THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING Department of Chemical and Metallurgical Engineering Pr ogre s s Report THE SYSTEM MnTe - MnS To'Yo Tien LT, H. Van Vlack. R. J Martin ORA Project 07137 under contract with' SELENIUM-TELLURIUM DEVELOMENT ASSOCIATION) INCo NEW YORK9 NEW YORK administered through: 71'FFI CE OF RESEARCH ADMINISTRATION ANN ARBOR September 1967

ABSTRACT The phase relationships in the system MnTe - MnS were studied. All phase changes were found to be rapid, therefore differential thermal analysis was used to determine the phase diagram. A eutectic wa.s located at about 90 mole % MnTe at 900~C, Although no solid solution formation could be detected at the MnS side of the system, 8 mole % of MnS ent;ered the solid solution in the high temperature (Na,Cl) form of MnTe at 800~C; a eutectoid was found at 700~C, where 5 mole lo MnS were dissolved in the low temperature (NiAs) modification. The MnTe solid solutions exsolved rapidly during coolingo ii

I.o INTRODUCTION The system, MnTe - MnS, was studied to provide background information on the influence of nonmetallic inclusions upon the mechanical deformation characteristics of steel o This paper, however, is limited to the phase relationships. MnS exis'ts in three different crystal forms,2 the zince blend and wurtzite structure!>- in which. Mn2+ ions are tetrahedrally coordinated, and the rock salt structure'which has octchedral coordinationo Since the Na,Cl phase is the only stable modification,3 the two tetrahedrall]y coordinated polymorphs shall not'be conside:red in this investigatlono MnTe has a N.iAS struoture at room temperature and transforms to the cubic, Na.l st~ructure at o1040oCo 7The phase transition is rapid for'both heating and cooling so that the high temperature crystalline form could not be retained at ro om tempera't ure u Because MnO, MnS. and MnSe all crystallize in the NaCl structure and MnTe has a NaCi. modification at high temperatures^,5 solid solution formation amnong these compounds might be expectedo It is interesting to note, however, that no extensive solid solution was observed in the system MnO- MnS.6 There is a, complete series of solid solutions in the sy.stem, MnS - MnSe7 A solid so.lution series of the Na,Cl structure was observed at high temperatures of the system. MnSe = MnTeo These solid solutions exsolved at low temperatures.~ The results implied that the bond type plays a, more important role in solid solution formation rather than the ionic size. MnO is most ionic among all the compounds o Which did not, form. solid solutions with compounds is more covalent in naaiture (Mn5). From. the above results, one would probably postulate that the systems MnO - MnSe and MnO - MnTe. -will have simple entectic relationships. However, it is hard to speculate on the nature of the MnTe - MnS system. II. EXDFERIMENTAL.1. SAMPLE PREPAPRATION The samples were prepared by mixing and compacting MnTe and MnS powders. The MnTe was previously prepa-red by mixing and compacting double vacuum distilled metallic manganese and high purity tellurium in stoichiometric ratio modified with 1 wt o excess tellurium.o The compacted powders were put in graphite crucible which was sealed in an evacuated Vycor tubeo The free space 1

in the Vycor tube was made minimal to reduce the loss of tellurium. The sealed assembly was then heated slowly to about 500~C where the free manganese and tellurium reacted vigorously,. melting the MnTe which formed. Only one phase, MnTe, was detected by x-ray powder patterns and metallographic techniques. The MnS was previously prepared through the sulfur reduction of MnSO4y.l Each compact of MnTe - MnS was placed in a. graphite crucible and was then sealed in a.n evaculated Vycor tube. The samples were heated at 1250~C for four hours and furnace cooledo Although no chemica.l analysis was performed on the mixture, microscopic examination revealed no third phase. This indicated that the weighed components were all preserved at the final producto A typical microstructure is presented in Figure 1o 2. PHAOSE'.I:_;ENT IFICATION X-ray powder patterns were obtained using 1146 mm Debye-Scherrer camera and Fe K- P radiation. Mixtures of cubic MnS and hexagonal MnTe were observed in all of the compositions preparedo No lattice parameter change was noticed among different compositions, indicating no solid solution could be retained at room temperatures between these two end-members. The lattice parameter of MnS was obtained'by measuring the back reflection line and extrapolated using Nelson and Riley m.ethod.9 The hexagonal MnTe cell dimension was obtained using the lines (006) and (220) which appeared at about 120.30 and 138. 27 degree 2Q0 respecPtiv-ely o At room. temperature the.lattice paraterers are: MnS: 5 244MnTe: a, = 4.1.45A c = 60708A These values agree well with the values reported by Taylor and Kagle.10 35 DIFERTENIAL THERMAL ANALYSIS The high temperature equilibrium conditions could not be retained for examinations at room temperature even when the sealed samples (0.5 gmin) were quenched in water~ Therefore, we used a. differential thermal analysis procedure to determine the phase relationshipso The samples for differential, thermal a.nalysis were sealed in an evacuated, 4 mm V\ycor tube with a recess in the bottom to accept a thermocouple. An A1203 reference was similarly prepa.red and the two were placed within a. piece of insulating fice barick to dampen spurious temperature changes within the furnace. 2

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The furnace was controlled by a mechanically driven rheostat which increased the temperature at a, rate of about 15~C per minute The differential thermal effect was recorded on a. strip chart recorder and calibrated to with ~5~C against known-phase changes in the system Pb-Sn,ll 12 and agai.nst the a to P aqu.artz inversion, 2 -II. RES! LTS The results of both heating and cooling curves of DTA runs were used to const.rulct, the phase digaram as shown in Figure 2o From the pFints observed, there are n-. poioibl.e alternatives t o the diagram as drawn, ACKNOLEDGiME1NT The financial support of the Seleniun and Tellurium Development Associat:ion is a pp reciat.edc 4

1600 - 1400 - - I ~o~~~~: 1200 -' < L 3 - * Cooling I. > 1000^^ IP9 A Heating w F-I I I I II 800 A 600o 0 10 20 30 40 50 60 70 80 90 100 MnTe MOLE % MnS MnS Figure 2. The system, MnTe - MnS.

REFERENCES 1. H. C. Chao and L. H. Van Vlack, "Deformation of Orientated MnS Inslusions in Low Carbon Steel," Trans. AIME, 233 (7) 1227-31 (1965). 2. L. Mo Corliss, N. Elliott, and J. M. Hasting, "Magnetic Structures of the Polymophic Forms of Manganous Sulfide," Phys. Rev. 104 [4] 924-8 (1956). 5. H. Haraldson and W. Klemm, "Uher die Manganochalkogenide," Z. amorg. Allg. Chem. 220, 183 (1934). 4. W. P. Johnston and Do E. Sestrich, "The MuTe - GeTe Phase Diagram," J. Ionorog, Nucl. Chem. 19, 229-36 (1961). 5. For instance, see, A. Fo Wells, "Strutural Inorganic Chemistry," Third Edition, Oxford at the Clarendon Press (1962). 6. H. C. Chao, Y. E. Smith, and L. H. Van Vlack, "The MnO - MnS Phase Diagram," Trans. AIME, 227(6) 769-7 (1963). 7. J. Mo Metha, P. Go Riewald, and L. H. Van Vlack, "The System MnSe - MnS," J. Am. Chramo Soc. 50 [31 164-5 (1967)o 8. A. J. Panson and W. D. Johnston, "The MnTe - MnSe System," J. Inorg. Nucl, Chem. 26. 701-3 (1964). 9. J. Bo Nelson and D. P. Riley, "An Experimental Investigation of Extrapolation Methods in the Derivation of Accurate Unit-Cell Dimentions of Crystals," Proc. Phys. Soc. (London) 57, 160-177 (1945). 10, Ao Taylor and M. J. Kagle, "Crystallographic Data on Metal and Alloy Structures," Dover Publication, Inc., New York (1963). 1.. M. Hansen, "Constitution of Binary Alloys," McGraw-Hill Book Co., New York (1958). 12. R. B. Sosman, "New and Old Phases of Silica," Trans Brit. Ceram. Soc. 549 655 ( 1955)6 6