U. S. Department of Commerce Research Paper RP2111 National Bureau of Standards Volume 45, July 1950 Part of the Journal of Research of the National Bureau of Standards High-Temperature X-Ray Study of the System Fe 30 4-Mn30 4 By H. F. McMurdie, Barbara M. Sullivanl and Floyd A. Mauer A series of compositions in the system Fea04-Mna04 was prepared and examined at high temperatures by X-ray diffraction methods. Although Fea04 and Mna0 4 form a con­ tinuous series of substitutional solid solutions stable at room temperatures, t hose composi­ tions co ntaining less than 60 percent of Mna04 are cubic, of the spinel type, and the rest are tetragonal. Those compositions t hat are tegragonal at room temperature pass through a two-phase region when heated and acquire the cubic spinel-type structure at high tempera­ tures. The temperature range of t he t wo-phase (exsei~ n ) region varies with composition. The relation to t hi s system of the mincrals vredenburgite and hausmannite is discussed. I. Introduction :Mason prcpared a series of compounds by co­ A few years ago, while the oxides of manganese pl'ecipi tating l11Lxtures of the hydroxide of iron were being studied in this laboratory (1] / it was and manganese and heating the precipitate at found that Mn304, on being heated, inverts from approximately 1,200 ° C to obtain the proper the tetragonal structure to a cubic form of the s tate of oxidation. In this way, homogeneous spinel type at 1,170° C . preparations covering the range from 20 to 100 The low temperature form of Mn30 4 (hausman­ mole percent of Mn30 4 were obtained. nite) is tetragonal, with a= 5.75 A and c=9.44 A (2]. A larger unit cell, which is more readily 1200 r----,--,---,---,--,--r---,---r--.--~ compared in volume and atomic arrangement to 11 0 0 that of the spinel-type (3] Mn30 4 obtained at 1000 - higher temperatures, may be constructed by con­ ONE PHASE sidering a equal to the diagonal of the cross sec­ 900 tion of the unit cell normal to c. When the two , , forms are compared in this way, it is found that 800 , , , , ,, the inversion involves only a slight distortion of ,, ,, :;> , , 700 ,, ,, the structure. As the change from tetragonal ,, \, w' , , to cubic at 1,170° C. requires little change in ,/ \ ~ 600 , \ structure, it is rapid and reversible as would be !<[ ,, , Q: , , 500 expected. ~ ,:' II ~ : I Since Fe304 and Mn304 are known to combine .....w , , 400 I TWO I to give cubic spinel-type structures (jacobsite), PHASES : the inversion of pure 1V1n304 described above 300 , suggests that a study of the system Fe30 4- : I 200 r Mn 0 4 would be of interest. I 3 : ,I II. Previous Studies 100 , The main previous work on the system Fe30C 0 i Mna0 4 is that of Mason [4], from whose disserta­ Fe 304 10 20 30 40 50 60 70 80 90 Mn 304 tion the diagram in figure 1 is reproduced. Mn 304 • MOL % FIG URE Diagram of the system Fe Or MD 0 4 after 1 Figures in brackets indicate the literature references at the eud of this 1. a a paper. Mason [4J. 35 These preparations were studied by means of that Mason expected to find. Later [7], he did powder photographs. The samples containing find minerals in this range of composition that less than 60 percent of Mn304 gave cubic patterns, consisted of a single tetragonal phase. whereas those containing more than 60 percent of Von Eckermann [8J suggested that the change Mn304 were tetragonal. The tetragonal form from the cubic to the tetragonal structure might showed a gradual increase in axial ratio with take place at a different composition if the temper­ increase in manganese content. The axial ratio atme were increased. Basing his hypothesis on varied from unity (at 60 percent of Mn304) to 1.16 Mason's paper, and on earlier work by Montoro (for pme Mn304)' Mason assumed that inasmuch [9], he suggested that compositions high in Mn30 4 as these compounds were formed near 1,200° C, might remain tetragonal up to the melting point, they represented the conditions at that temper­ whereas those richer in iron would change on ature and were not in equilibrium at room tem­ heating from cubic to tetragonal at approximately peratme. This interpretation seemed to be 1,000° C. As Mason pointed out in a later paper confirmed by the existence in natme of the mineral [10], such a change to lower symmetry on heating vredenburgite. This mineral has been found by seems unlikely. X-ray diffraction and microscopy [5,6] to be com­ When work in this laboratory disclosed that posed of two phases, one cubic and the other heating pme Mn30 4 brought about a change from tetragonal. The presence of these two phases the tetragonal to the cubic structme at 1,170° C, indicates that the mineral is formed by exsolution. it became evident that further work was needed According to Mason, this two-phase mineral to clarify the pictme of the high-temperatme represents equilibrium conditions at room tem­ portion of the system. Because of the rapid peratme, and on the basis of an analysis of the inversions that occur in this system, it is not two phases present, he indicated an area of im­ practical to investigate the phase relations by miscibility in the portion of the system between examination of quenched samples. High-tem­ 54 and 91 percent of Mn304' His approximation peratme X-ray diffraction studies seemed to be of the region in which exsolution would occur is the only practical method of attacking the prob­ outlined in figure 1. He did not indicate the posi­ lem, and, since suitable equipment was available tion of the houndary between the cubic and the [11], work was undertaken to revise the diagram tetragonal phases in the one phase region. of the system. In Mason's smvey of the minerals comprising the system Fe304-Mn304, he fixed limits of com­ III. Method and Materials position for four minerals: magnetite, jacobsite, vredenbmgite, and hausmannite. According to The high-temperatme X -ray diffraction appara­ him, "Vredenburgite signifies those substances tus described by Van Valkenburg and McMmdie whose composition lies within the area of im­ [11] was employed to hold the specimens at various miscibility, i. e., between 54 and 91 percent temperatmes while the diffraction patterns were Mn30 4'" The name hausmannite was given to being made. Unfiltered FeK radiation was used, substances in this system containing from 91 to and the patterns were automatically recorded on 100 percent of Mn304' The remainder of the a Geiger-counter spectrometer. The equipment system, from pme Fe30 4 to 54 percent of Mn304, used can record only those lines for which the was arbitrarily divided at a composition corre­ Bragg angle is less than 45°. For this reason, it sponding to 50 percent of Fe30 4 and 50 percent of was not possible to determine the cell dimensions MnFe204, that is, at 16.7 percent of Mn30 4. with the accuracy attained by Mason. Substances containing less than 16.7 percent of A series of mixtmes at intervals of 5 percent Mn304 were called magnetite, and those containing by weight 2 from 60 to 100 percent of Mn304 was from 16.7 to 54 percent of Mn30 4 were designated prepared by a method similar to that of Mason. jacobsite. The two-phase mineral vredenbmgite Manganous and ferrous sulfates were weighted was believed by Mason to represent equilibrium out in the proper proportions and dissolved. conditions at room temperatme and was therefore The hydroxides were coprecipitated with N aOH called i1-vredenbmgite. The name a-vreden­ 2 Although Mason's compositions are ex pressed in mole percent and those bmgite was reserved for a high-temperatme form in this paper in weight percent, the difference iu this system is very slight. 36 Journal of Research and washed by centrifuging until free from sul­ to assume that the samples wer e at equilibrium fates. The precipitates were then heated at when held at one temperature for half an hour. 1,200 0 0 for approximately 1 hours to convert In order to prevent oxidation of the sample at them to the desired state of oxidation. Analysis temperatures between 200 0 and 1,000 0 0, it was of several of the samples for iron and manganese necessary to pass a stream of nitrogen through showed that the composition was as originally the furnace. In no case where nitrogen was used planned in each case. did lines of the higher oxide appear. X-ray diffraction patterns of each of the samples were recorded at room temperature and at various IV. Results elevated temperatures. vVhen a pattern was Table 1 shows the phases present and the unit­ recorded at a high temperature, the sample was cell dimensions 3 for eaeh of the samples at various usually held at that temperature for about half temperatures . These data show that, at room an hour. Recording the pattern required about temperature, each sample consists of a single 40 minutes, after which the sample was raised to phase, the one containing 60 percent of Mn30 4 the next temperature without intermediate cooling. being cubic while all others are tetragonal. When In a few cases samples were held at one tempera­ the samples containing 65 to 95 percent of Mn30 4 ture for periods up to 24 hours. The change in the relative amounts of the two phases during 3 As mentioned previously.