Magnetic, transport, and thermodynamic properties of CaMn
2
O
4
single crystals
B. D. White,
1
J. A. Souza,
1,
* C. Chiorescu,
2
J. J. Neumeier,
1,†
and J. L. Cohn
2
1
Department of Physics, Montana State University, P.O. Box 173840, Bozeman, Montana 59717-3840, USA
2
Department of Physics, University of Miami, Coral Gables, Florida 33124, USA
Received 9 December 2008; revised manuscript received 12 February 2009; published 24 March 2009
Physical properties including magnetic susceptibility, room-temperature electrical resistivity, thermal con-
ductivity, heat capacity, and thermal expansion are reported for high quality single-crystal samples of marokite
CaMn
2
O
4
. We determined that CaMn
2
O
4
is highly electrically insulating and exhibits long-range antiferro-
magnetic order below T
N
= 217.5 0.6 K with easy axis along a. Anisotropic thermal expansion, similar to that
of crystallographically layered materials, is observed, suggesting that the crystal structure of CaMn
2
O
4
is also
assembled from previously undescribed layers. An extensive thermodynamic study of the antiferromagnetic
transition was undertaken resulting in a heat-capacity critical exponent = 0.082 0.007 and calculated pres-
sure derivative dT
N
/ dP = 5.154 0.174 K / GPa.
DOI: 10.1103/PhysRevB.79.104427 PACS numbers: 75.47.Lx, 75.50.Ee, 71.70.Ej, 65.40.De
I. INTRODUCTION
Initial studies of CaMn
2
O
4
were published shortly after its
discovery in 1963 in Morocco as a naturally occurring
mineral.
1
It was subsequently named marokite for its country
of origin and was found to crystallize in an orthorhombic
structure with space group
2
Pbcm and exhibit antiferromag-
netic AFM ordering below 225 K.
3
But in the decades fol-
lowing these preliminary investigations, further studies are
completely absent in the literature until 2001, when
CaMn
2
O
4
was studied as an impurity phase in the solid-state
synthesis of manganese oxides exhibiting colossal magne-
toresistance behavior.
4
CaMn
2
O
4
has since enjoyed some re-
newed interest; however, recently published studies have
been primarily limited to reporting on aspects of the mag-
netic ordering
4–6
and crystal structure.
4,5,7
An extensive
physical property characterization of CaMn
2
O
4
has yet to be
conducted.
Compounds with chemical formula AB
2
O
4
frequently
crystallize in the cubic spinel structure with space group
Fd3m or a tetragonally distorted spinel structure with space
group I4
1
/ amd. This is true of several manganese oxide
compounds: NMn
2
O
4
with N =Ni, Cu, Mn, Zn, Cd, Mg, and
Li.
8,9
The crystal structure of CaMn
2
O
4
is displayed in Fig. 1
as generated by JAVA STRUCTURE VIEWER.
10
It is primarily
constructed from a complex network of MnO
6
octahedra. We
can formulate a convenient description of this network by
considering zigzag layers of edge-sharing octahedra running
parallel to b, which are coupled along c by vertex sharing of
apical oxygen sites. All MnO
6
octahedra are oriented with
one of two tilted orientations with respect to the c axis and,
while all octahedra display the same tilt orientation within
each layer, successive zigzag layers alternate between the
two. Tunnels along a between the zigzag layers are occupied
by Ca
2+
ions. In stoichiometric CaMn
2
O
4
, all manganese
ions are nominally high-spin Mn
3+
3d
4
, t
2g
3
e
g
1
. This elec-
tronic configuration is well known to result in highly Jahn-
Teller JT distorted MnO
6
octahedra. Neutron-diffraction
studies observe 26% elongation of the axial Mn-O bond dis-
tances relative to equatorial in the octahedra.
4
The magnitude
of this distortion certainly excludes the possibility of a cubic
spinel structure for CaMn
2
O
4
but does not directly prevent
the stabilization of a tetragonally distorted spinel structure.
However, the large Ca
2+
ions do inhibit tetragonal symmetry.
The A site in the spinel structure is located in a position
which forms AO
4
tetrahedra. Comparison of the ionic radii
of all divalent ions occupying a tetrahedral site reveals that
Ca
2+
has the largest ionic radius of any with only Pb
2+
and
Hg
2+
being slightly smaller.
11
Most other divalent ions in this
situation those generally occupying the A site of compounds
which do crystallize in the spinel structure have ionic radii
50%–70% as large as Ca
2+
1.00 Å.
11
The structure of
CaMn
2
O
4
is stabilized when Ca
2+
incorporates itself into
eightfold coordination with CaO
8
polyhedra instead of CaO
4
tetrahedra.
4,5
Many cubic spinel compounds exhibit a struc-
tural phase transition under pressure to one of three related
orthorhombic structures—those of CaMn
2
O
4
Pbcm,
CaFe
2
O
4
Pnma, and CaTi
2
O
4
Cmcm.
8
This observation
FIG. 1. Color online Crystal structure of CaMn
2
O
4
viewed
roughly along a perspective has small c-axis component to visually
clarify details of the three-dimensional 3D structure. MnO
6
octa-
hedra are explicitly displayed within a single unit cell revealing a
complex network with large Ca
2+
ions occupying tunnels along a
between zigzag layers built along b.
PHYSICAL REVIEW B 79, 104427 2009
1098-0121/2009/7910/1044279 ©2009 The American Physical Society 104427-1