used to construct the corresponding phase diagrams. In the equilibrium state, the system contains one sillenite phase with Bi₂O₃ : P₂O₅ = 12 : 1 and four other bismuth phosphates (2 : 1, 3 : 1, 5 : 1, and 1 : 1). In a metastable state, resulting from solidification of metastable melts, there exist * solid solutions (0–25 mol % P₂O₅) based on the high-temperature form of Bi₂O₃. At lower temperatures, the * phase transforms eutectoidally into the metastable phase, which has the same structural basis as the high-temperature solid solutions. At room temperature, the phase exists in a narrow composition range around 15 mol % P₂O₅. At lower P₂O₅ contents, the * phase decomposes at 868 K by a monotectoid reaction to form a mixture of the metastable and phases. The phases 3Bi₂O₃ · P₂O₅ () and 2Bi₂O₃ · P₂O₅ (), melting incongruently at 1193 and 1223 K, respectively, appear in both the equilibrium and metastable phase diagrams.     

Phase Equilibria in the Li–W–Mn–O System

An isothermal compositional subsolidus phase equilibrium diagram of the Li–W–Mn–O system is constructed. Possible solid-state transformations in the system at a variable pressure and constant temperature are presented and phase transformations involving melt in temperature and pressure ranges of interest are analyzed.     

Phase Relations in the SrO–Bi2O3–Fe2O3 System

The phase relations in the composition region SrFeO_{3 –} –Fe₂O₃–BiFeO₃ are studied in air by x-ray diffraction and electron microscopy. The 1000°C phase compatibility diagram is constructed. Sr_{1 – x} Bi _x FeO_{3 –} solid solutions are prepared in the range 0 < x 0.8. Their lattice parameter is found to vary nonlinearly with x. Two new phases were identified: (Sr,Bi)₃Fe₄O _y (tetragonal lattice, a= 3.907(2) Å, c= 27.30(2) Å) and Sr_0.6Bi_0.4FeO_{3 –} (tetragonal lattice,a = 5.555(2) Å, c= 11.848(5) Å).     

Phase Relations in the BaO · 2B2O3–La2O3 · 3B2O3 System

The BaO · 2B₂O₃–La₂O₃ · 3B₂O₃ join of the ternary system BaO–La₂O₃–B₂O₃ is studied using differential thermal analysis, x-ray diffraction, and density measurements. The join is shown to be pseudobinary, with eutectic phase relations. In the composition range 5–99.8 mol % La₂O₃ · 3B₂O₃, glassy materials are obtained, which seems to be associated with the existence of the congruently melting compound BaO · La₂O₃ · 5B₂O₃.     

Calculation of Phase Equilibria in the Th(NO3)4-HNO3-H2O System at 25°C

Pitzer's binary parameters of thorium nitrate were calculated from critically selected published data on the water vapor pressure in the Th(NO₃)₄-H₂O system at 25°C. From data on the water vapor pressure in the Th(NO₃)₄-HNO₃-H₂O ternary system, the Pitzer's ternary parameters were calculated at 25°C. The branch of thorium nitrate hexahydrate crystallization in the ternary system was calculated in the region from 0 to 15 m HNO₃ using complete set of Pitzer's binary and ternary parameters. The calculation results coincide with the experimental data on the solubility.     

Phase Relations in the SnSe2–Er2Se3 System

The phase diagram of the SnSe₂–Er₂Se₃ system is studied using differential thermal analysis, x-ray diffraction, microstructural analysis, and microhardness and density measurements. The SnSe₂–Er₂Se₃ system is pseudobinary and contains two eutectics and two intermediate phases, Er₂Sn₃Se₉ and Er₂SnSe₅. At room temperature, the SnSe₂-based solid solution extends to 4.5 mol % Er₂Se₃, and the SnSe₂ solubility in Er₂Se₃ is 2 mol %.     

Phase Transformations in the Y2Cu2O5–BaCuO2 System

The sequence of phase transformations in the Y₂Cu₂O₅–BaCuO₂ pseudobinary system was studied during heating and cooling in the range 1170–1310 K. The results demonstrate that the reaction zone in BaCuO₂/Y₂Cu₂O₅ diffusion couples consists of BaCuO₂/YBa₂Cu₃O_{7 –} /Y₂BaCuO₅/Y₂O₃/Y₂Cu₂O₅ layers, corresponding to the sequence of chemical changes during YBa₂Cu₃O_{7 –} crystallization between 1170 and 1220 K. In the range 1260–1310 K, BaCu₂O₂ is formed. During cooling of a Y₂Cu₂O₅ + 4.3BaCuO₂ mixture, YBa₂Cu₃O_{7 –} crystallizes in a wide temperature range, between 1240 and 1190 K. The process depends on the presence of BaCu₂O₂ on the surface of Y₂BaCuO₅ grains in the high-temperature solution and the oxygen supply from the gas phase.     

Phase Relations along the Y2O3–CuWO4Join in the Y2O3–WO3–CuO System

The phase relations in the Y₂O₃–WO₃–CuO system were studied by x-ray diffraction and thermal analysis. The results were used to construct the 800°C section of the phase diagram. Based on the new and earlier data on the liquidus relations, the section through the Y₂O₃–WO₃–CuO phase diagram along the Y₂O₃–CuWO₄join was mapped out.     

Mechanochemical Synthesis and Thermal Behavior of Metastable Mixed Oxides in the CaO–Sb2O3–Bi2O3 System

The phase composition of crystalline mechanochemical synthesis products in the CaO–Sb₂O₃–Bi₂O₃ system was determined. Of the known phases in this system, only three could be prepared mechanochemically: Ca₂Sb₂O₅, CaSb₂O₄, and CaBiO_2.5 (fcc). A new metastable phase, "-Bi₂O₃, with an orthorhombic structure close to that of the high-temperature, fluorite phase -Bi₂O₃, was obtained by mechanical processing at 30°C. A number of new metastable fluorite solid solutions of binary and ternary oxides were obtained as single-phase powders by mechanochemical synthesis. The mechanochemical yield of primary crystalline products was shown to be several times higher than that of secondary products. A broad composition range was revealed in which perovskite and fluorite phases are in mechanochemical equilibrium. The composition dependence of the lattice parameter of the metastable fluorite phase Bi_{2 – x} Sb _x O₃ was found to be the opposite of the one predicted by Vegard's law. Metastable mixed oxides undergo phase transformations during heating (starting at 280°C in the case of the ternary perovskite phase). Bi_{2 – x} Ca _x O_{3 – 0.5x} fluorite solid solutions experience a transformation at 400°C, accompanied by oxygen loss. During heating in air, Sb₂O₃-containing fluorite phases partially stabilize owing to oxidation but, nevertheless, undergo structural transformations above 480°C. The transformation of Sb_{2 – x} Ca _x O_{3 – 0.5x} metastable fluorite solid solutions near 500°C in air is accompanied by the formation of needle-like Sb₂O₃ crystals. A mechanism is proposed for the extremely rapid growth of such crystals: extrusion of the Sb₂O₃ resulting from fluorite decomposition in agglomerates through triple junctions of aggregates and through cracks in the surface layer of agglomerates.     

Phase Relations in the Na2O–Al2O3–Nb2O5and CaO–Al2O3–Nb2O5Systems

Phase relations in the Na₂O–Al₂O₃–Nb₂O₅and CaO–Al₂O₃–Nb₂O₅systems were studied. The Na₂O system was found to contain neither ternary compounds nor niobate–aluminate solid solutions. In the CaO system, a ternary compound of composition 4CaO · Al₂O₃·Nb₂O₅was identified (cubic structure, a= 7.628 Å, Z= 2, _meas= _x= 4.43 g/cm³).     

Phase Relations in the NiFe2O4–NiCr2O4–CuCr2O4System

The phase relations in the NiFe₂O₄–NiCr₂O₄–CuCr₂O₄system were investigated experimentally and theoretically. X-ray diffraction data were used to construct the phase diagram of the system and elucidate the structural mechanisms of the transitions from the cubic spinel structure to the tetragonal (I42d, c/a< 1 and I4₁/amd, c/a> 1) and orthorhombic (Fdd2) structures.     

Phase transformation in the Al2O3–ZrO2 system

Co-precipitation methods have been used to produce 20 mol% Al2O3–80 mol% ZrO2 mixed oxides, from aqueous solutions of zirconium oxychloride and aluminium chloride, followed by precipitation with ammonia. The resulting gel was calcined at increasing temperatures, and X-ray diffraction confirmed that the structure remained amorphous up to 750°C and then crystallized as a single-phase cubic zirconia solid solution, but with a reduced unit-cell dimension. At higher temperatures, the unit-cell dimension increased and, above 950°C, this phase started to transform to a tetragonal zirconia (t-ZrO2) phase, again of reduced cell dimensions compared with t-ZrO2, with simultaneous appearance of small amounts of -Al2O3. Above 1100°C, the tetragonal phase transformed to monoclinic zirconia on cooling, and the amount of -Al2O3 increased. Above 1200°C, the -Al2O3 transformed to the stable -Al2O3. These results confirm that aluminium acts as a stabilizing cation for zirconia up to temperatures of about 1100°C. © 1998 Chapman & Hall     

Phase Relations in the NaVO3–Ca(VO3)2 System

The phase diagram of the NaVO₃–Ca(VO₃)₂ system is mapped out. The system is shown to contain NaVO₃-based solid solutions (0–5 mol % Ca(VO₃)₂), Ca(VO₃)₂-based solid solutions (0–17 mol % NaVO₃), and the double metavanadate Na₂Ca(VO₃)₄. The results on the sequence of phase changes during heat treatment of an Na₂CO₃ + CaCO₃ + 2V₂O₅ mixture and on the direction of diffusion flows (studied by the Tubandt method) indicate that the phase formation in the NaVO₃–Ca(VO₃)₂ system is mainly due to the diffusion of Na⁺ ions.     

Phase Relations in the Systems Al2TiO5–Fe2O3 , Al2O3–TiO2–Fe2O3 , and Al2TiO5–Cr2O3

Phase relations in the systems Al₂TiO₅–Fe₂O₃, Al₂TiO₅–Cr₂O₃, and Al₂O₃–TiO₂–Fe₂O₃ are investigated, and the composition ranges of pseudobrookite Al_{2 – 2x} M_2x TiO₅ (M = Fe, Cr) solid solutions are determined.     

Phase equilibria in the system Na2O-B2O3-Ga2O3 at 600°C

Phase equilibria in the system Na₂O-B₂O₃-Ga₂O₃ were investigated at 600°C using quenching and X-ray powder techniques. Many, but not all the binary phases reported previously were confirmed, no new binary phases were found. The ternary phase diagram was solved and a new phase, 3Na₂O·4B₂O₃·Ga₂O₃, discovered. The X-ray powder data are given for this and also for -Ga₂O₃ and Na₂O·Ga₂O₃. Solid solution effects were investigated for the primary and binary phases by comparison of patterns; no solid solution effects were detected. These results are compared with those of investigations of two pseudo-binaries reported previously, both of which reported other ternary phases which could not be confirmed. Although there is some conflict between the results, it is pointed out that the validity of one pseudo-binary is doubtful due to chemical analysis of one ternary phase, while the report on the other pseudo-binary gives an X-ray powder pattern of a ternary phase which is probably impure.     

Phase Equilibria and Structure of Solid Solutions in the La–Co–Fe–O System at 1100°C

Phase equilibria in the La–Co–Fe–O system are studied at 1100°C in air using samples prepared by the citrate, nitrate, and conventional ceramic routes. The stability regions and structures of solid solutions in the La–Co–Fe–O system are determined by x-ray powder diffraction: LaCo_{1 – y} Fe _y O_{3 –} (0 < y 0.25, sp. gr. R c; 0.775 y< 1, sp. gr. Pbnm), Co_{1 – y} Fe _y O (0 < y 0.13, NaCl-type structure, sp. gr. Fm3m), and Fe_{3 – x} Co _x O₄ (0.84 x 1.38, sp. gr. Fd3m). The structural parameters of phase-pure solid solutions are determined by the Rietveld method. The composition dependences of lattice parameters are presented for LaCo_{1 – y} Fe _y O_{3 –} (0 < y 0.25) and Fe_{3 – x} Co _x O₄ (0.84 x 1.38). The 1100°C isotherm of the pseudoternary system La₂O₃–CoO–Fe₂O₃ in air is constructed.     

Ion-Conducting Defect Pyrochlore Phases in the K2O–Sb2O3–WO3 System

Potassium tungstate antimonates were prepared by calcining K₂CO₃ + Sb₂O₃ + 2(1 – – )WO₃ mixtures in air. Data on the phase composition of the obtained materials were used to locate the pyrochlore phase region in the Sb₂O _z –W₂O₆ – K₂O composition triangle at 1170 K. The distributions of the constituent ions and lattice defects over the crystallographic positions of space group Fd3m were inferred from structural and gravimetric data. The ac conductivity of the potassium tungstate antimonates was measured from 600 to 1000 K. The conductivity and its activation energy were shown to be correlated with the concentrations of cation (position 16d) and anion (position 8b) defects. The concentration and mobility of K⁺ ions involved in charge transport were determined.     

Phase equilibria in the system ZnO-B2O3-SiO2 at 950° C

Phase equilibria in the system ZnO-B₂O₃-SiO₂ investigated at 950°C using quenching and X-ray powder diffraction techniques. The binary phases reported reviously were confirmed but no ternary phases were found. Solid-solution effects were investigated for the primary and binary phases by comparison of patterns; no solid solutions were detected. There is a large Quid area in the high-B₂O₃ corner of the phase diagram. h is found that zinc orthosilicate,-2ZnO · SiO₂, dissolves in this to form a zinc-oxide-rich borosilicate liquid but 4ZnO · 3B₂O₃ does not. Them is also a liquid eutectic at approximate composition 65ZnO-25B₂O₃-10-SiO₂ (wt%). The rate of volatilization and loss of B₂O₃ was also investigated for 4ZnO · 3B₂O₃ and it was concluded that although volatilization occurs, the loss was insufficient at the firing times and temperature used to invalidate the major features of the diagram.