Activity–Composition Relations in MnCr 2 O 4 –CoCr 2 O 4 Solid Solutions and Stabilities of MnCr 2 O 4 and CoCr 2 O 4 at 13001C N. Koc w and M. Timucin* Metallurgical and Materials Engineering Department, Middle East Technical University, Ankara, Turkey Phase equilibria in the system MnO–CoO–Cr 2 O 3 were investi- gated at 13001C under controlled oxygen partial pressures by using the gas equilibration technique. The CoO activities in var- ious phase assemblages of the system were measured by deter- mining the partial pressures of oxygen in the gas phase for coexistence with metallic cobalt. The activity data revealed that at 13001C, MnO–CoO and MnCr 2 O 4 –CoCr 2 O 4 solid solutions exhibit mild positive departures from ideal behavior. The activ- ities in the stoichiometric spinel solutions were found to be in good agreement with those predicted from a model based on cation distribution equilibria. The standard free energy of for- mation of the compound CoCr 2 O 4 from its oxide components at 13001C was determined as 37636 J/mol, while that for MnCr 2 O 4 was found as 44 316 J/mol. I. Introduction C ERAMICS with spinel structure have attracted considerable research interest owing to their importance in a variety of technological applications, particularly in the area of electronic ceramics, refractories, and, more recently, in in situ manufac- tured structural cermets. 1 The processing of spinel-containing ceramics, particularly those involving transition metal oxides, often requires a thorough understanding of equilibrium rela- tions under controlled oxygen partial pressures at subsolidus temperatures in the oxide system of the spinel phase under con- sideration. The aim of the present work was to establish such relationships and to study the thermodynamics of phases in the system MnO–CoO–Cr 2 O 3 at 13001C. At 13001C, the system contains two distinct series of solid solutions, as indicated in Fig. 1. MnO and CoO form the (Mn 1x Co x )O solutions with rock-salt structure, while the sec- ond solid solution series is of spinel type, with MnCr 2 O 4 and CoCr 2 O 4 as the end members. The conjugation lines, shown in Fig. 1, define the compositions of coexisting rock-salt and spinel solid solutions. The thermodynamic properties of MnO–CoO solid solution series were determined in numerous previous studies. 2–7 The CoO activities, measured in the interval 6001–12501C, exhibited positive departures from ideality, with differing degrees of de- viation. In the spinel solid solution field of the MnO–CoO– Cr 2 O 3 ternary, Jacob and Fitzner 8 ascertained the activities of MnCr 2 O 4 and CoCr 2 O 4 along the MnCr 2 O 4 –CoCr 2 O 4 join at 11001C, through the heterogeneous equilibrium formalism de- veloped by Wagner. 9 Their results revealed slight positive de- partures from ideal behavior. In general, in earlier studies on spinel-containing systems, the spinel compounds were treated as perfectly stoichiometric. However, a recent investigation on the extent of spinel field in the MnO–Cr 2 O 3 system 10 indicated that manganese chro- mite, MnCr 2 O 4 , can dissolve excess manganese owing to the partial substitution of Mn 31 ions for the Cr 31 ions in octahedral sites of the spinel lattice. The extent of the nonstoichiometry thus induced was found to depend on the temperature and on the partial pressure of oxygen in the environment. In the present work, the equilibria and thermodynamics of the MnO–CoO–Cr 2 O 3 system were investigated at 13001C, with particular emphasis on the effect of nonstoichiometry in the spinel area. II. Experimental Procedure The thermodynamic data were generated by measuring the CoO activities in homogeneous solid solution fields and in heteroge- neous phase assemblages of the system through the gas equili- bration method. Phase boundaries of the spinel field were determined by quenching experiments. Details of the experi- mental procedure are explained below. (1) Materials and Sample Preparation Samples in the system were synthesized by high-temperature calcination of compacts prepared from reagent-grade powders of CoO, Cr 2 O 3 , and Mn 3 O 4 . For each sample, these oxides were weighed in the desired proportions, and then they were blended under acetone in an agate mortar and pestle with a grinding ac- tion for B1 h. The dried mixtures were pressed into 2 cm di- ameter cylindrical pellets, and the compacts were heated in a vertical tube furnace at 13001C for thermal synthesis of the phase assemblages. The reactive sintering of each sample lasted 24 h with one intermediate grinding. During the last 12 h of the treatment, a controlled atmosphere with an oxygen partial pres- sure of p O2 ¼ 10 8 atm was maintained over the pellets so that sufficiently reducing conditions were provided for the formation of required phase assemblages, precluding, however, the possi- bility of metallic cobalt precipitation. This procedure ensured homogenization of samples and shortened the duration of sub- sequent equilibration runs. (2) Equilibration Runs The equilibration experiments were carried out with the aim of determining the CoO activities throughout the system, and also for delineating the phase boundaries of the spinel field. An equi- libration run consisted in exposing a fragment of a sintered com- pact, in the hot zone of a tube furnace, to the action of a reducing gas until equilibrium-condensed phases developed. At the end of equilibration, which generally lasted 20 h, the sample was quen- ched rapidly and the phases present were determined by optical microscopy supplemented with X-ray diffraction (XRD). A cus- tom-made vertical tube furnace heated with silicon carbide resist- ance elements was available for conducting equilibration runs. A commercial PID control unit actuated with a Pt–Pt13%RH ther- mocouple helped maintain the temperatures constant within 711C of the set values. An impervious alumina tube served as the reaction chamber. The bottom of this tube was sealed with special glass joints containing separate side arms for gas J ournal J. Am. Ceram. Soc., 88 [9] 2578–2585 (2005) DOI: 10.1111/j.1551-2916.2005.00489.x r 2005 The American Ceramic Society 2578 D. W. Johnson—contributing editor *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: nkoc@metu.edu.tr Manuscript No. 11416. Received October 28, 2004; approved April 11, 2005.