Raman spectra of birnessite manganese dioxides

Raman spectra of birnessite manganese dioxides

C. Juliena,*, M. Massotb, R. Baddour-Hadjeanc, S. Frangerd,S. Bachd, J.P. Pereira-Ramosd

aLaboratoire des Milieux Desordonnes et Heterogenes, CNRS-UMR 7603, Universite Pierre et Marie Curie,

4 place Jussieu, case 86, 75252 Paris cedex 05, FrancebLaboratoire de Physique des Milieux Condenses, CNRS-UMR 7602, Universite Pierre et Marie Curie,

4 place Jussieu, case 77, 75252 Paris cedex 05, FrancecLaboratoire de Dynamique, Interactions et Reactivite, CNRS-UMR 7075, 2 rue Henri Dunant, 94320 Thiais, France

dLaboratoire d’Electrochimie, Catalyse et Synthese Organique, CNRS-UMR 7582, 2 rue Henri Dunant, 94320 Thiais, France

Received 25 July 2002; received in revised form 13 January 2003; accepted 24 January 2003

Abstract

Structural features of layered manganese dioxides of the birnessite family are studied using Raman scattering spectroscopy.

This local probe is capable of analysing directly the near-neighbour environment of oxygen coordination around manganese and

lithium cations. Four types of sol–gel birnessite (SGB) are considered: lithium birnessite (Li-Bir), sodium birnessite (Na-Bir),

sol–gel birnessite (SG-Bir), and sol–gel Co-doped birnessite (SGCo-Bir). Thus, in a first approach, we consider the overall

spectral features of birnessites such as the superposition of the spectra of local structures, while the lattice modes are discussed

in the spectroscopic symmetry. Results show the specific spectroscopic fingerprints of SG-Bir single phases, the site occupancy

of Co ions in the substituted SGCo-Bir compound, and vibrations due to lithium ions with their oxygen neighbours in Li-Bir,

Li0.32MnO2�0.6H2O. A correlation between the interlayer d-spacing and the stretching mode frequencies of birnessite oxides has

been established.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Manganese dioxides; Birnessite; Raman spectroscopy; Lattice dynamics

1. Introduction

Manganese dioxides (MDO) are important in areas

such as dry-cell batteries, catalysts, and advanced

rechargeable batteries. Having good electrochemical

performance, they are attractive as positive electrode

materials for lithium cells because manganese has

economic and environmental advantages over com-

pounds based on cobalt or nickel [1]. Many of the

MDO materials with similar gross structural features,

namely the MnO6 octahedral units building the MDO

framework, nevertheless show a diversity of structural

and physico-chemical properties depending on the

specific synthetic route [2]. The structural differences

are commonly attributed to variations in particle size

and the type of defect chemistry.

Despite the largest part of studies related to lithium

intercalation in manganese oxides that have been

devoted to those with spinel-type lattice, other porous

0167-2738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0167-2738(03)00035-3

* Corresponding author. Tel.: +33-1-44-27-45-61; fax: +33-1-

44-27-45-12.

E-mail address: [email protected] (C. Julien).

www.elsevier.com/locate/ssi

Solid State Ionics 159 (2003) 345–356

structures have been considered such as layered bir-

nessite (y-MnO2) or phyllomanganates [3–9]. The

birnessite-type manganese oxides are a class of lay-

ered manganese oxides which are both found in nature

(soils, ore deposits, marine nodules, etc.) and pro-

duced synthetically (hydrothermal, sol–gel, etc.).

Fig. 1. (a) Layered structure of sodium birnessite. The interlayer distance d= 7.1 A corresponds to two consecutive superimposed MnO6

octahedral sheets. The trigonal prismatic sites are occupied by sodium ions. (b) Layered structure of alkali-free birnessite. The interlayer distance

d= 7.25 A corresponds to two consecutive nonsuperimposed MnO6 octahedral sheets. The interlayer space defines trigonal antiprismatic and

trigonal pyramidal sites. (c) Schematic representation of metal ion inserted sites in the interlayer of a hydrated birnessite structure (adapted from

Ref. [4]).

C. Julien et al. / Solid State Ionics 159 (2003) 345–356346

They are characterized by a layered structure com-

prised of edge-sharing MnO6 octahedra, with water

molecules and/or metal cations occupying the inter-

layer region as shown in Fig. 1 (adapted from Ref.

[4]).

In the area of positive electrodes for lithium-ion

batteries, extensive investigations on the requirements

of optimum-ideal electrode systems have shown that

the structural characterization and the ultimate goal of

correlating structural features with physical and chem-

ical properties demand the use of a broad range of

techniques that could provide the information to

establish the sought correlation. Vibrational spectro-

scopy such as Raman scattering (RS) is one of the

most powerful techniques available for materials

characterization [10–16]. Because it is sensitive to

amorphous components and those with short-range

order, RS yields a more complete and reliable descrip-

tion of materials such as the manganese oxides, where

crystalline disorder may be expected.

Since Raman scattering spectroscopy affords a

convenient and powerful local probe for the structural

evaluation of oxide materials, we extended our inves-

tigations to MDO birnessites prepared by sol–gel

synthesis. RS spectra of sol–gel birnessites are

reported and compared with those of other lithiated

MDO. This local probe is capable of analysing

directly the near-neighbour environment of oxygen

coordination around manganese and alkali cations.

Thus, as a first approximation, we consider the

birnessite vibrational spectra such as the superposition

of the spectra of local structures. Samples include

lithium birnessite (Li-Bir), sodium birnessite (Na-Bir),

sol–gel birnessite (SG-Bir), and sol–gel Co-doped

birnessite (SGCo-Bir). The lattice modes are dis-

cussed in the spectroscopic symmetry and the vibra-

tions due to lithium ions with their oxygen neighbours

are identified in the Li-birnessite lattice.

2. Experimental

2.1. Sample preparation

There is a wide variety of layered Mn(IV) oxides,

like birnessite [17–20], 2-D ranceite [21], and bur-

esite [22,23], which differ by the interlayer distance,

the ion-exchange capacity, the interlayer cation con-

tent, the manganese oxidation state, the IR features. A

way of synthesis for birnessite compounds (that we

called «classical birnessite») is the Stahli’s method

[18], consisting in an oxidation of Mn(OH)2 with O2

or Cl2 in NaOH or LiOH salt, which leads to the Na-

birnessite with the formula Na4Mn14O27�9H2O or

Na0.32MnO2�0.67H2O, named Na-Bir, or to the lithi-

ated compound Li0.32MnO2�0.67H2O, named Li-Bir,

that can be obtained. Sol–gel birnessites, doped with

or without cobalt named SGCo-Bir and SG-Bir,

respectively, were prepared from an acid treatment

of mixed oxides AMnO2 or AM1� xCoxO2 (A=Na+,

K+). During this treatment, a disproportionation of

Mn3 + into insoluble Mn4 + and soluble Mn2 + takes

place and alkali ions (A) are released into the solution.

The MDO birnessite compounds were synthesized

according the sol–gel procedure described elsewhere

[4,5]. Li-birnessite was prepared using the procedure

mentioned by Li and Pistoia [24]. This method con-

sists in the direct synthesis using Mn(NO3)2 in a 2 M

LiOH solution kept below 5 jC. Stirring in air was

effective in transforming the Mn hydroxide into

hydrated birnessite. Water was removed by heating

at 250 jC. The Li0.52MnO2.1 oxides were prepared by

ion-exchange reaction via the Na0.45MnO2�0.6H2O

sol–gel precursor. The LT-Li0.52MnO2.1 compound

obtained from the precursor heated at 300 jC consists

in the mixture of a hexagonal birnessite and a spinel

phase. The HT-Li0.52MnO2.1 compound obtained from

the precursor heated at 600 jC indicates a single

spinel phase with a = 8.21 A.

2.2. Instruments

The structure of the samples was characterised by

X-ray powder diffraction (XRPD) using a diffractom-

eter (Philips model PW1830) with nickel-filtered

CuKa radiation (k = 1.5406 A). The diffraction pat-

terns were taken at room temperature in the range of

5j< 2h < 80j using step scans.

Raman scattering spectra were taken between 4 and

1200 cm� 1 at room temperature in a quasi-backscat-

tering configuration. A Jobin-Yvon (model U1000)

double monochromator with holographic gratings

and a computer-controlled photon-counting system

was used. The laser light source was the 514.5-nm

line radiation from a Spectra-Physics 2020 argon-ion

laser. To have a high signal-to-noise ratio, each RS

C. Julien et al. / Solid State Ionics 159 (2003) 345–356 347

spectrum is the average of 12 successive scans ob-

tained at a spectral resolution of 2 cm� 1. The fre-

quency stability and the accuracy of the apparatus were

checked recording the Raman spectrum of silicon. To

avoid sample photodecomposition or denaturation, RS

spectra were recorded using a low excitation power of

10 mW. An increase in lattice temperature generally

results in a shift of Raman peak wavenumber up to the

formation of Mn3O4 oxide. All powders were pellet-

ized to obtain a mirror-like surface sample for Raman

analysis.

Raman spectra were fitted using the GRAM/386

software from Galatic Industries. The curve analysis is

based on the original algorithm of nonlinear peak

fitting described by Marquardt [25] and known as

the Levenberg–Marquardt method. The fitting calcu-

lation was done assuming a linear baseline for the

spectra and that all the Raman lines introduced in the

fit have a Lorentz line shape [26].

3. Results and discussion

3.1. Structure and symmetry

Fig. 2 shows the XRPD diagrams of the four

varieties of manganese dioxides exhibiting the birnes-

site-type structure. The X-ray diffraction patterns of

birnessite-type MnO2 powders are of rather poor

quality and consist at best of a small number of sharp

and broad lines on top of a diffuse background. This

experimental fact suggests a lattice with structural

defects. At low 2h angles, i.e. around 2h = 12j, thepredominant feature of XRPD corresponds to the

basal spacing of the lamellar structure. The peak

position of the (001) Bragg line shows a marked shift

with the insertion of alkali ions in the interlayer

region.

Depending on the synthesis route, a combination

of Mn(IV)/Mn(III) or Mn(IV)/Mn(II) is found in

birnessite. For example, the mean oxidation state of

manganese in sol–gel materials and the classical

birnessites (prepared from Stahli’s method) varies

in the range 3.6 < ZMn < 3.8. In the case of SG-Bir

and SGCo-Bir [20,27], we have a mixture of Mn(IV)

and Mn(II) while a combination of Mn(IV) and

Mn(III) is found for the classical birnessites. In both

cases, the precise determination of the structure of

material has never been performed. Nevertheless,

these phases are known to exhibit a hexagonal or

monoclinic symmetry, one like the layered CdI2-type

structure consisting of single sheets of edge-sharing

[MnO6] octahedra and water molecules between

layers with Mn2 + or Mn3 + located between the water

layer and oxygen of the [MnO6] slab in order to

counterbalance the charge defects in MnO6 sheets.

The sequence along the c axis is then

O�MnIV � O�MnIIIðþÞ or IIð*Þ � H2O

�MnIIIðþÞ or IIð*Þ � O�MnIV � O; ð1Þ

where (+) denotes classical birnessite and (*) SG-Bir

compounds. The orthogonal distance between two

consecutive slabs of [MnO6] is around 7 A. In the

case of SGCo-Bir, we have previously shown that

Co3 + ions are incorporated into the MnO6 sheets as

Co3 + is substituting for Mn ions [28]. The chemical

formula of the different birnessite compounds are

reported in Table 1.

Sodium ions are localized in trigonal prismatic

(TP) sites between the superimposed MnO6 octahe-

dral layers (Fig. 1). Concerning the SG-Bir com-

pound, MnO1.86�0.6H2O, the structure is trigonal

(P3� m1 S.G.) with the unit cell parameters a = b =

2.85 A and c = 7.25 A. The increase of the interlayer

distance di with respect to the sodium-containing

phase is consistent with the removal of all alkali ions

from the interlayer region. The preferential orientation

shown by the strong appearance of the (001) and

(002) Bragg lines corresponds to the alignment of

MnO6 layers parallel to the (a,b) plane. For this

compound, the sheets formed by the MnO6 octahedra

are not superimposed. Thus, two types of site appear:

trigonal antiprismatic (TAP) and trigonal pyramidal

(TY) sites [29]. Upon insertion of lithium ions, the

hexagonal birnessite phase transforms into monoclinic

phase (C2/m S.G.) with a = 5.13 A, b = 2.85 A, c =

7.12 A, and b = 103j for x(Li) = 0.35. The interlayer

distance decreases linearly with increasing Li content

due to the 7% shrinkage of the host lattice. With a d-

spacing of 6.94 A, the Li0.32MnO2�0.6H2O compound

is typical of a 7-A birnessite material. Based on

structural consideration the decreasing interlayer dis-

tance is attributed to the less electrostatic repulsion

between MnO6 sheets.


Fig. 2. X-ray powder diffraction diagrams of birnessite-type manganese dioxides. (a) Li-Bir, Na-Bir and SG-Bir, (b) SGCo-Bir.


3.2. Raman spectra of birnessite compounds

First, it should be noted that Raman technique is

useful for analysing the local structure of manganese

dioxides, especially for samples with poor crystallin-

ity, for which it is difficult to apply the Rietveld

refinement of the XRD data. Recently, Julien et al.

[30] have shown that elucidation of the quantitative

determination of the structural disorder present in g-

MnO2 is accurate by Raman scattering spectroscopy.

The Raman data of manganese dioxides with the g-

type structure were treated by a local environment

model, which allowed to consider a relationship

between the band frequency and the pyrolusite inter-

growth that corresponds to the structural De Wolff

defects. Among the RS spectra of manganese oxides

reported in the literature, we did not find any work

dedicated to birnessite-type MnO2.

Second, a clear assignment of Raman lines to

lattice vibrations has been possible in two cases, i.e.

the symmetric stretching vibrations of MnO6 above

600 cm� 1, and the low-frequency Raman features

associated with Li–O vibrations. We tentatively try to

improve the mode identification by using a micro-

scope Raman apparatus, but the submicrometer size of

sol–gel powders did not allow to orientate properly

the samples.

Fig. 3 shows the Raman spectra of four manganese

oxides having the birnessite-type structure. The gen-

eral peculiarity of the vibrational features of birnes-

site-type MnO2 is their low Raman activity. Three

major features can be recognised at 500–510, 575–

585 and 625–650 cm� 1. The two high-wavenumber

bands are dominating all spectra, while bands in the

low-frequency region appear with a rather weak

intensity. One observes significant spectral modifica-

tions as a function of the framework structure, i.e. the

nature of ions located in between the basal MnO6

sheets. Raman frequencies of the birnessite-type

MnO2 are presented in Table 2.

Some general remarks can be done concerning the

interpretation of the vibrational spectrum of birnes-

site-type MnO2. First, we can analyse the vibrational

spectra from two points of view, which of course do

not exclude each other. Either determine the symmetry

properties of the vibrational bands or try to assign the

observed frequencies to vibrations of defined atoms or

groups of atoms or consider local vibrations and

mixed vibrations. Second, a local vibration corre-

sponds to a vibrational mode of localized atom or

group of atoms (such as a complex anion), which is

enough uncoupled from the rest of atoms in the lattice

for easy determination, while a mixed vibration is not

localized on a given atom or small group of atoms.

Third, as the manganese atom is about five times

heavier than oxygen atom, the vibrations of the Mn–

O groups are supposed to involve mainly the oxygen

atoms.

According to the group theoretical calculation,

MDO materials with monoclinic C2/m space group

(Z = 4), i.e. C2h3 spectroscopic symmetry, are predicted

to show 9 Raman-active modes with 3 Ag + 6 Bg

species and 12 infrared-active modes with 4 Au + 8

Bu species. Experimental results show that seven peaks

at 280, 378, 410, 490, 510, 585, and 625 cm� 1 are

detected in the RS spectrum of the monoclinic Li-Bir

phase, while three bands at 500, 578, and 640 cm� 1

are recorded in the RS spectrum of Na-Bir (Fig. 3).

The discrepancy between group factor analysis and

Raman data is attributed to the defect chemistry and

Table 1

Designation and chemical formula of the Li–Mn–O compounds studied in this work

Designation Compound Formula

Na-Bir Na0.32MnO2�0.6H2O Na0.32[Mn4 +]0.68[Mn3 +]0.32O2�0.6H2O

Li-Bir Li0.32MnO2�0.6H2O Li0.32[Mn4 +]0.68[Mn3 +]0.32O2�0.6H2O

SG-Bir MnO1.86�0.6H2O [Mn4 +]0.84[Mn2 +]0.16O1.84�0.6H2O

SGCo-Bir Mn0.85Co0.15O2�0.6H2O [Co3 +]0.15[Mn4 +]0.72[Mn2 +]0.13O1.80�0.6H2O

spinel E-LiMn2O4 Li[Mn4 +Mn3 +]O4

NMD romanechite (R)2Mn5O10�xH2O

EMD g-MnO2 MnO2�0.16H2O

m-LMO LiMnO2 monoclinic phase Li[Mn3 +]O2

LT-LMO Li0.52MnO2.1 layered phase

HT-LMO Li0.52MnO2.1 spinel + hexagonal phase


the local disorder of the birnessite structure but a

trivial explanation would just be that some peaks have

too little intensity to be detected. The presence of

vacancies in the MnO6 sheets of MnO2�nH2O com-

pounds breakdowns the crystal symmetry and lowers

the spectroscopic symmetry. RS spectra of synthetic

birnessites show variation in band position and, to a

larger extent, relative band intensity (Fig. 3).

Nevertheless, the general similarity of the spectra

suggests that samples are characterized by the same

basic structure. The basic structure of the birnessite

group has been inferred to be similar to that of

chalcophanite [31]. MnO6 octahedral layers have a

vacancy in one out of every six octahedral sites. These

are separated by layers of lower-valent cations (Li+ or

Na+) and by layers of water. Lithium birnessite is

distinguished from other compounds by the presence

of well-resolved Raman bands and the appearance of a

low-wavenumber peaks located at 280 and 410 cm� 1.

Assignments for this peak at 280 cm� 1 will be

discussed in Section 3.3. The presentation of the

Raman features of birnessite compounds follows the

classification based on the nature of the polymer-

isation of MnO6 units, in which six oxygens surround

a central manganese cation in approximately octahe-

dral coordination. As the degree of polymerisation can

be determined by Raman spectroscopy, it has been

shown that Raman shifts are correlated with the M–O

bond order and bond lengths [32].

Table 2

Raman peak wavenumber (in cm� 1) of birnessite-type manganese

dioxides

Mode SG-Bir SGCo-Bir Na-Bir Li-Bir

m1 730 (w) 732 (w) – –

m2 646 (s) 638 (s) 640 (s) 625 (s)

m3 575 (s) 585 (s) 578 (s) 585 (s)

m4 506 (m) 515 (m) 500 (m) 510 (m)

m5 485 (sh) 486 (sh) – 490 (sh)

m6 – – – 410 (m)

m7 – 386 (m) – 378 (w)

m8 296 (w) 300 (w) – 280 (m)

RS band intensity: s = strong, m=medium, w =weak, sh = shoulder.

Fig. 3. Raman spectra of birnessite-type manganese oxides. (a) MnO1.86�0.6H2O, (b) Mn0.85Co0.15O1.86�0.6H2O, (c) Na0.32MnO2�0.6H2O, and

(d) Li0.32MnO2�0.6H2O.


The RS spectrum of SG-Bir, MnO1.86�0.6H2O,

supports its proposed layered structure. The correla-

tion between the frequency position of the Raman-

active modes and the degree of polymerisation shows

that MnO1.86�0.6H2O birnessite has an average of 4.8

shared edges per MnO6 octahedron [33]. The Raman

band at 646 cm� 1 can be viewed as the symmetric

stretching vibration m2(Mn–O) of MnO6 groups [34].

It is assigned to the A1g symmetric mode in the C2h3

spectroscopic space group. The band located at 575

cm� 1 usually attributed to the m3(Mn–O) stretching

vibration in the basal plane of [MnO6] sheets is

particularly strong in birnessite compounds compared

with the literature data related to lithiated spinels. This

feature can be related to the high rate of Mn(IV) in the

birnessite family.

The RS spectrum of SGCo-Bir, Mn0.85Co0.15O1.86�0.6H2O, where cobalt partly substitutes for manga-

nese, displays similar features to MnO1.86�0.6H2O.

Manceau et al. have shown that CoII can be easily

oxidized to CoIII by phyllomanganates resulting in the

Mn substitution into the MnO2 network [32]. Our

spectroscopic results show the same trends as the

XRD studies suggesting that Mn3 + ions are probably

substituted by Co3 + ions in MnO2 layers during the

ion-exchange synthesis [35,36]. The most important

Raman change is observed on the m3(Mn–O) stretch-

ing frequency, which presents a shift of 10 cm� 1

towards the high-wavenumber side for the SGCo-Bir.

This result indicates a strengthening of the Mn–O

bond along the chains in the Co-doped SG-Bir, as a

consequence of the difference between ionic radii, i.e.

r(Co3 +) = 0.63 A and r(Mn3 +) = 0.68 A. This result

gives evidence for the stronger network energy of the

Co-doped material. When Co ions are incorporated in

the SG-Bir lattice, there is also a slight frequency shift

of the m2 stretching frequency at 646 cm� 1 towards

lower frequencies, which suggests a softening of the

Mn–O bond along the interlayer direction. Both RS

features indicate probably a distortion in the SGCo-

Bir compound, which can be viewed as an elongation

of the MO6 basal octahedra perpendicularly to the

chains.

Because Raman spectroscopy is not a primary

structural technique like X-ray diffraction it is neces-

sary to ‘‘calibrate’’ it against well-crystallized materi-

als. For comparison, the Raman spectrum of the spinel

E-MnO2 and that of the natural romanechite and the

EMD g-MnO2 phase are shown in Fig. 4. Raman

features of several manganese oxides are listed in

Table 3. Despite the large variety of MDO phases, the

spectral features m2 and m3 in the high-frequency

region of internal vibrations are always assigned to

symmetric stretching vibrations of the MnO6 octahe-

dra with the Ag phonon species for the band above

600 cm� 1. For instance, the peak observed at 625

cm� 1 in Li-birnessite corresponds to that of Mn–O

stretching vibration recorded at 643 cm� 1 for roma-

nechite and at 631 cm� 1 for g-MnO2 with a pyrolu-

site intergrowth rate Pr = 29% (Fig. 4). The Mn–O

symmetric stretching vibration appears at 625 cm� 1

in the RS spectrum of LiMn2O4 spinel while it is

shifted at 592 cm� 1 when Li ions are extracted from

the spinel lattice to form E-MnO2 [12]. Some sim-

ilarity exists with the Raman spectra of other MnO2

phases, but there are detectable peak shifts toward the

low-energy side that could originate from the different

chemical bonding environments for both compounds.

The spectroscopic features of MDO compounds show

firstly that distortion of MnO6 octahedra is strongly

correlated with the nature of the MDO framework, i.e.

the polymerization of the Mn–O chains associated

with the MnO6 edge sharing. Secondly, the RS band at

ca. 570 cm� 1 is the specific fingerprint of the Mn–O

vibration along the chains in the manganese dioxide

framework. This mode appears at 572 cm� 1 in nsutite

g-MnO2 while observed at 570 cm� 1 in romanechite

(Fig. 4).

Fig. 5 shows the frequency shift of the high-wave-

number Mn–O stretching mode as a function of the

interlayer spacing in birnessite-type manganese diox-

ides. The removal of Na+ or Li+ ions from the

interlayer space and the accommodation of water

molecules between MnO2 sheets are responsible for

the increase of the distance between two successive

slabs. The increase of the Mn–O stretching mode

wavenumber upon alkali-ion extraction is attributed

to the change in the manganese oxidation state and to

the weakness of the interactions between layers. The

net effect of the alkali-ion incorporation into the

interlayer space is a local lattice distortion, which

results in shorter Mn–O chemical bonds and change

in the covalency of MnO2 slabs, and a partial reduction

of the Mn4 + ions, which results in a decrease of the

vibrational mode frequencies that is inherent to the

presence of Mn3 + ions. Assuming a harmonic oscil-


lator model, the reason for the increase of the Mn–O

stretching mode wavenumber is a lattice stiffening that

corresponds to a compression of the MnO6 slabs.

Nevertheless, it is an experimental fact that such

variations do not occur for those manganese dioxides

with a three-dimensional (3D) network. The opening

of the structure of 3D manganese dioxides results in

the modification of the average MnO6 octahedral

polymerisation. Thus, we observed a decrease of the

Mn–O stretching vibration frequency with the increas-

Table 3

Spectroscopic characteristics of various manganese oxides

Compound Formula Space group Edge sharing Raman peaks

Pyrolusite h-MnO2 P42/mnm (136) 2 538 (s), 665 (s)

Ramsdellite R-MnO2 Pnma (62) 4 294 (w), 518 (s), 580 (s),

630 (w), 740 (w)

Nsutite g-MnO2 hex. 3 + 2/3 379 (w), 491 (w), 520 (w),

572 (s) 631 (s), 738 (w)

Hollandite K2Mn8O16�xH2O I4/m (87) 4 259 (w), 507 (s), 585 (s), 630 (w)

Romanechite (R)2Mn5O10�xH2O B2/m (12) 4 + 2/5 376 (w), 515 (w), 570 (s),

643 (s), 721 (w)

Todorokite (R)Mn6O18�xH2O P2/m (10) 4 + 2/3 590 (w), 641 (s)

Spinel E-MnO2 Fd3m (227) 6 460 (w), 497 (w), 592 (s)

Manganosite MnO Fm3m (225) 8 523 (w), 574 (w), 647 (s)

Raman peak frequency given in cm� 1 and intensity s = strong, w=weak.

Fig. 4. Raman spectra of manganese dioxides (a) spinel E-Mn2O4, (b) natural romanechite, and (c) EMD g-MnO2 with a pyrolusite intergrowth

rate Pr = 27%.


ing number of edges shared, Nshe, by each MnO6

octahedron. As an example, m2 = 665 cm� 1 is recorded

for the pyrolusite with a (1�1) network tunnel and

Nshe = 2, while m2 = 643 cm� 1 for the romanechite with

a (1� 3) lattice tunnel and Nshe = 4.4.

3.3. Raman spectra of lithiated manganese dioxides

Fig. 6 presents the Raman spectra of four forms

of lithiated manganese dioxides: (a) birnessite Li0.32MnO2�0.6H2O, (b) monoclinic LiMnO2, (c) HT-

Li0.52MnO2.1, and (d) LT-Li0.52MnO2.1 phase. These

oxides display high-wavenumber RS bands attributed

to the Mn–O stretching vibrations while the low-

wavenumber region differs according to the crystal

chemistry. Lithium birnessite and monoclinic LiMnO2

are distinguished from the Li0.52MnO2.1 spinel-like

Fig. 5. Frequency shift of the high-wavenumber stretching mode as

a function of the interlayer spacing in birnessite-type manganese

dioxides.

Fig. 6. Raman spectra of lithiated manganese dioxides. (a) Birnessite Li0.32MnO2�0.6H2O, (b) monoclinic LiMnO2, (c) LT-Li0.52MnO2.1, and (d)

HT-Li0.52MnO2.1.


compound by the presence of a well-resolved Raman

band around 280 cm� 1. The RS spectrum of LT-

Li0.52MnO2.1 exhibits the disordered character of this

low-temperature Li-birnessite structure with the pre-

dominant features of the spinel-containing phase.

Taking into account the structural considerations

of Le Goff et al. [29] on the most suitable position for

alkali ions in trigonal prismatic sites of interlayer

space (Fig. 1) and of a weak bonding for the cations

in the MnO2 lattice, we could consider the low-

wavenumber Raman band as the stretching mode of

LiO6 groups in Li-birnessite. It has been demonstra-

ted that, for several lithium metal oxide materials, the

vibrational mode of LiO4 tetrahedron occurs in the

range 350–500 cm� 1 [37], while the vibrational

mode of LiO6 octahedron appears at lower wave-

numbers in the range 200–300 cm� 1 [15,16]. The

Li–O stretching mode at 260 cm� 1 has been recog-

nised to be due to the occupation of Li ions in

octahedral sites of LiCoO2. This low-frequency mode

shifts to 245 cm� 1 in LiNiO2. The higher frequency

(280 cm� 1) observed in the birnessite Li0.32M-

nO2�0.6H2O phase is consistent with the somewhat

shorter bond distances and higher reduced mass due

to insertion of Li ions in the octahedral sites of the

empty channels.

The spinel structure E-LiMn2O4 is primarily char-

acterized by structural groups as follows. (1) MnO6

octahedra connected to one another in three dimen-

sions by edge sharing; (2) LiO4 tetrahedra sharing

each of their four corners with a different MnO6 unit

but essentially isolated from one another; (3) a three-

dimensional network of octahedral (16c) and tetra-

hedral (primarily 8a) sites, through which lithium

ions can move through the lattice. The RS vibra-

tional spectrum of the Li0.52MnO2.1 spinel phase can

be compared with that of E-LiMn2O4 spinel. Analy-

sis of Raman and IR spectra of various lithiated

oxides was done in terms of localized vibrations

[34,38]. Considering the 6Li–7Li isotopic shifts evi-

denced in E-LiMn2O4 spinel, the IR band located at

420 cm� 1 was attributed to the asymmetric stretch-

ing vibration of LiO4 tetrahedra in the LiMn2O4

spinel structure, while the corresponding Raman-

active symmetric stretching mode was observed at

382 cm� 1 [34].

The vibrational frequency of the LiO4 tetrahedron

has appeared out at 437 cm� 1 in the Li0.52MnO2.1

spinel-like compound (Fig. 6). These results definitely

prove that the low-frequency bands have a main

contribution which can be assigned to Li–O vibra-

tions or, more precisely, to the valence frequency of

LiO4 tetrahedra, i.e. all the lithium is four-fold coor-

dinated in LiMn2O4. This leads to the frequency at ca.

437 cm� 1 for oscillation of the Li+ ion with O2� near

neighbours in Li0.52MnO2.1.

4. Conclusion

The vibrational features of birnessite-type manga-

nese dioxides have been reported for the first time.

In this work, Raman scattering spectroscopy has

proven to be a useful tool for the local environment

identification of layered structures such as Li-Bir,

Na-Bir, SG-Bir, and SGCo-Bir. Structural properties,

like the degree of polymerisation in the case of

manganese dioxides or the degree of distortion of

MnO6 polyhedra, confirm that birnessite occurs in an

infinity of structural varieties all based on the same

crystal lattice but differing in lattice order, manga-

nese oxidation state, and cation substitution. Close

similarities have been evidenced between birnessites

and other lithiated manganese oxides such as mono-

clinic LiMnO2 and spinel Li0.52MnO2.1. From these

results, we have identified several structural patterns:

(i) the specific spectroscopic fingerprints of the SGB

single phases, (ii) the site occupancy of Co ions in

the substituted SGCo-Bir compound, (iii) vibrations

due to lithium ions with their oxygen neighbours in

birnessite Li0.32MnO2�0.6H2O are observed at 280

cm� 1, (iv) the Raman-active symmetric stretching

mode of LiO4 tetrahedron was observed at 437

cm� 1 in Li0.52MnO2.1, and (v) a correlation between

the interlayer d-spacing and the stretching mode

frequencies of birnessite oxides.

Acknowledgements

The authors would like to thank Dr. B. Yebka for

providing samples of manganese dioxides and Dr. J.C.

Boulliard (Collection de Mineralogie) for providing

romanechite samples. Mr. Michel Lemal is gratefully

acknowledged for his careful work in performing the

XRPD measurements.


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Raman spectra of birnessite manganese dioxides