Structure and Magnetism in λ-MnO
2
. Geometric
Frustration in a Defect Spinel
J. E. Greedan,* N. P. Raju, A. S. Wills, C. Morin, and S. M. Shaw
Brockhouse Institute for Materials Research and Department of Chemistry,
McMaster University, Hamilton, Ontario L8S 4M1, Canada
J. N. Reimers
Moli Energy Ltd., 20000 Stewart Cresent, Maple Ridge, B.C. V2X 9E7, Canada
Received March 23, 1998. Revised Manuscript Received June 16, 1998
λ-MnO
2
, a metastable form of manganese dioxide, retains the cubic spinel structure upon
lithium removal from LiMn
2
O
4
by soft chemical methods, either electrochemical or acid
leaching. The minimum lithium content, achieved by the latter route at pH 1, is Li
0.10
-
MnO
2
, which is in reasonable agreement with previous reports. For lithium contents near
the minimum value, long-range antiferromagnetic order sets in below T
N
) 32 K, and Curie-
Weiss susceptibility behavior is found above 125 K, with fitting constants, θ
c
)-104(4) K
and C ) 1.97(2) emu-K/mol. This value of C is consistent with the lithium content found
analytically. The susceptibility is remarkably field dependent in the temperature range
near T
N
for some samples with larger lithium contents, which might be understood in terms
of field-induced short-range ferromagnetic correlations. Neutron diffraction studies show a
complex magnetic order described by a propagation vector k ) (
1
/
2
1
/
2
1
/
2
)(128 Mn moments
per magnetic unit cell) and confirm the T
N
) 32 K. A model for the magnetic structure is
proposed that is consistent with the neutron intensities. The complexity of the magnetic
structure is consistent with the geometric frustration inherent in the Mn sublattice, which
is comprised of a three-dimensional array of corner-sharing tetrahedra. The properties of
λ-MnO
2
are compared and contrasted with those of -MnO
2
, with the rutile structure, and
the pyrochlore Y
2
Mn
2
O
7
, with the same topology for the Mn(4+) sublattice.
Introduction
The methods of soft chemistry or “chimie douce” allow
the synthesis and study of solid materials that are
thermodynamically unstable under standard prepara-
tion conditions dictated, as they typically are, by solid-
state reaction kinetics. The subject of the work de-
scribed here is λ-MnO
2
, one of the metastable forms of
manganese dioxide, for which the thermodynamically
stable form is -MnO
2
or pyrolusite with the rutile
structure. λ-MnO
2
, first reported to result from acid
leaching of lithium from the spinel LiMn
2
O
4
, retains the
structure of the parent phase.
1
Further systematic
studies confirmed that acid concentrations near pH 1
were most efficient at lithium removal with final
products of composition Li
0.03
MnO
2
.
2
λ-MnO
2
also re-
sults from electrochemical removal of Li from LiMn
2
O
4
cathodes in solid-state lithium battery cells during
standard charge/discharge cycling. Powder X-ray dif-
fraction (PXRD) confirms the retention of the spinel
framework, Fd3m, with a
o
) 8.029(1) Å to be compared
with the cell constant of the parent LiMn
2
O
4
,a
o
) 8.245-
(1) Å.
2
The Mn ions occupy the 16d sites in Fd3m and
thus form a three-dimensional (3D) array of corner-
sharing tetrahedra (Figure 1a). This exact magnetic
sublattice is found in pyrochlore structure oxides, which
also crystallize in Fd3m symmetry. Corner-sharing
tetrahedral magnetic lattices satisfy one of the condi-
tions for geometric magnetic frustration, the other being
the presence of dominant nearest neighbor antiferro-
magnetic exchange interactions.
3,4
Clearly (see Figure
1b) it is not possible to arrange four spins at the corners
of a regular tetrahedron in a mutually antiparallel
pattern, two spins will always be frustrated.
Geometrically frustrated systems often exhibit un-
usual magnetic properties that stem from their enor-
mous ground-state spin degeneracies and the concomi-
tant difficulty in selecting a unique ground state.
4-6
To
contrast geometrically frustrated and conventional an-
tiferromagnets, it is useful to review the characteristics
of the latter. The thermal evolution of such systems
can be discussed conveniently in terms of three regimes;
they are, the paramagnetic, the long-range ordered, and
the short-range ordered or “critical” regimes.
At the highest temperatures is found the paramag-
netic state in which the spin-spin correlations are
spatially random and dynamically fluctuating. At the
lowest temperatures there is the long-range ordered
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(2) Mosbah, A.; Verbaere, A.; Tournoux, M. Mater. Res. Bull. 1983,
18, 1375.
(3) Anderson, P. W. Phys. Rev. 1956, 102, 1008.
(4) Villain, J. Z. Phys. 1978, B33, 31.
(5) Reimers, J. N.; Berlinsky, A. J.; Shi, A.-C. Phys. Rev. 1991, B43,
865.
(6) Reimers, J. N. Phys. Rev. 1992, B45, 7287.
3058 Chem. Mater. 1998, 10, 3058-3067
10.1021/cm9801789 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/16/1998