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Oxide solid solutions derived from homogeneous carbonate precursors: The CaO-MnO solid solution
dournal of the Less-Common
Metals,
116 (1986)
219 - 221
219
OXIDE SOLID SOLUTIONS DERIVED FROM HOMOGENEOUS CARBONATE PRECURSORS: THE CaO-MnO SOLID SOLUTION* K. R. POEPPEL~EIER~,
H. S. HOROWITZ2:
Exxon Research and Engineering
and J. M. LONGOO
Company, Annandale,
NJ 08801 (U.S.A.)
Summary Thermal decomposition of the mixed carbonate solid solution Cal_,Mn,CO, (0.0 g 3t < 1.0) at 1273 K in hydrogen results in the respective oxide solid solutions Ca,._.Mn,O. Thermolysis at temperatures less than 1173 K of the homogeneous carbonate phase, Cai_,Mn,COa (x = 0.5) results in incomplete solid solution and a calcium and manganese oxiderich mixture. The miscibility gap extends from 25 to 85 mol% MnO at 873 K. The decomposition reactions were conducted in a hydrogen atmosphere in order to maintain manganese in the divalent state. These results are compared with earlier decomposition studies conducted at reduced pressures of carbon dioxide and in uucuo. The important role of the decomposition mechanism in the application of the solid solution precursor method to solid state reactions is emphasized.
1, Introduction The four binary systems CaO-MnO, CaS-MnS, Case-MnSe and CaTeMnTe share common features because all the compounds are isomorphous. The compounds have the sodium chloride structure at all temperatures with the exception of MnTe, which has the NiAs structure below 1313 K. Full solid miscibility below their liquidus-solidus curves exists in each system [ 1, 21. The three systems CaS-MnS, Case-MnSe and CaTe-MnTe have a miscibility gap at lower temperatures [ 23, Convincing evidence for a miscibility gap in the CaO-MnO system has not been reported. Early work on CaO-MnO compositions [3] prepared by thermal ‘decomposition of coprecipitated hydroxides did report incomplete solid solution at 873 K. Others [2] have suggested that a miscibility gap *Dedicated
to Professor
J. D. Corbett
on the occasion
of his 60th
birthday.
‘Present address: Department of Chemistry, Northwestern University, Evanston, IL 60201, U.S.A. $Present address: E. I. DuPont de Nemours & Co., Inc., Central Research & Development_Dept., Experimental Station, Wilmington, DE 19898, U.S.A. h Present address: Exxon Production Research Co., Houston, TX 77001, U.S.A. 0022-5088186/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
220
should form based on the similarity of the solvus curves for the sulfides, selenides and tell~ides, and the large difference in the radii of Ca*+ and Mn2+ ions This ‘paper addresses the question of a miscibility gap below 1273 K in the CaO-MnO system. The solid solution precursor technique [4] was used to prepare homogeneous carbonate starting materials Ca,_,Mn,CO, (0.0 < x < 1.0). Their ,decomposition was studied over the tempe~t~e range 873 - 1273 K. The decomposition reaction was conducted in hydrogen in order to maintain manganese in the divalent state. Our results are compared with earlier decomposition studies conducted at reduced carbon dioxide pressures [ 51 and in uacuo [ 6,7 1.
2. Experimental
details
2.1. Preparation of the precursors Solid solution precursors were prepared from aqueous solutions of calcium and manganese nitrate. These solutions were prepared by dissolving CaC03 and MnCO, in dilute nitric acid. All solutions were purged with nitrogen gas to remove dissolved oxygen and prevent manganese oxidation. The CaC03 was reagent grade and had been treated at 873 K in an atmosphere of CO2 to remove any trace of non-carbonate phases that might have been present. The pure C&O3 was then stored in an inert atmosphere until used. Reagent grade MnC03 was unacceptable because it always was found to contain a significant fraction of oxidized manganese and other noncarbonate phases. The MnCOs used in our experiments was precipitated from a manganese nitrate solution with an excess of ammonium carbonate, dried at 423 K in a vacuum oven, and stored in an inert atmosphere. Approximately 2 M ammonium carbonate was used to precipitate the solid solution carbonate. The precipitate was formed by the addition of the appropriate premixed solution of calcium and manganese nitrate to a large molar excess of the ammonium carbonate solution with good mixing. The precipitates were washed with water under a nitrogen atmosphere, dried at 423 K in a vacuum oven, and then stored in an inert atmosphere.
2.2. Solubility limits in the Cal _xMn, 0 solid solution The solid solution precursors Cai_,Mn,COa were decomposed in alumina boats at temperat~es from 873 to 1273 K. The reaction atmosphere was a lO%H-90%He mixture. Rapid heating rates or preheated furnaces were used in order to avoid phase separation into CaCO, and MnO. The temperature was measured using a chromel-alumel thermocouple situated inside the reactor near the sample. In general, 3 h was sufficient for the thermolysis of the carbonate precursors and their conversion to pure oxides. After each firing, the samples were cooled rapidly to prevent any back reaction by removing the reactor tube from the furnace while maintaining the reducing gas environment, Trace amounts of carbonate were sometimes
221
detectable in samples decomposed between 873 and 923 K, especially by IR spectroscopy. A second firing after grinding would eliminate the residual carbonate. Below 873 K the decompositions were sluggish and often incomplete. 2.3. Characterization The solid solution precursors were examined by powder X-ray diffraction using a Philips diffractometer with Cu Ka radiation and equipped with a graphite crystal diffracted beam monochromator. The diffraction patterns of all the carbonate precursors were those expected [8] for single-phase materials (calcite structure). The fired oxides derived from the decomposition of the carbonates were characterized using powder X-ray diffraction and IR spectroscopy. With diffraction they were characterized to be either single-phase, or twophase mixtures of oxides, with the rock salt structure. The lattice parameter of the cubic structure was determined using six slow scanned (0.25” min-‘) reflections from the 28 interval 50” - 130”. IR spectra were recorded on all fired products to detect the presence of unreacted carbonate. Spectra were recorded from 4000 - 400 cm-’ using a Digilab FTS spectrometer and pressed KBr pellets.
3. Results and discussion The solid solution precursor technique is an effective method that can be used to study the subsolidus region of the CaO-MnO system if conditions that result in unmixing of carbonates, partial decomposition and recarbonization [5] are avoided. Earlier studies in air or oxygen [4] showed that a variety of new low temperature phases, such as Ca$lnsOs, CaMnsO,, of CaMn40s and CaMn@iz, could be formed by oxidative decomposition the appropriate solid solution carbonate precursors. These high and mixed valent manganese-containing phases had not previously been identified because their low thermal stabilities (below 1273 K) are exceeded by the temperatures typically required for conventional solid state synthesis. The solid solution precursor technique also proved effective for preparing known, higher temperature phases in the CaO-MnO, (x > 1.0) system, such as CaMnO,, Ca2Mn0, and CaMn204, at lower temperatures and in higher surface area form than is generally possible by conventional methods. In the present study our interest was in the CaO-MnO system; thus it was important to avoid reaction conditions that could lead to the oxidation of divalent manganese. Using a flowing atmosphere of hydrogen gas, the decomposition of the mixed carbonate precursors Cai_xMnxCOs could be carried out without the complications associated with a valence change. We found that all traces of carbonate could be removed by heating at temperatures greater than 873 K. From our experiments it is not possible to describe the reaction mechanism that results in oxide formation, but it is
222
clear that the homogeneous character of the mixed calcite precursors is an important factor in the decomposition reaction. We will return to this point later in the discussion. Decomposition of the mixed carbonates Cai_xMnxCOs at 1273 K resulted in single-phase oxides Cal _,Mn,O with green-olive color. On exposure to air the samples would darken but their X-ray patterns remained unchanged, which would indicate only minor surface oxidation had occurred. Likewise, green MnO would darken noticeably with time when exposed to air. In contrast, we observed that pure CaO reacts with humid air to form a considerable amount of hydroxide [ 9 - 111. Examination of quenched Ca, _,Mn,O samples prepared at 1273 K showed that they were pure single-phase oxides with the rock salt, or sodium chloride structure. As the composition of the solid solution changed there was a linear variation of the cubic unit cell parameter characteristic of a disordered rock salt structure. These results are shown in Fig. 1. They are in agreement with previous studies of our own and others [12, 131. I
5.0
-
4.6
-
4.4
I 0
I
1
1
I
20
,
I
I
I
40 MOLE
I
I
,
,
60 PERCENT
I
,
,
1,
60
1w
Mn
Fig. 1. The variation in lattice parameter for the solid solutions CaTe-MnTe, Case-MnSe and CaS-MnS after Leung and Van Vlack [ 21 and for CaO-MnO, (this study).
223
Consequently, the deviation from precisely linear variation in cell volume AV will be negative and vary in a predictable way with the degree of substitution. Recently Otero and Arean and Stone [6] observed an anomalously large negative deviation in cell volume AV at approximately 65 mol.% MnO while employing in uucuo decomposition. The authors suggest a solution model that incorporates the larger Ca2+ ions with anion displacements in the first coordination sphere. They suggest this results in a significant and experimentally measurable less rapid increase in the cubic cell parameter a, and cell volume. The covalent character of the Mn-0 bond was advanced to account for the observed deviation, and the ability of MnO to accommodate, in a substitutional fashion, the larger calcium ion without the expected lattice expansion. Other explanations such as oxidation of some manganese and cation segregation were considered but thought unlikely. We find over the entire range of composition, including 65 mol.% MnO, only the expected negative deviation from precisely linear variation in cell volume owing to a linear variation in the cubic cell parameter. Thermal decomposition reactions of solid solutions in the CaO-MnOCO2 system are affected by the reaction conditions. It is recognized [5] that the pressure-temperature (CO,) curve for pure MnCO, decomposition (no valence change) establishes at a given temperature a carbon dioxide pressure that will prevent decomposition in the CaO-MnO-CO2 system. That is, CaCOs requires a higher temperature for decomposition at a given carbon dioxide pressure than does MnCOs. It is generally known [5, 81 that thermal decomposition in air, oxygen or reduced partial pressures of C02, and at temperatures lower than the decomposition temperature of CaCOs results in partial decomposition, i.e. calcium-rich carbonate and manganese-rich oxide. In contrast, in vucuo decomposition [14] might be expected to lead to separate phases of CaO and MnO as has been observed for the mineral dolomite CaMg(CO,),. We have looked at the decomposition reaction of the 1:l precursor Cai_xMnxCOs (x = 0.5) from 873 to 1273 K in flowing hydrogen gas. We described earlier that at 1273 K we find only a single-phase oxide Ca,_,Mn,O (x = 0.5) with a disordered rock salt structure. However, over the temperature range 873 - 1173 K we find incomplete solid solution. Decomposition of the carbonate at or above 1173 K resulted in a single-phase material that remained single phase on cooling and long-term annealing at temperatures less than 1173 K. Therefore construction of the solvus curve shown in Fig. 2 was limited to data derived from carbonate decompositions. Decompositions were carried out for 3 h in hydrogen at 873, 923, 973, 1023, 1073 and 1123 K to construct the solvus curve shown in Fig. 2. The compositions were estimated from their lattice parameters assuming a linear variation. Incomplete solid solution between CaO and MnO was first observed by Natta and Passerini [3]. We find immiscibility extending from approximately 25 to 85 mol.% MnO at 873 K. In Fig. 3 we compare the X-ray
224
1600
-
1400
-
1200
-
1000
-
600
-
0 0 SOLID SOLUTION
z ii 2 d
0 00
00
0
K F
I 0
I 20
Cd0
Fig. 2. The subsolidus
I
I
I
40 MOLE PERCENT
I 60 MnO
region and miscibility
I
I 80
I 100 MnO
gap in the CaO-MnO
system.
diffraction patterns after decomposition at 873 and 1273 K. The narrow diffraction peaks of the two-phase mixture indicate that the composition limits of the solvus curve are accurately represented when calculated from the lattice constants of the individual phase. We also observed, as did Natta and Passerini, that little interparticle reaction occurred on heating at the lower temperatures (873 - 1123 K), until rapid sample homogenization and sintering occurred between 1123 - 1173 K. Fubini and Stone [7] report that at 755 K the decomposition of Cal_XMnxCOs (X = 0.5) in uacuo leads to some formation of Mn3+ ions as evidenced by an intervalence Mn 3+-Mn2+ absorption which then disappears at 955 K. In this intermediate temperature range they have suggested that some decomposition to CO occurred by reduction of carbonate and oxidation of Mn2+ ions in the lattice. With annealing at 925 K in uucuo they found solid solution formation Cai_,Mn,O (X = 0.5). In contrast, in our decomposition studies in hydrogen of the 1 :l precursor Cal_nMnxC03 (X = 0.5) from 873 to 1273 K we find what appears to be a miscibility gap between CaO and MnO.
4. Conclusions The thermal decomposition of Cal_,MnwC03 (3~= 0.5) in hydrogen has been shown to give rise to a miscibility gap in the CaO-MnO system below 1173 K. In contrast, Fubini and Stone [7] reported that at 925 K
225
I
44
I
I
I
I
I
I
I
42
40
38
36
34
32
30
-28
Fig. 3. X-ray diffraction patterns of Cal-,Mn,O (x = 0.5) formed at 12’73 K (bottom) and 873 K (top). The Bragg (111) and (200) reflections are shown for a single-phase and two-phase mixture respectively.
this decomposition reaction, when conducted in uucuo, led to a single-phase product. Their observation of Mn3+ species during in vucuo decomposition, unlikely under the reducing conditions we have used to decompose carbonate precursor, may explain our different results. Other subtle changes in the reaction conditions can sign~icantly alter the reaction product. Excessively slow heating rates to the decomposition temperature, for example, can cause phase separation yielding CaCOs and MnO, just as is observed when low decomposition temperatures are employed. Clearly the decomposition
226
mechanism in the present investigation deserves more study. We note that the decomposition mechanism of dolomite CaMg(C03)2, where oxidation is not a problem, has also been shown to be complex. Kinetic as well as thermodynamic arguments have been used [15] to explain conflicting experimental results reported in the dolomite studies, The solvus boundary shown in Fig. 2 probably represents an equilibrium situation in the CaO-MnO system. The miscibility gap between CaO and MnO that we have observed is asymmetric with a slightly greater solid solubility toward the CaO region of the diagram. The possibility that the miscibility gap is due solely to sequential decomposition of the homogeneous state, first to a manganese-rich oxide and calcium-rich carbonate which subsequently decomposes to a calcium-rich oxide that does not back react at these temperatures to form a solid solution because of poor interparticle diffusion, cannot be ruled out. The substantial solid solubility of CaO in MnO observed in this study is perhaps noteworthy in this respect, in contrast to the negligible solid solution of CaO in MnO observed [5] at reduced CO2 pressures in the CaO-MnO-CO* system. The solid solution precursor method is a useful technique that can achieve solid-state reaction without repeated grinding and firing. Because low temperatures can be used during the decomposition reaction, the precursor method has the added advantage that materials in a sufficiently divided state can be prepared for applications where higher surface areas are important. In order to take full advantage of the precursor approach in solid-state syntheses, it is important that the decomposition mechanism be considered so as to ensure that the atomic-scale mixing which characterizes the precursor is retained throughout the decomposition process. Towards this objective, further decomposition studies are needed on metal carbonates, oxalates, hydroxides and other suitable compositions that can form isomorphous components.
Acknowledgments The authors thank Mr. J. Dunn for the preparation used in this work, and Dr. J. Brown for the FTIR spectra.
of the materials
References 1 H. Schenk, M. G. Frohberg and R. Nunninghoff, Arch. Eisenhuettenwes., 35 (1964) 269. 2 C. Leung and L. H. Van Vlack, J. Am. Ceram. Sot., 62 (1979) 613. 3 G. Natta and L. Passerini, Gazz. Chim. Ital., 59 (1929) 129. 4 H. S. Horowitz and J. M. Longo,Mater. Res. Bull., 13 (1978) 1359. 5 J. R. Goldsmith and D. L. Graf, Geochim. Cosmochim. Acta, I1 (1957) 310. 6 C. Otero Arean and F. S. Stone, J. Chem. Sot., Faraday Trans. 1, 75 (1979) 2285. 7 B. Fubini and F. S. Stone, J. Chem. Sot., Faraday Trans. 1, 79 (1983) 1215.
221
8 J. M. Longo, H. S. Horowitz and L. R. Clavenna, in S. L. Holt, J. B. Milstein and M. Robbins (eds.), Solid State Chemistry: A Contemporary Overview, Adv. Chem. Ser., 186, American Chemical Society, Washington, DC, 1980, p. 139. 9 J. Ewing, D. Beruto and A. W. Searcy,J. Am. Ceram. Sot., 62 (1979) 580. 10 D. Beruto, L. Barco, A. W. Searcy and G. Spinolo, J. Am. Ceram. Sot., 63 (1980) 439. 11 D. Beruto, L. Barco and A. W. Searcy, J. Am. Ceram. Sot., 66 (1983) 893. 12 A. H. Jay and K. W. Andrews, Nature (London), 154 (1944) 116; J. Iron Steel Inst., 152 (1945) 15. 13 F. P. Glasser, J. Am. Ceram. Sot., 45 (1962) 242. 14 H. Hashimoto, E. Komaki, F. Hayashi and T. Uematsu, J. Solid State Chem., 33 (1980) 181. 15 G. Spinolo and D. Beruto, J. Chem. Sot., Faraday Trans. 1, 78 (1982) 2631.
Metals,
116 (1986)
219 - 221
219
OXIDE SOLID SOLUTIONS DERIVED FROM HOMOGENEOUS CARBONATE PRECURSORS: THE CaO-MnO SOLID SOLUTION* K. R. POEPPEL~EIER~,
H. S. HOROWITZ2:
Exxon Research and Engineering
and J. M. LONGOO
Company, Annandale,
NJ 08801 (U.S.A.)
Summary Thermal decomposition of the mixed carbonate solid solution Cal_,Mn,CO, (0.0 g 3t < 1.0) at 1273 K in hydrogen results in the respective oxide solid solutions Ca,._.Mn,O. Thermolysis at temperatures less than 1173 K of the homogeneous carbonate phase, Cai_,Mn,COa (x = 0.5) results in incomplete solid solution and a calcium and manganese oxiderich mixture. The miscibility gap extends from 25 to 85 mol% MnO at 873 K. The decomposition reactions were conducted in a hydrogen atmosphere in order to maintain manganese in the divalent state. These results are compared with earlier decomposition studies conducted at reduced pressures of carbon dioxide and in uucuo. The important role of the decomposition mechanism in the application of the solid solution precursor method to solid state reactions is emphasized.
1, Introduction The four binary systems CaO-MnO, CaS-MnS, Case-MnSe and CaTeMnTe share common features because all the compounds are isomorphous. The compounds have the sodium chloride structure at all temperatures with the exception of MnTe, which has the NiAs structure below 1313 K. Full solid miscibility below their liquidus-solidus curves exists in each system [ 1, 21. The three systems CaS-MnS, Case-MnSe and CaTe-MnTe have a miscibility gap at lower temperatures [ 23, Convincing evidence for a miscibility gap in the CaO-MnO system has not been reported. Early work on CaO-MnO compositions [3] prepared by thermal ‘decomposition of coprecipitated hydroxides did report incomplete solid solution at 873 K. Others [2] have suggested that a miscibility gap *Dedicated
to Professor
J. D. Corbett
on the occasion
of his 60th
birthday.
‘Present address: Department of Chemistry, Northwestern University, Evanston, IL 60201, U.S.A. $Present address: E. I. DuPont de Nemours & Co., Inc., Central Research & Development_Dept., Experimental Station, Wilmington, DE 19898, U.S.A. h Present address: Exxon Production Research Co., Houston, TX 77001, U.S.A. 0022-5088186/$3.50
@ Elsevier
Sequoia/Printed
in The Netherlands
220
should form based on the similarity of the solvus curves for the sulfides, selenides and tell~ides, and the large difference in the radii of Ca*+ and Mn2+ ions This ‘paper addresses the question of a miscibility gap below 1273 K in the CaO-MnO system. The solid solution precursor technique [4] was used to prepare homogeneous carbonate starting materials Ca,_,Mn,CO, (0.0 < x < 1.0). Their ,decomposition was studied over the tempe~t~e range 873 - 1273 K. The decomposition reaction was conducted in hydrogen in order to maintain manganese in the divalent state. Our results are compared with earlier decomposition studies conducted at reduced carbon dioxide pressures [ 51 and in uacuo [ 6,7 1.
2. Experimental
details
2.1. Preparation of the precursors Solid solution precursors were prepared from aqueous solutions of calcium and manganese nitrate. These solutions were prepared by dissolving CaC03 and MnCO, in dilute nitric acid. All solutions were purged with nitrogen gas to remove dissolved oxygen and prevent manganese oxidation. The CaC03 was reagent grade and had been treated at 873 K in an atmosphere of CO2 to remove any trace of non-carbonate phases that might have been present. The pure C&O3 was then stored in an inert atmosphere until used. Reagent grade MnC03 was unacceptable because it always was found to contain a significant fraction of oxidized manganese and other noncarbonate phases. The MnCOs used in our experiments was precipitated from a manganese nitrate solution with an excess of ammonium carbonate, dried at 423 K in a vacuum oven, and stored in an inert atmosphere. Approximately 2 M ammonium carbonate was used to precipitate the solid solution carbonate. The precipitate was formed by the addition of the appropriate premixed solution of calcium and manganese nitrate to a large molar excess of the ammonium carbonate solution with good mixing. The precipitates were washed with water under a nitrogen atmosphere, dried at 423 K in a vacuum oven, and then stored in an inert atmosphere.
2.2. Solubility limits in the Cal _xMn, 0 solid solution The solid solution precursors Cai_,Mn,COa were decomposed in alumina boats at temperat~es from 873 to 1273 K. The reaction atmosphere was a lO%H-90%He mixture. Rapid heating rates or preheated furnaces were used in order to avoid phase separation into CaCO, and MnO. The temperature was measured using a chromel-alumel thermocouple situated inside the reactor near the sample. In general, 3 h was sufficient for the thermolysis of the carbonate precursors and their conversion to pure oxides. After each firing, the samples were cooled rapidly to prevent any back reaction by removing the reactor tube from the furnace while maintaining the reducing gas environment, Trace amounts of carbonate were sometimes
221
detectable in samples decomposed between 873 and 923 K, especially by IR spectroscopy. A second firing after grinding would eliminate the residual carbonate. Below 873 K the decompositions were sluggish and often incomplete. 2.3. Characterization The solid solution precursors were examined by powder X-ray diffraction using a Philips diffractometer with Cu Ka radiation and equipped with a graphite crystal diffracted beam monochromator. The diffraction patterns of all the carbonate precursors were those expected [8] for single-phase materials (calcite structure). The fired oxides derived from the decomposition of the carbonates were characterized using powder X-ray diffraction and IR spectroscopy. With diffraction they were characterized to be either single-phase, or twophase mixtures of oxides, with the rock salt structure. The lattice parameter of the cubic structure was determined using six slow scanned (0.25” min-‘) reflections from the 28 interval 50” - 130”. IR spectra were recorded on all fired products to detect the presence of unreacted carbonate. Spectra were recorded from 4000 - 400 cm-’ using a Digilab FTS spectrometer and pressed KBr pellets.
3. Results and discussion The solid solution precursor technique is an effective method that can be used to study the subsolidus region of the CaO-MnO system if conditions that result in unmixing of carbonates, partial decomposition and recarbonization [5] are avoided. Earlier studies in air or oxygen [4] showed that a variety of new low temperature phases, such as Ca$lnsOs, CaMnsO,, of CaMn40s and CaMn@iz, could be formed by oxidative decomposition the appropriate solid solution carbonate precursors. These high and mixed valent manganese-containing phases had not previously been identified because their low thermal stabilities (below 1273 K) are exceeded by the temperatures typically required for conventional solid state synthesis. The solid solution precursor technique also proved effective for preparing known, higher temperature phases in the CaO-MnO, (x > 1.0) system, such as CaMnO,, Ca2Mn0, and CaMn204, at lower temperatures and in higher surface area form than is generally possible by conventional methods. In the present study our interest was in the CaO-MnO system; thus it was important to avoid reaction conditions that could lead to the oxidation of divalent manganese. Using a flowing atmosphere of hydrogen gas, the decomposition of the mixed carbonate precursors Cai_xMnxCOs could be carried out without the complications associated with a valence change. We found that all traces of carbonate could be removed by heating at temperatures greater than 873 K. From our experiments it is not possible to describe the reaction mechanism that results in oxide formation, but it is
222
clear that the homogeneous character of the mixed calcite precursors is an important factor in the decomposition reaction. We will return to this point later in the discussion. Decomposition of the mixed carbonates Cai_xMnxCOs at 1273 K resulted in single-phase oxides Cal _,Mn,O with green-olive color. On exposure to air the samples would darken but their X-ray patterns remained unchanged, which would indicate only minor surface oxidation had occurred. Likewise, green MnO would darken noticeably with time when exposed to air. In contrast, we observed that pure CaO reacts with humid air to form a considerable amount of hydroxide [ 9 - 111. Examination of quenched Ca, _,Mn,O samples prepared at 1273 K showed that they were pure single-phase oxides with the rock salt, or sodium chloride structure. As the composition of the solid solution changed there was a linear variation of the cubic unit cell parameter characteristic of a disordered rock salt structure. These results are shown in Fig. 1. They are in agreement with previous studies of our own and others [12, 131. I
5.0
-
4.6
-
4.4
I 0
I
1
1
I
20
,
I
I
I
40 MOLE
I
I
,
,
60 PERCENT
I
,
,
1,
60
1w
Mn
Fig. 1. The variation in lattice parameter for the solid solutions CaTe-MnTe, Case-MnSe and CaS-MnS after Leung and Van Vlack [ 21 and for CaO-MnO, (this study).
223
Consequently, the deviation from precisely linear variation in cell volume AV will be negative and vary in a predictable way with the degree of substitution. Recently Otero and Arean and Stone [6] observed an anomalously large negative deviation in cell volume AV at approximately 65 mol.% MnO while employing in uucuo decomposition. The authors suggest a solution model that incorporates the larger Ca2+ ions with anion displacements in the first coordination sphere. They suggest this results in a significant and experimentally measurable less rapid increase in the cubic cell parameter a, and cell volume. The covalent character of the Mn-0 bond was advanced to account for the observed deviation, and the ability of MnO to accommodate, in a substitutional fashion, the larger calcium ion without the expected lattice expansion. Other explanations such as oxidation of some manganese and cation segregation were considered but thought unlikely. We find over the entire range of composition, including 65 mol.% MnO, only the expected negative deviation from precisely linear variation in cell volume owing to a linear variation in the cubic cell parameter. Thermal decomposition reactions of solid solutions in the CaO-MnOCO2 system are affected by the reaction conditions. It is recognized [5] that the pressure-temperature (CO,) curve for pure MnCO, decomposition (no valence change) establishes at a given temperature a carbon dioxide pressure that will prevent decomposition in the CaO-MnO-CO2 system. That is, CaCOs requires a higher temperature for decomposition at a given carbon dioxide pressure than does MnCOs. It is generally known [5, 81 that thermal decomposition in air, oxygen or reduced partial pressures of C02, and at temperatures lower than the decomposition temperature of CaCOs results in partial decomposition, i.e. calcium-rich carbonate and manganese-rich oxide. In contrast, in vucuo decomposition [14] might be expected to lead to separate phases of CaO and MnO as has been observed for the mineral dolomite CaMg(CO,),. We have looked at the decomposition reaction of the 1:l precursor Cai_xMnxCOs (x = 0.5) from 873 to 1273 K in flowing hydrogen gas. We described earlier that at 1273 K we find only a single-phase oxide Ca,_,Mn,O (x = 0.5) with a disordered rock salt structure. However, over the temperature range 873 - 1173 K we find incomplete solid solution. Decomposition of the carbonate at or above 1173 K resulted in a single-phase material that remained single phase on cooling and long-term annealing at temperatures less than 1173 K. Therefore construction of the solvus curve shown in Fig. 2 was limited to data derived from carbonate decompositions. Decompositions were carried out for 3 h in hydrogen at 873, 923, 973, 1023, 1073 and 1123 K to construct the solvus curve shown in Fig. 2. The compositions were estimated from their lattice parameters assuming a linear variation. Incomplete solid solution between CaO and MnO was first observed by Natta and Passerini [3]. We find immiscibility extending from approximately 25 to 85 mol.% MnO at 873 K. In Fig. 3 we compare the X-ray
224
1600
-
1400
-
1200
-
1000
-
600
-
0 0 SOLID SOLUTION
z ii 2 d
0 00
00
0
K F
I 0
I 20
Cd0
Fig. 2. The subsolidus
I
I
I
40 MOLE PERCENT
I 60 MnO
region and miscibility
I
I 80
I 100 MnO
gap in the CaO-MnO
system.
diffraction patterns after decomposition at 873 and 1273 K. The narrow diffraction peaks of the two-phase mixture indicate that the composition limits of the solvus curve are accurately represented when calculated from the lattice constants of the individual phase. We also observed, as did Natta and Passerini, that little interparticle reaction occurred on heating at the lower temperatures (873 - 1123 K), until rapid sample homogenization and sintering occurred between 1123 - 1173 K. Fubini and Stone [7] report that at 755 K the decomposition of Cal_XMnxCOs (X = 0.5) in uacuo leads to some formation of Mn3+ ions as evidenced by an intervalence Mn 3+-Mn2+ absorption which then disappears at 955 K. In this intermediate temperature range they have suggested that some decomposition to CO occurred by reduction of carbonate and oxidation of Mn2+ ions in the lattice. With annealing at 925 K in uucuo they found solid solution formation Cai_,Mn,O (X = 0.5). In contrast, in our decomposition studies in hydrogen of the 1 :l precursor Cal_nMnxC03 (X = 0.5) from 873 to 1273 K we find what appears to be a miscibility gap between CaO and MnO.
4. Conclusions The thermal decomposition of Cal_,MnwC03 (3~= 0.5) in hydrogen has been shown to give rise to a miscibility gap in the CaO-MnO system below 1173 K. In contrast, Fubini and Stone [7] reported that at 925 K
225
I
44
I
I
I
I
I
I
I
42
40
38
36
34
32
30
-28
Fig. 3. X-ray diffraction patterns of Cal-,Mn,O (x = 0.5) formed at 12’73 K (bottom) and 873 K (top). The Bragg (111) and (200) reflections are shown for a single-phase and two-phase mixture respectively.
this decomposition reaction, when conducted in uucuo, led to a single-phase product. Their observation of Mn3+ species during in vucuo decomposition, unlikely under the reducing conditions we have used to decompose carbonate precursor, may explain our different results. Other subtle changes in the reaction conditions can sign~icantly alter the reaction product. Excessively slow heating rates to the decomposition temperature, for example, can cause phase separation yielding CaCOs and MnO, just as is observed when low decomposition temperatures are employed. Clearly the decomposition
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mechanism in the present investigation deserves more study. We note that the decomposition mechanism of dolomite CaMg(C03)2, where oxidation is not a problem, has also been shown to be complex. Kinetic as well as thermodynamic arguments have been used [15] to explain conflicting experimental results reported in the dolomite studies, The solvus boundary shown in Fig. 2 probably represents an equilibrium situation in the CaO-MnO system. The miscibility gap between CaO and MnO that we have observed is asymmetric with a slightly greater solid solubility toward the CaO region of the diagram. The possibility that the miscibility gap is due solely to sequential decomposition of the homogeneous state, first to a manganese-rich oxide and calcium-rich carbonate which subsequently decomposes to a calcium-rich oxide that does not back react at these temperatures to form a solid solution because of poor interparticle diffusion, cannot be ruled out. The substantial solid solubility of CaO in MnO observed in this study is perhaps noteworthy in this respect, in contrast to the negligible solid solution of CaO in MnO observed [5] at reduced CO2 pressures in the CaO-MnO-CO* system. The solid solution precursor method is a useful technique that can achieve solid-state reaction without repeated grinding and firing. Because low temperatures can be used during the decomposition reaction, the precursor method has the added advantage that materials in a sufficiently divided state can be prepared for applications where higher surface areas are important. In order to take full advantage of the precursor approach in solid-state syntheses, it is important that the decomposition mechanism be considered so as to ensure that the atomic-scale mixing which characterizes the precursor is retained throughout the decomposition process. Towards this objective, further decomposition studies are needed on metal carbonates, oxalates, hydroxides and other suitable compositions that can form isomorphous components.
Acknowledgments The authors thank Mr. J. Dunn for the preparation used in this work, and Dr. J. Brown for the FTIR spectra.
of the materials
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