Experimental Study of Phase Equilibria in the MnO–“TiO2

1. Introduction

Manganese and titanium are both transition metals andthey have several valences in oxides; Mn2�, Mn3�, Mn4�,Ti2�, Ti3� and Ti4� depending on temperature, oxygen par-tial pressure and phase in which they reside. Although theirproportions in oxide systems are still not clear, it is accept-able that Mn2� is predominant under moderate reducingcondition among several valences of manganese. On theother hand, Ti4� is stable in molten oxide even understrongly reducing condition (where solid TiO2 reduces toTi3O5), which can be achieved in usual experimental tech-nique.1)

Phase relations in the MnO–“TiO2”–“Ti2O3” ternary sys-tem are still unclear. These systems are main system ofnon-metallic inclusions in steel product. For example, in-clusions in Mn/Si deoxidized steel with Ti alloying such asHSLA (High Strength Low Alloyed) steel consist mainlyMnO–TiOx–SiO2 and these act as nucleation site of intra-granular ferrite transformation.2,3)

The purpose of the present study is to experimental-ly determine phase equilibria and liquidus of theMnO–“TiO2”–“Ti2O3” ternary system under various oxy-gen partial pressures and temperatures. This study is a partof thermodynamic and experimental study on theMnO–SiO2–“TiO2”–“Ti2O3” system. Extension of experi-mental study to the MnO–SiO2–“TiO2”–“Ti2O3” systemwill be presented, subsequently.4) Thermodynamic model-ing in this system will be followed.5,6)

2. Previous Works

Previous studies for the MnO–“TiO2” system is criticallyreviewed by Eriksson and Pelton.7) Experimental informa-tion for liquidus measurements are listed in Table 1. Phaseequilibria of the MnO–“TiO2” system was first investigatedby Grieve and White8) who used thermal analysis tech-nique and reflected light microscope under N2 or vacuumatmosphere. Two intermediate compounds, MnTiO3 andMn2TiO4 were observed and the former melts incongruent-ly at 1 360°C while the latter melts congruently at 1 450°C. However, a later phase diagram study under N2

by Leusmann9) showed that MnTiO3 melts congruently at1 410�10°C. Also he reported that melting temperature ofMn2TiO4 was 1 420�10°C from differential thermal analy-sis (DTA). Another compound Mn2Ti3O8 was found byJoubert and Durif,10) but its stability was limited up to330°C. The existence of another compound “MnTi2O5” wasdiscussed by Evans and Muan.11) They estimated Gibbs en-ergy of formation of the pseudobrookite phase “MnTi2O5”at 1 250°C from activity measurement in the MnO–NiO–TiO2 system, although they did not observe that compound.No evidence on the existence of this particular compoundhas been confirmed.

For MnO–“TiO2”–“Ti2O3” system at low pO2, Grey etal.12,13) performed phase equilibrium study at 1 200°C andreported that pseudobrookite solid solution emanating fromTi3O5 toward “MnTi2O5” direction exist in equilibrium withrutile and/or pyrophanite solid solution. The pseudo-brookite (s.s.) extends from 0 to 65 mol% of the hypotheti-

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Experimental Study of Phase Equilibria in theMnO–“TiO2”–“Ti2O3” System

Youn-Bae KANG and Hae-Geon LEE1)

Formerly Graduate Student, Dept. of Materials Science and Engineering, Pohang University of Science and Technology(POSTECH), now at Centre de Recherche en Calcul Thermochimique (CRCT), École Polytechnique, Montreal, Quebec, H3C3A7, Canada. 1) Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology(POSTECH), Pohang, 790-784, Korea. E-mail: [email protected]

(Received on March 22, 2005; accepted on July 7, 2005 )

Phase equilibria and liquidus in the system MnO–“TiO2”–“Ti2O3” under controlled atmosphere have beeninvestigated in the temperature range from 1 300 to 1 550°C and in the range of log pO2 (in atm) from �7.2(pCO/pCO2�1) to �16.6 (C–CO equilibration). High-temperature equilibration, quenching and electron probemicroanalysis (EPMA) were employed to obtain equilibrium compositions of liquid and several solid solu-tions. The following phases have been observed; molten oxide, manganosite (MnO (s.s.)), rutile (TiO2–d(s.s.)), spinel (Mn2TiO4–MnTi2O4), pyrophanite (MnTiO3–Ti2O3) and pseudobrookite (“MnTi2O5”–Ti3O5) solidsolutions. Liquidus of manganosite and rutile were measured and compared with previous investigations.“MnTi2O5” compound was confirmed to be unstable phase in the MnO–“TiO2” system. It was found in thepresent study that sub-solidus phase equilibria are affected considerably by oxygen partial pressure.

KEY WORDS: MnO–“TiO2”–“Ti2O3” system; phase equilibria; oxygen partial pressure; spinel; pyrophanite;pseudobrookite; oxide metallurgy; inclusion.

cal “MnTi2O5” in Ti3O5 at 1 200°C depending on the oxy-gen partial pressure. Pyrophanite (s.s.) has a large miscibili-ty gap at 1 200°C extending from 10–75 mol% Ti2O3 (re-ferred as tagirovite (s.s.)). However, the phase equilibria be-tween pyrophanite (Mn rich) and tagirovite (Ti rich) wasnot so clear due to difficulty of experimental condition.

Lecerf and Hardy14) reported the compound MnTi2O4 inMnO–“Ti2O3” system. This compound had an almost nor-mal spinel-type structure and it was later confirmed byLambert et al.15) Hardy et al.16) also reported Mn2TiO4,which has tetragonal structure below 770°C and transformsto cubic spinel phase above 770°C. From X-ray diffractionstudy, they concluded that the Mn2TiO4 is inverse typespinel (Mn2� in tetrahedral site). Further, they17) revealedfrom lattice parameter measurement that MnTi2O4 andMn2TiO4 form a continuous spinel solid solution at1 200°C.

Apart from the phase diagram study by Grieve andWhite8) and Leusmann,9) liquidus compositions at severaltemperatures in the MnO–“TiO2” system have been report-ed by a number of researchers by employing molten oxideequilibration with primary phase pellet and wet-chemicalanalysis. Phase diagram of the MnO–“TiO2” system wasdrawn by thermodynamic optimization of Eriksson andPelton7) and is shown in Fig. 1 with the reported experi-mental data. The optimization by Eriksson and Pelton7) wasdone using mainly the data of Leusmann.9) However, asseen in the figure, all reported data1,8,9,18–20) are not consis-tent with each other. Temperatures for melting or eutecticreactions are different between Grieve and White8) andLeusmann.9) Melting behavior of MnTiO3 is incongruentaccording to Grieve and White while Leusmann reportedMnTiO3 melts congruently. Liquidus of MnO measured byseveral researchers are not in agreement with each otherand its shape is unreasonable. Liquidus of TiO2 measuredby Grieve and White and Leusmann, in which thermal

analysis technique were used in both investigations, are farfrom each other.

Inconsistency of liquidus position in the MnO–“TiO2”system makes it difficult to characterize the phase diagramand thermodynamics of the MnO–“TiO2”–“Ti2O3” system.Moreover, since stability of titanium containing oxide couldbe affected by oxygen partial pressure, its careful controlduring experiment is very important to understand phaseequilibria of the system. Most experimental reports on theMnO–“TiO2”–“Ti2O3” system did not specify the actualoxygen partial pressure, as described above. Information onthe oxide solid solutions in MnO–“TiO2”–“Ti2O3” system israre and most investigators did not consider possible solidsolutions except for the work of Grey et al.13) Further care-ful experimental studies are, therefore, required to provideaccurate phase diagram and develop thermodynamic data-

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Table 1. Details of experimental methods used for determining phase equilibria in the MnO–TiOx system.

Fig. 1. Reported phase equilibrium data in the MnO–“TiO2” sys-tem. Lines are calculated from Eriksson and Pelton.7)

Experimental conditions for each study are listed in Table1. nTi and nMn represent number of moles of Ti and Mn,respectively.

base. The aim of the present study is to provide new experi-mental data on the phase equilibria in the MnO–“TiO2”–“Ti2O3” system including liquidus and sub-solidus phaserelations. This newly provided experimental data will beused by the present authors for thermodynamic optimiza-tion to develop a consistent thermodynamic data set.5,6)

3. Experimental

The general experimental procedures used in the presentstudy are high temperature equilibration, quenching andfollowed by electron probe X-ray microanalysis (EPMA).The present investigation was carried out in a controlled at-mosphere using CO/CO2 gas mixture or C/CO equilibra-tion. Initial oxide mixtures were prepared from reagentgrade oxides powders. The MnO (99.9 mass%, Kosundo)and TiO2 (99.9 mass%, Kosundo) were dried at 110°C, andweighed in the desired proportions. Those powders werethen mixed by ball-milling under ethyl alcohol for 12 h anddried at 200°C in air.

Each sample was weighed from 0.5 to 1.0 g, pelletizedand placed in a boat made from thin molybdenum foil(0.07 mm thick) or a platinum crucible (OD 10 mm�H10 mm�T 0.1 mm) depending on temperature and oxygenpartial pressure during experiment. The samples were thenplaced in the hot zone of a recrystallized alumina tubeequipped for gas-mixing, which is tightly sealed by water-cooled brass end cap. A vertical MoSi2 resistance furnacewas used for heating samples. Temperatures were con-trolled to �2°C using a B type Pt/Rh thermocouple placedexternally to the alumina tube, and were continuously mon-itored throughout the runs using another B type thermocou-ple placed immediately above the sample.

Oxygen partial pressure was controlled using a flowingCO/CO2 gas mixture or CO with graphite holder. Figure 2shows calculated phase diagram of Ti–O system5,7) withtemperature and oxygen partial pressure as axes. Calculatedoxygen partial pressure under three different conditions(pCO/pCO2�1, 9 and C/CO equilibration) using F*A*C*Tdatabase21) were also shown in the figure. Moistures in CO,CO2 gases were removed by passing over CaSO4 and CO2

impurities in CO gas was purified by passing sodium hy-droxide. In former two conditions (pCO/pCO2�1 and 9) inthe temperature range between 1 200°C to 1 550°C, the sta-ble form of Ti oxide is rutile (TiO2–d). On the other hand,under the condition of C/CO equilibration, rutile reduces to Ti3O5 (below 1 457°C) and to Ti2O3 (above 1 457°C).7)

Experiments in the present study were conducted at thosethree different gas conditions to reveal effect of oxygen par-tial pressure on the phase equilibria in the MnO–“TiO2”–“Ti2O3” system.

Total flow rate was 400 cm3/min and the volume ratio ofeach gas was controlled. Mass flow controllers for CO andCO2 gas were preliminarily calibrated using soap-bubble-column-technique. Oxygen partial pressures were moni-tored in some runs using an yttria-stabilized zircornia sen-sor with air as the reference.

After equilibration, runs were terminated by rapidquenching the sample into ice water. Whole part of eachsample was then mounted in epoxy resin, and polished forexamination by optical microscopy and electron micro-

probe analysis.The electron microprobe analysis was carried out on a

JEOL 8100-JXA in the WDS mode. Operating conditionswere 15 kV accelerating voltage, 20 nA probe current. Datawere reduced using ZAF correction routine. For standards,pure MnO and TiO2 (rutile) crystals supplied from JEOLLtd. were used for manganese and titanium concentrations.Additionally, the calibration was also checked using somelarger crystals of MnTiO3. The MnTiO3 sample was pre-pared from commercially available MnTiO3 powder(99.9 mass%, Kosundo). The powders were made into pelletunder a pressure of 200 MPa. The prepared pellet was sin-tered at 1 400°C under pCO/pCO2�9 for 3 d. The composi-tions of samples were measured by EPMA as describedabove (using MnO and TiO2 as standard crystals).Measured cationic ratio (nTi/(nMn�nTi)) was 0.501�0.002 so that pure MnO and pure TiO2 seem suitable asstandards in the present study.

At least five analyses were made of each phase in a sam-ple and averaged. The STD of the measurement was within1 mass% in most cases. Because the liquid phase in theMnO–TiOx was very fluid and hence gave rise to the forma-tion of large numbers of precipitates, even during rapidquenching, EPMA analysis were carried out more than tenanalyses with a broad probe size (�30 mm), as done byMuan in the similar systems NiO–TiO2.

22) When Mo wasused for sample holder, Mo dissolution into the oxide waschecked during EPMA measurement. At high temperaturewith high oxygen partial pressure, noticeable amount of Modissolution into molten oxide was observed. In that case,the sample was rejected for further analysis and platinumcrucible was used as container.

The XRD analysis was used to confirm reduction of TiO2

to Ti3O5 or Ti2O3 at C/CO equilibrium condition. The XRDanalysis was carried out with a RIGAKU D/max 2500H X-ray diffractometor with a graphite monochrometer usingCu Ka radiation.

Achievement of equilibrium has been confirmed by samefinal phases and compositions with different starting com-positions, well grown facets of crystals. At least one com-


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Fig. 2. Stable regions of titanium oxides as functions of tempera-ture and oxygen partial pressure.5,7) Thick lines are oxy-gen partial pressure employed in the present study.Several lines between Rutile and Ti3O5 regions meanphase boundaries of several Magnéli phases (TinO2n-1,n�4).

mon final equilibrium assemblages starting from differentinitial compositions in each run were confirmed.

The EPMA provides information only on the metalcation content present in a phase and does not distinguishthe proportion of the cations of the same element with dif-ferent valences. In the investigated oxides, titanium is pre-sent as Ti4� and Ti3� (while all manganese could be as-sumed in divalent state). In the present study, for construc-tion of the phase diagram in the ternary for MnO–“TiO2”–“Ti2O3”, all the titanium was recalculated to theTi4� oxidation state. This is equivalent to projecting thephase composition from the oxygen corner of theMnO–“TiO2”–“Ti2O3” ternary onto the MnO–“TiO2” line.This projection is shown by an arrow in Fig. 3, where thecompositions of the real oxide are schematically illustratedwith a dashed line.

4. Results

Details of the compositions of the liquid and the solidphases determined in the present studies for the MnO–“TiO2”–“Ti2O3” system are given in Table 2.

The phase diagram deduced from the experimental datain Table 2 is presented in Fig. 4 (at pCO/pCO2�1), Fig. 5(at pCO/pCO2�9) and Fig. 6 (at C/CO equilibration).Oxygen partial pressures at each temperature are specifiedin the figures. Solid lines in the figures are only estimatedbased on experimental data in Table 2 and melting temper-atures of manganosite and rutile, which can vary with oxygen partial pressure, were taken from F*A*C*T data-base.21)

Under the conditions investigated in the present study, the following phases have been observed: moltenoxide, manganosite (MnO (s.s.)), rutile (TiO2–d (s.s.)),spinel (Mn2TiO4–MnTi2O4), pyrophanite (MnTiO3–Ti2O3)and pseudobrookite (“MnTi2O5”–Ti3O5) solid solutions.Pseudobrookite (s.s.) was appeared in the limited tempera-ture range under pCO/pCO2�9 while it disappears underpCO/pCO2�1.

The backscattered electron images of typical microstruc-tures in the system MnO–“TiO2”–“Ti2O3” are shown inFigs. 7 through 10. Figures 7 and 8 illustrate the mi-crostructures of the manganosite (MnO s.s.) and rutile(TiO2 s.s.) crystalline phases in equilibration with liquidoxide under controlled CO/CO2 gas mixture, respectively.

Equilibrium morphologies of spinel (s.s) with manganosite(s.s.) can be seen in Fig. 9. Pseudobrookite in equilibriumwith pyrophanite is shown in Fig. 10. Each solid phase wasidentified on the basis of the atomic compositions obtainedfrom EPMA.

5. Discussion

Earlier studies of phase diagram measurement by Grieveand White8) and Leusmann9) have been carried out by dif-ferential thermal analysis and optical microscopy to deter-mine phase transformation temperatures and to identify thepresence of particular phases. However, those methods donot provide equilibrium phase composition so that it wasnot possible to characterize phase equilibria between solidsolutions in the system. Also, liquidus compositions at sev-eral temperatures were measured by several investiga-tors1,18–20) employing chemical equilibration by primaryphase saturation followed by wet chemical analysis. Inthose studies, they only analyzed compositions of liquidphase. This method also does not provide the equilibriumcompositions of solids. Moreover, identification of equilib-rium phases was not conducted directly.

On the other hand, the experimental techniques used inthe present study differ from those used by previous investi-gators. The compositions of liquid and solid phases formedhave been directly measured after equilibration. The use ofEPMA provides the equilibrium compositions of co-exist-ing phases, either liquid or solid, simultaneously so that it ispossible to measure solid solubility as well as liquid com-position. This is more important in studying sub-solidusphase equilibria. Also phase identification during analysisis possible by electron microscope in back scattered elec-tron mode. The oxygen partial pressure in equilibrium canbe known by the equilibration of CO/CO2 gas mixture orC/CO equilibration. This makes phase equilibria in the sys-tem be characterized accurately while using Ar or N2 gasdoes not provide the exact oxygen partial pressures.Homogeneity of particular phase can be confirmed by ana-lyzing several positions or line analysis on the sample byEPMA. General advantages of using EPMA in phase dia-gram measurement are discussed in more detail by Jak etal.23) and Jak and Hayes.24)

5.1. Phase Diagram of the MnO–“TiO2”–“Ti2O3” Sys-tem under pCO/pCO2�1

Figure 4 shows phase diagram of the MnO–“TiO2”–“Ti2O3” system at pCO/pCO2=1 with Ti cationic ratio(nTi/(nTi�nMn)) as compositional axis. For reference, equi-librium oxygen partial pressures calculated fromFactSage21) were given at several temperatures. Five phaseshave been found in the temperature range of 1 350 to1 550°C in this gas atmosphere: molten oxide, manganosite(s.s.), spinel (s.s.), pyrophanite (s.s.) and rutile (s.s.). Ti dis-solves in manganosite s.s. up to 2.5 mole percent. Spinelshows almost stoichiometric Mn2TiO4. Melting behavior ofspinel is not clear (either congruently melts or incongruent-ly melts) from the result in the present study because com-position of liquidus of manganosite at the melting tempera-ture of spinel is very close to the composition of spinelphase (nTi/(nTi�nMn)�1/3). Pyrophanite shows the congru-


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Fig. 3. Representation of experimental data of the MnO–“TiO2”–“Ti2O3” onto MnO–“TiO2” side from oxygencorner.


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Table 2. Results of Experiments on the System MnO–“TiO2”–“Ti2O3” in Equilibrium with Various Gas Conditions.


© 2005 ISIJ 1548

Fig. 4. Phase diagram of the MnO–“TiO2”–“Ti2O3” system at pCO/pCO2�1, measured in the present study.

Fig. 5. Phase diagram of the MnO–“TiO2”–“Ti2O3” system at pCO/pCO2�9, measured in the present study.

Fig. 6. Phase diagram of the MnO–“TiO2”–“Ti2O3” system at C/CO equilibrium condition, measured in the present study. N.B. The solid dot lines(– – –) indicate the boundaries above which all oxides are no longer stable and hence reduced to metallic liquid. The dotted phase bound-aries ( · · · · ) are, therefore, only estimated.

ent melting behavior about 1 450°C. It is close to stoichio-metric MnTiO3 composition in equilibration with spinel s.s.On the other hand, Ti cationic ratio of pyrophanite increasesfrom 0.5 to 0.53 at 1 400°C in equilibration with rutile.Rutile does not show any noticeable solubility of Mn.

5.2. Phase Diagram of the MnO–“TiO2”–“Ti2O3” Sys-tem under pCO/pCO2�9

Phase equilibrium data of the MnO–“TiO2”–“Ti2O3” sys-tem at pCO/pCO2�9 are shown in Fig. 5. The oxygen par-tial pressure at pCO/pCO2�9 is lower than the oxygen par-tial pressure at pCO/pCO2�1.0 by about two order of mag-nitude at the same temperature. Most noticeable change inthis phase diagram is appearance of pseudobrookite (s.s.),which is not stable at pCO/pCO2�1 in the temperaturerange of 1 350 to 1 550°C. Pseudobrookite shows a peritec-tic melting behavior and pyrophanite melts congruently. Onthe other hand, melting behavior of spinel is still not clear(either congruently melts or incongruently melts) from theresult in the present study. The melting temperature of py-rophanite seems slightly higher than those atpCO/pCO2�1.0. This means that solid phases becomemore stable against liquid phase with the decrease of oxy-gen partial pressure. Small solubility of Mn possibly inTiOx was observed in the present study. Grey et al.13) re-ported that Mn dissolves in several Magnéli phases up to2% in terms of cationic ratio. Based on the phase stabilityshown in Fig. 2, all Magnéli phases are not stable at theseconditions (temperature and oxygen partial pressure) ifthere is no manganese in system. However, according toGrey et al.,13) several Magnéli phases become stable atslightly higher oxygen partial pressure when Mn dissolvesin the phases. In the present study, however, it was not con-firmed whether dissolution of Mn was taken place in rutileor Magnéli phases.

5.3. Phase Diagram of the MnO–“TiO2”–“Ti2O3” System under C/CO Equilibrium

Figure 6 shows phase diagram of the MnO–“TiO2”–“Ti2O3” system under C–CO equilibration. At this reducing


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Fig. 7. Microstructure of manganosite in equilibrium with liquidoxide. Legend: L�molten oxide, M�manganosite s.s.The specimen was held at 1 500°C under 0.9CO–0.1CO2

atmosphere with initial composition of 80mass%MnO�

20mass%TiO2.

Fig. 8. Microstructure of rutile in equilibrium with liquid oxide.Legend: L�molten oxide, R�rutile s.s. The specimenwas held at 1 500°C under 0.5CO–0.5CO2 atmospherewith initial composition of 20mass%MnO�80mass%TiO2.

Fig. 9. Microstructure of manganosite in equilibrium withspinel. Legend: SP�spinel s.s., M�manganosite s.s. Thespecimen was held at 1 450°C under 0.9CO–0.1CO2

atmosphere with initial composition of 80mass%MnO�

20mass%TiO2.

Fig. 10. Microstructure of pyrophanite in equilibrium withpseudobrookite. Legend: IL�pyrophanite s.s., PB�

pseudobrookite s.s. The specimen was held at 1 300°Cunder 0.9CO–0.1CO2 atmosphere with initial composi-tion of 35mass%MnO�65mass%TiO2.

condition, phase diagram of the MnO–“TiO2”–“Ti2O3” sys-tem shows very different shape compared to previous phasediagrams. Rutile s.s is no more stable at this gas conditionso that it disappears while pyrophanite and pseudobrookitesolid solutions, Ti rich in both phases, are placed in pure Tioxide side. Spinel and pyrophanite solid solutions shift toTi side. This can be understood that more dissolution ofMnTi2O4 in spinel s.s. and Ti2O3 in pyrophanite, in whichTi3� presents, both, because oxygen partial pressure de-ceases. Also, pseudobrookite s.s. is composed of mainlyTi3O5 with relatively lower content of Mn. Up to 1 500°C,no liquid oxide phase was observed. Therefore, liquiduscompositions could not be measured in the present study.Therefore, in the Fig. 6, liquidus lines (dotted line) are onlyestimated. Moreover, from thermodynamic analysis,5) lowoxygen partial pressure results in reduction of several oxidephases to metallic phase (liquid alloy). This is shown asdashed line in Fig. 6 and, above this dashed line, metallicliquid appears and oxide becomes no longer stable.

The schematic diagram of the MnO–“TiO2”–“Ti2O3”system is shown in Fig. 11. Long-dashed lines in the figurerepresent schematic phase equilibrium composition in theMnO–“TiO2”–“Ti2O3” system under a fixed oxygen partialpressure. As seen in Table 2 and Figs. 4–6, Ti solubility in solid solutions such as manganosite, spinel and pyro-phanite increases with decreasing oxygen partial pressure.This results from that the phase equilibria in theMnO–“TiO2”–“Ti2O3” system shift from MnO–“TiO2” to-ward MnO–“Ti2O3” with the decrease of oxygen partialpressure. This is illustrated with an arrow in Fig. 11. Theappearance of pseudobrookite at lower oxygen partial pres-sure could be explained in this manner because “MnTi2O5”is unstable phase in MnO–“TiO2” system, as discussed sub-sequently.

5.4. Stability of “MnTi2O5”

Evans and Muan11) estimated the Gibbs energy of forma-tion of “MnTi2O5” with pseudobrookite structure from the-

oretical calculations based on their experiment in theNiO–MnO–TiO2 system.

MnTiO3�TiO2�“MnTi2O5”

DG°�2.93�3.35 (kJ/mol at 1 250°C).............(1)

Due to the long extrapolation for the evaluation, they men-tioned that the value of Gibbs energy of formation wasquite uncertain.

Grieve and White did not report about this compound.However, Leusmann9) reported that no evidence of thiscompound was obtained after careful observation.Navrotsky25) also suggested that “MnTi2O5” may be unsta-ble due to the large cation radius of Mn2� ion in octahedralsite of pseudobrookite structure. Grey et al.13) measured lat-tice parameter change of the pseudobrookite at 1 200°C andconfirmed that replacement of 2Ti3� (0.670 Å) by Mn2�

(0.830 Å)�Ti4� (0.605 Å)26) leads to an expansion of theunit cell. This results in breakage of pseudobrookite struc-ture to form pyrophanite and rutile. Grey et al.13) reportedthe highest Mn concentration at 1 200°C was 0.216 in termsof nMn/(nTi�nMn). As seen in Figs. 4 and 5, pseudobrookitephase is unstable under pCO/pCO2�1 while it appears inthe limited temperature range under pCO/pCO2�9. Fromthis observation, “MnTi2O5” is not stable in MnO–“TiO2”system under relatively high oxygen partial pressure.

5.5. Comparison with Earlier Studies

Melting behavior of pyrophanite (MnTiO3) has been re-ported either incongruent melting at 1 360°C,8) congruentmelting at 1 404°C27) or congruent melting at 1 410�10°C9)

from thermal analyses. However, in the present study, it wasfound that melting temperature of pyrophanite is higher(�1 450°C) compared to previous studies and it is affectedby oxygen partial pressure.

For the spinel phase (Mn2TiO4 rich), Grieve and White8)

reported that it melts at 1 450°C congruently, whileLeusmann9) showed 1 420�10°C to be its congruent melt-ing temperature. In the present study, melting temperatureof spinel phase is slightly higher than 1 450°C. Although itis not easy to be concluded whether spinel s.s. melts con-gruently or incongruently from the experimental data in thepresent study, thermodynamic modeling study by presentauthors5) suggested that the spinel phase melted congruent-ly.

In the study of Grieve and White,8) they used molybde-num crucible as a container of samples under nitrogen orvacuum. However, for MnO–“TiO2” system which lies inrelatively high oxygen partial pressure, Mo dissolves intomolten oxide, as mentioned in Sec. 3, and it might besource of experimental error. Leusmann9) did not mentionabout crucible material in his investigation.

Experimental method employed by previous studies8,9)

were thermal analysis, which is a “dynamic method” whilethe experimental method used in the present study is chem-ical equilibration at high temperature, which is a “staticmethod”. For the phase diagram determination, “staticmethod” is prior to the “dynamic method”. Moreover,phase equilibria in the MnO–“TiO2”–“Ti2O3” system arestrongly dependent on the oxygen partial pressure in gasphase, the “dynamic method” might not present actual


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Fig. 11. Schematic diagram in the MnO–“TiO2”–“Ti2O3” sys-tem. Short-dashed lines represent spinel s.s., pyrophan-ite s.s. and pseudobrookite s.s., respectively while longdashed lines mean hypothetical equilibrium positions ofthe MnO–”TiO2”–”Ti2O3” system under fixed oxygenpartial pressures. The arrow means direction of movingphase equilibrium positions with decreasing oxygen par-tial pressure.

equilibrium state due to kinetic problem. Thus, results inthe present study seem more reliable compared to the previ-ous results.

As seen in Fig. 1, reported liquidus data by several inves-tigations1,8,9,18–20) do not agree each other. Although thosemeasurements were conducted under different oxygen par-tial pressures, liquidus of manganosite and rutile do notchange significantly by oxygen partial pressure which wasemployed in those investigations. Liquidus of rutile report-ed by Ohta and Morita19) at 1 600°C and by Amitani et al.1)

at 1 500°C seems in reasonable agreement with those in thepresent study. Liquidus of manganosite measured by Ohtaand Morita19) at 1 600°C and Amitani et al.1) at 1 500°Cshow higher MnO concentration compared to those in pre-sent study. The reason for this inconsistency is not clear atthe present stage. However, it might be considered thatequilibration molten oxide with MnO pellet causes separa-tion of small particles of MnO from pellet during dissolu-tion of MnO to molten oxide. In chemical analysis ofmolten oxide, those particles might result in overestima-tion of MnO concentration in molten oxide. Trend ofmanganosite liquidus in the MnO–SiO2–“TiO2” systemmeasured by the present authors, which will be presented innext article of this series,4) also lend strong support to theexperimental result in the present study.

Reported liquidus data by Ohta and Morita19) at 1 400°C,Ito et al.18) at 1 400°C and Kim et al.20) at 1 450°C do notagree with result obtained in the present study. At 1 400°C,liquid phase was not observed in whole concentration in thepresent study. Also, at 1 450°C, manganosite and spinel twophase region was observed in the present study while Kimet al.20) reported liquid oxide and MnO two phases.Although XRD analysis was not carried out to confirmphase relations, electron images of liquid�manganositeequilibrated at 1 500°C (Fig. 7) and spinel�manganositeequilibrated at 1 450°C (Fig. 9) with same starting compo-sition showed very different shapes (globular shape of large(hundreds of micrometers) manganosite crystal in formercase while irregular shape of small (several tens of microm-eters) manganosite crystal in latter). Given that Ohta andMorita,19) Ito et al.18) and Kim et al.20) did not check the ac-tual phase relations in their samples, the data obtained inthe present study are preferred in the construction of thephase diagram.

Amitani et al.1) reported that “2MnO·TiOx” was ob-served in the MnO–“Ti2O3” system at 1 500°C and men-tioned that “its X-ray pattern was identical to that of2MnO·TiO2. Hence, 2MnO·TiOx is thought to have thesame crystal structure as 2MnO·TiO2, forming solid solu-tion”. In their Ti4�/Ti3� analysis in the “2MnO·TiOx”, theyreported that Ti4�/Ti3� decreases with decreasing oxygenpartial pressure and finally equimolar amount of Ti4� andTi3� was observed at log pO2��17.5. Thus, it seems thatthe “2MnO·TiOx” is a part of spinel solid solution and atlog pO2��17.5, particularly, 2/3 mole of Mn2TiO4 (Ti4�)and 1/3 mole of MnTi2O4 (Ti3�) mix to form spinel solidsolution. And “2MnO·TiOx” is not actually exist in theMnO–“Ti2O3” system since it contains Ti4� predominantlyand MnTi2O4 is the only intermediate compound in the

MnO–“ Ti2O3” system.14)

6. Conclusions

The phase relations, liquidus as well as solid solubility inseveral solid solutions in the MnO–“TiO2”–“Ti2O3” systemhave been determined experimentally under controlled atmosphere in the temperature range from 1 300°C to1 550°C. High-temperature equilibration under controlledatmosphere followed by EPMA was employed to measureequilibrium compositions in equilibrium phases. The fol-lowing phases have been observed; molten oxide,manganosite, spinel, pyrophanite, pseudobrookite and ru-tile, in the present study. Stable region of pseudobrookitephase has been observed, that is, limited temperature andoxygen partial pressure. Non-existence of “MnTi2O5” inMnO–“TiO2” system was confirmed. Decreasing oxygenpartial pressure in the MnO–“TiO2”–“Ti2O3” system resultsin increasing melting temperatures of intermediating phasessuch as spinel and pyrophanite and dissolution of Ti inthese solid solutions.

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Experimental Study of Phase Equilibria in the MnO–“TiO2