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A CLOSE LOOK AT ELECTROLYTIC MANGANESE DIOXIDE (EMD) AND THE γ-MnO2 & ε-MnO2 PHASES USING RIETVELD MODELING D. E. Simon, R. W. Morton, and J. J. Gislason DES Consulting, 5561 Chickering Court, Bartlesville OK 74006 Route 1 Box 343, Bartlesville OK 74003 4733 Dartmout Drive, Bartlesville OK 74006 ABSTRACT Electrolytic Manganese Dioxide (EMD) material was analyzed by Rietveld refinement of x-ray diffraction patterns to answer the question, “Is EMD composed of gamma- or epsilon-MnO2?” An electron diffraction study of EMD using a 20 nanometer spot size electron beam reported observing only the ε-MnO2 structure and no γ-MnO2 structure. However, a Transmission Electron Microscopy (TEM) study of EMD observed only γMnO2 structure as revealed by atom planes with a 0.4 nanometer spacing along with crystal twinning. Rietveld refinement results of EMD x-ray diffraction patterns indicate that EMD can be adequately described using both the gamma- and epsilon-manganese dioxide (γ-MnO2 and ε-MnO2) phases with an occasional occurrence of pyrolusite (βMnO2). It is proposed that the ε-MnO2 structure observed in both electron and x-ray diffraction patterns is only a signature of a disordered manganese occupancy of the long range hexagonal oxygen framework and not a discrete phase, and EMD material predominately composed of short range ordered γ-MnO2. INTRODUCTION X-ray diffraction (XRD) has been one of the physicochemical properties frequently used to probe the secrets of the excellent battery activity exhibited by EMD. Much of the voluminous literature on this subject is cited in several reviews [1-4]. The EMD’s employed in alkaline cells typically exhibit poor quality powder patterns, which are described at best as a small number of broad peaks on top of an undulating background (Figure 1). The peak positions and widths vary among samples deposited under different conditions. The pattern characteristics observed signify disorder, which has been thought to be the origin of the battery activity, especially since the closely related polymorphs of EMD, i.e., pyrolusite (β-MnO2) and ramsdellite (an uncommon mineral), possess a high degree of order but poor alkaline battery activity. Battery activity has also been associated with chemical non-stoichiometry, in which Mn3+ ions and protons substitute for Mn4+ ions in the MnO2 lattice .
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The crystal structure of EMD is closely related to the beta-, epsilon-, and gammapolymorphs of MnO2, all comprised of a hexagonally close packed lattice of O2- anions with the Mn4+ cations filling one-half the octahedral sites in the oxygen lattice. The difference in the above polymorphs lies in the arrangement of the Mn4+ within the octahedral sites. In both forms, the [MnO6] octahedrons link to other [MnO6] octahedrons so as to produce [MnO6] chains parallel to the c-axis and forms tunnels between these chains. In pyrolusite (Figure 2), the [MnO6] units form (1x1) tunnels, whereas, in ramsdellite (Figure 3), characterized by Bystrom , the [MnO6] octahedral units form (1x2) tunnels. De Wolff  analyzed some line-rich γ-MnO2 samples and suggested that γ-MnO2 could be described as a random intergrowth of layers of ramsdellite (Figure 4) and pyrolusite (Figure 5) where Prr is the probability of two pyrolusite being adjacent to each other in the structure. With certain assumptions, De Wolff accounted for the line shifts.
SSA --- 29.6 m 2/gm
2t heta (degrees )
Figure 1. Typical X-ray diffraction pattern of an EMD material.
The γ-MnO2 and ε-MnO2 concepts were coupled, well before Chabre and Pannetier’s work, giving a useful correlation between the XRD pattern of EMD and the deposition current density. This was done through the introduction of the “Q ratio” of the peak heights at 2Θ = 22.0o and 37.0o (Cu Kα radiation), these being the most characteristic peaks of γ- and ε-MnO2, respectively . Preisler  and later investigators [11,12] found that the Q ratio (referred to as the γ-/ε- character) decreases as the deposition current density increases. The B.E.T. surface area and other related features of porosity (e.g., pore volume & bulk density) also monotonically change as the deposition current density increases, and hence these properties correlated with the Q ratio . Simon, Andersen, and Elliott  described an EMD model using Rietveld refinement analysis of EMD x-ray diffraction patterns where it was proposed that EMD material is composed of γ-MnO2, and ε-MnO2 plus or minus β-MnO2 phases. They showed that their model worked very well with samples having specific surface areas ranging from 10 to 86 m2/gm. Their model assumes that EMD is characterized as a binary mixture of γMnO2 and ε-MnO2 crystallites with different crystallite domain sizes based on Rietveld refinement analysis. The major conclusion reached was that the crystallite domain size of the ε-MnO2 is approximately 3 times that of the γ-MnO2. Also, the ratio of ε-MnO2 to γMnO2 was essentially constant at a value of 1.5 throughout the surface area range. In a second paper, Simon, Andersen, and Elliott  described a revised structural model for EMD material composed of only small crystallite domain sized crystals of γ-MnO2 in the range of 15 to 50 angstroms. The ε-MnO2 diffraction pattern portion was described as the signature of the oxygen framework with a crystallite domain size approximately 3 times that of the γ- MnO2 and probably not a discrete phase in EMD materials. The goals of this study were (a) to apply Rietveld refinement by assuming a binary mixture of manganese dioxide phases and thus to treat such a model quantitatively and (b) to describe a physical model of the EMD structure (“in light the reported” isn’t good English) that explains the reported contradiction between transmission electron microscopy and electron diffractometry. EXPERIMENTAL Sample Preparation: Eight commercially available EMD samples were chosen for this study and ground to less than 2.5 nanometer particle size for use in x-ray diffraction. Patterns were obtained with a Siemens D-500 Experimental X-Ray Patterns: diffractometer, with Cu Kα radiation from a long, fine-focus tube. A curved graphite monochrometer served to suppress the Kβ and background radiation. Settings were: tube voltage = 40 kV; tube current = 35 mA; detector voltage = 1010 V; 0.1 degree receiving slit, 1 degree scatter slit; 0.02 degree 2Θ /step; 2 sec/step time interval; scan range = 4120 degrees 2Θ . The collected data were directly used in the Rietveld refinement.
Rietveld Procedure: The instrumental background profile was determined from an x-ray scan of a “low background holder” without a mounted sample. The background profile is similar to the background encountered for the pyrolusite diffraction pattern (Figure 6). Once the background shape is numerically defined, the net x-ray intensity above background is assumed to be contributions from the phases in the sample. Crystallographic descriptions of the phases were as follows: Bystrom’s  data for ramsdellite were used for γ-MnO2 and the data of De Wolff et al.  were used for εMnO2. These references provided the space group assignments and initial input data for the unit cells, atom positions and thermal parameters. Rietveld refinement was applied to each sample, and the lattice parameters and peak width parameters were refined to fit the pattern. The thermal parameters for the Mn and O were set equal in both phases. Output from the refinement, in addition to the XRD patterns for the component MnO2 phases, includes the lattice parameters, weight percent, and crystallite domain size of each phase. The peak width at half-maximum height is related to the crystallite domain size through the Scherrer equation . Williamson Hall  calculations were perfomed using the peak width relationship with respect to 2Θ obtained from the Rietveld refinement to separate the strain contribution to observed peak width from the crystallite domain contribution to the observed peak width. Both γ-MnO2 and ε-MnO2 structures (and β-MnO2 when present) contribute to x-ray diffraction intensity, and together with background, make up the entire pattern. The intensity contribution from each phase and background are additive for every 2Θ data Table 1. Data obtained from the Rietveld refinement of x-ray diffraction patterns on eight commercially available EMD materials.
point. The lattice parameters of both phases are allowed to vary along with the peak width parameters from which the crystallite domain sizes of both phases are estimated. The background contribution is modeled as a smooth declining curve with increasing 2Θ angle and is based on the characteristics similar to x-ray diffraction patterns of β-MnO2 (Figure 6). RESULTS AND DISCUSSION General Pattern Features Figure 7 shows a typical Rietveld refinement of an EMD material and includes the raw and refined XRD patterns, the difference pattern (raw – refined), and the refined patterns for the two individual phases modeled. Results of the Rietveld refinement are tabulated in Table 1. Noteworthy features from the refinement plot and data in Table 1 are as follows: a. The calculated model defines the experimental patterns very well. b. The background is low and uniform, in contrast to what was considered to be a wavy undulating background is actually the lower relative intensity diffraction peaks from the small crystallite domain γ-MnO2 phase. c. The γ-MnO2 peaks are much broader than the ε-MnO2 peaks. d. The reported weight ratio of γ-MnO2 and ε-MnO2 is roughly 1:1. When present, β-MnO2 does not appear to change this relationship. e. The γ-MnO2 crystallite domain size (CDSs) using the Scherrer Equation varies from 30 to 40 Å. This is in agreement with crystallite domain sizes using Williamson-Hall calculations (CDSwh), based on the refined peak width parameters, show that the γ-MnO2 CDSwh is 30 to 40 angstroms and are strain free crystals. For ε-MnO2, the CDSs is approximately 120 Å. However, the CDSwh for ε-MnO2 calculates at greater than 2000 angstroms with high strain values evident (1.2 – 1.9 x 10-3 ∆ L/L values). Typical Williamson-Hall plots for both γ-MnO2 and ε-MnO2 phases are shown in Figures 8 and 9. Transmission Electron Microscope Image Study Transmission Electron Microscopy (TEM) was performed on several EMD samples and reported earlier by Simon, Andersen, and Elliott . The samples were ultrasonically dispersed and then transferred to a carbon grid and lattice fringe images obtained. Figure 10 is an example of an EMD sample from this work. This TEM indicates that the εMnO2 structure is not observed; i.e., the TEM image reveals only γ-MnO2 structure, as
By definition, ε-MnO2 is composed of a framework of hexagonally close-packed O-2 anions with one-half of the octahedral sites randomly filled with Mn4+ cations. In contrast, γ-MnO2 has one-half of the octahedral sites filled with Mn4+ cations in an ordered configuration creating 1x2 tunnels between the MnO6 octahedrons. Likewise, (β-MnO2) has Mn4+ cations arranged in one-half of the octahedral sites in another ordered configuration, creating 1x1 tunnels between the MnO6 octahedrons. This ordering or disordering of the MnO6 octahedrons can lead to different diffraction patterns depending on the area being observed (greater or less than 10 nanometers). Figure 11 show a schematic of the long range order expected in a γ-MnO2 crystal having a large crystallite domain size of hexagonal close packed oxygen framework (this schematic shows only the Mn4+ ions at one-fourth above and below the plane of O2- ions at zero and one-half, respectively). X-ray diffraction patterns of this material would typically have narrow high intensity peaks. However, in contrast, Figure 12 shows how micro-twinning by Mn4+ creates small ordered crystallite domains of γ-MnO2 within the hexagonal close-packed O-2 ion framework. X-ray diffraction patterns of this material would typically have broad low intensity peaks typical of short range order. However, since long range order of the O2- ions exists in EMD material, as evidenced by the CDSwh > 200 nanometers, one could suspect that a superlattice signature may be present in the x-ray diffraction pattern. In EMD x-ray patterns, the ε-MnO2 pattern observed is considered to be this signature of the Mn4+ ion disorder throughout the long range oxygen framework (Figure 12). In other words, the ε-MnO2 phase found in EMD x-ray diffraction patterns results from the appearance of Mn+4 disorder within the long range ordered oxygen framework. However, the γ-MnO2 pattern is the result of the short range ordering of the Mn+4 ions with a pattern similar to that of ramsdellite (Figure 3) with micro-twinning being the boundaries of the short range order crystallites. These small ordered Mn4+ domains put strain on the O2- ion long range ordering as evidenced by the Williamson-Hall strain values ranging from 1.5 to 1.9 ∆L/Lx10-3 for the ε-MnO2 structure. Another criteria for this reasoning is the fact that the ε-MnO2 to γ-MnO2 concentration ratio is constant at 1:1. Thus mass is conserved between the two structures when refining the same crystalline arrangement with two different structures at the same time. CONCLUSION From the nature of electro-crystallization, we suggest that the large CDSwh disordered εMnO2 structure found in EMD X-ray diffraction patterns represents the dimensions of the hexagonal close-packed O2- framework, possibly due to the intersection of growth sites, which emanate from the nucleation sites. Within each growth site, the γ-MnO2 domain sizes represent the average distance between micro-twin boundaries.
Therefore, we propose that although the ε-MnO2 structure interpreted to be present in x– ray diffraction patterns of EMD, it is not a discrete phase for quantitative analysis. It is rather a superstructure signature of long range disordered Mn4+ ions in the ordered hexagonal close-packed oxygen framework. We consider EMD materials to be composed only of small ordered crystallites of γ-MnO2 crystallites that are related to each other by either micro-twinning within the large hexagonal close-packed oxygen framework. Therefore, depending on your point of view, EMD material must be composed of either the short range ordered γ-MnO2 or the long range disordered ε-MnO2 phase and not both phases. ACKNOWLEDGEMENTS The authors gratefully acknowledge Kerr McGee Chemical LLC for supplying the samples used in this study. REFERENCES 1. R. G. Burns, in Battery Material Symposium, Vol. 1, Brussels 1983 ed. By A. Kozawa and M. Nagayama, IBA, Cleveland, Ohio (1984) pp. 341-356. 2. J. B. Fernandes, B. D. Desai and V. N. K. Dalal, J. Power Sources 15 (1985) 209-237. 3. Y. Chabre and J. Pannetier, Progress in Solid State Chemistry 23 (1995) 1-130. 4. T. N. Andersen, Modern Aspects of Electrochem., Vol. 30, ed. by R. E. White, B. E. Conway and J. O’M. Bockris, Plenum Press, New York (1996) 313-413. 5. P. Ruetschi, J. Electrochem. Soc. 131(1984) 2737-2744. 6. A. Bystrom, Acta Chem. Scand., 3 (1949) 163; cf. ICDD # 39-375. 7. P. M. De Wolff, Acta Crystallogr., 12 (1959) 341-345. 8. P. M. De Wolff, J. W. Visser, R. Giovanoli and R. Brutsch, Chimia 32 (1978) 257259. 9. A. H. Heuer, A. Q. He, P.J. Hughes, and F. H. Feddrix, ITE Letters on Batteries, New Technologies & Medicine, 1 (2000) 76-80. 10. E. Preisler, J. Applied Electrochem. 19 (1989) 540-546. 11. T. N. Andersen, Prog. Batteries & Battery Materials 11 (1992) 105-129. 12. R. Williams, R. Fredlein, G. Lawrance, D. Swinkels and C. Ward, Progress in Battery & Battery Materials, 13 (1994) 102-112. 13. D. E. Simon, T. N. Andersen, and C. D. Elliott, ITE Letters on Batteries, New Technologies & Medicine, 1, (2000) 10-19. 14. D. E. Simon, T. N. Andersen, and C. D. Elliott, Poster presented at the 50th Annual Denver X-ray Conference, August 2001, Steamboat Springs, CO. 15. T. N. Andersen and R. G. Moody, in Prog. Batteries & Battery Materials, 13 (1994) 1-11. 16. B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Co., Inc., Reading, Mass, 1959
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