Copyright ©JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47.
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 [5].
267
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268
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[1]. In pyrolusite (Figure 2), the [MnO6] units form (1x1) tunnels,
whereas, in ramsdellite (Figure 3), characterized by Bystrom [6], the [MnO6] octahedral
units form (1x2) tunnels. De Wolff [7] 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.
450
SSA --- 29.6 m 2/gm
405
360
Intensity (cps)
Intensity, c/s
315
270
225
180
135
90
45
0
10
20
30
40
50
60
70
80
2t heta (degrees )
2Θ, Degrees
Figure 1. Typical X-ray diffraction pattern of an EMD material.
90
100
110
120
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Figure 2. XtalDraw© [20] rendition of pyrolusite – 1x1 tunnels.
Figure 3. XtalDraw© [20] rendition of ramsdellite – 1x2 tunnels.
Figure 4. XtalDraw© [20] rendition of a 2:1 De Wolff regular-interstratified EMD with
Prr = 0.0.
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Figure 5. XtalDraw© [20] rendition of a 2:1 De Wolff regular-interstratified EMD with
Prr = 0.5.
and broadenings in reasonable agreement with experimental data (cf. Ref. 3 for a lucid
analysis). The single chains (pyrolusite-like) of [MnO6] octahedrons in the ramsdellite
matrix are often termed “De Wolff Disorders”.
In other studies, a number of investigators found that EMD’s exhibited hexagonal
symmetry rather than the orthorhombic symmetry of γ-MnO2. In the first of these
studies, De Wolff, Visser, Giovanoli and Brutsch [8] interpreted EMD deposited at a
relatively high current density to be ε-MnO2, where the Mn4+ are distributed randomly in
one-half of the octahedral sites. The broad diffraction line at d ~ 0.42 nanometer could
not be indexed, and was rationalized as a consequence of a partial ordering between Mn4+
ions and vacancies within the octahedral sites (avoidance of simultaneous occupation by
Mn4+ ions at the center of octahedrons sharing faces). Similarly, Heuer, He, Hughes, and
Feddrix [9] interpreted transmission electron microscopy and electron diffraction patterns
of an EMD material to be ε-MnO2 and they proposed a model for ε-MnO2. Again, the
broad diffraction line at 0.42 nanometers could not be indexed and it was postulated that
this peak was due to possible vacancy ordering in the ε-MnO2.
Chabre & Pannetier [3] showed in a theoretical study that the micro-twinning of γ-MnO2
along the twin planes {021} and {061} may demonstrate features not explained by De
Wolff Disorders. The net effect of this work was to formulate a phase diagram with two
variables: percent De Wolff disorder versus percent micro-twinning. This phase diagram
accommodates ramsdellite, all chemically deposited manganese dioxide (CMD), EMD
and heat-treated EMD (which also have β-MnO2 peaks). Using numerical simulation
methods, Chabre and Pannetier [3] showed that De Wolff Disorders did not explain the
lack of resolution that characterizes the XRD pattern of most EMD’s and CMD’s. They
simulated the effect ascribed to giving the 0.42 nanometer line in the case of ε-MnO2, and
found that no line of the observed magnitude resulted; all that resulted was a step in the
diffuse background and not a peak. Therefore, they discredited ε-MnO2 as a structure in
EMD material.
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Copyright ©JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47.
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 [9]. Preisler [10] 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 [10].
Simon, Andersen, and Elliott [13] 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 [14] 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.
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272
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 [6] data for
ramsdellite were used for γ-MnO2 and the data of De Wolff et al. [8] 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 [16]. Williamson Hall [21] 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.
Weight %
CDSs - Ǻ
CDSwh – Ǻ
Strainwh –
∆L/Lx10-3
1
2
50 ± 1
26
29
50 ± 1
35
37
3
4
5
Gamma – MnO2
51 ± 1 48 ± 1 52 ± 1
35
36
30
36
35
34
< 0.01
< 0.01
< 0.01
< 0.01 < 0.01
Epsilon – MnO2
Weight % 50 ± 1 50 ± 1 49 ± 1 52 ± 1
CDSs - Ǻ
113
119
119
104
CDSwh – Ǻ > 2000 > 2000 > 2000 > 2000
Strainwh –
∆L/Lx10-3 1.59
1.58
1.46
1.86
Beta – MnO2
Weight %
ND
ND
ND
ND
6
7
8
47 ±1
37
42
52 ±1
34
38
48 ± 1
31
33
< 0.01
< 0.01
< 0.01
48 ± 1 53 ± 1 48 ± 1 46 ±1
113
101
106
103
> 2000 > 2000 > 2000 > 2000
1.67
1.77
1.87
1.86
ND
ND
ND
6 ± 0.5
Copyright ©JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47.
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 [21]. 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
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450
IB A # 6
R e f in e d P a t t e r n
400
B a c k g ro u n d
300
In ten sity (cp s)
Intensity, c/s
350
250
200
150
100
50
0
10
20
30
40
50
60
70
80
90
100
110
120
110
120
2 th e ta (d e g r e e s )
2Θ, Degrees
Figure 6. β-MnO2 XRD pattern showing smooth declining background.
450
S S A
--- 29 . 6 m 2/ gm
R e f in e d P a t t e r n
B ac k ground
D if f e r e n c e P a t t e r n
405
P t n : M n O 2 - e p s ilo n
P tn: M nO 2 - gam m a
Intensity (cps)
Intensity,
c/s
360
315
270
225
180
135
90
45
0
10
20
30
40
50
60
70
80
90
100
2 t h e ta (d e g re e s )
2Θ, Degrees
Figure 7. Contribution of background, ε-MnO2, and γ-MnO2 portions to the XRD pattern
of EMD.
Copyright ©JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47.
0.060
0.055
0.050
PWHM Cos(theta)
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4 s in(the ta)
Figure 8. Williamson Hall plot of PWHM data for the γ-MnO2 phase in EMD showing
no slope to the curve, indicating no strain in the structure.
0.060
0.055
0.050
PWHM Cos(theta)
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4 s in(the ta)
Figure 9. Williamson Hall plot of PWHM data for the ε-MnO2 phase in EMD showing
significant slope to the data, indicating strain in the structure.
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2 nm
Figure 10. TEM lattice fringe image of EMD sample [14] showing crystal lattices of 0.4
nanometers typical of γ-MnO2 crystal structure. No evidence of ε-MnO2 stucture is
observed.
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Figure 11. Theoretical long range ordering of γ-MnO2.
Figure 12. Short-range order crystallites of γ-MnO2 due to micro-twinning and De Wolff
disorder. Viewing of the whole area is representative of the ε-MnO2 disordered
superstructure.
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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.
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Copyright ©JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47.
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.
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