ORIGINAL ARTICLE
Thermodynamics of copper-manganese and copper-iron spinel
solid solutions
Sulata K. Sahu
1,2
| Alexandra Navrotsky
1
1
Peter A. Rock Thermochemistry
Laboratory and NEAT ORU, University
of California Davis, Davis, California
2
Department of Materials Science and
Engineering, Massachusetts Institute of
Technology, Cambridge, Massachusetts
Correspondence
Alexandra Navrotsky, Peter A. Rock
Thermochemistry Laboratory and NEAT
ORU, University of California Davis,
Davis, CA 95616. Email:
anavrotsky@ucdavis.edu
Funding information
Clean Diesel Technologies Incorporated;
U.S. Department of Energy, Grant/Award
Number: DE-FG02-03ER6053
Abstract
High-temperature oxide melt solution calorimetry has been performed to investi-
gate the energetics of spinel solid solutions in the Mn
3
O
4
-CuMn
2
O
4
and Fe
3
O
4
-
CuFe
2
O
4
systems. The spinel solid solutions were synthesized by a ceramic route
and calcined at appropriate temperatures to obtain single phase samples. The drop
solution enthalpies of the solid solutions in the Mn
3
O
4
-CuMn
2
O
4
system are the
same within experimental error as the enthalpies of drop solution of mechanical
mixtures of the end-members, indicating a zero heat of mixing, i.e., ideal mixing
in terms of enthalpy. In Fe
3
O
4
-CuFe
2
O
4
, the drop solution enthalpy of the solid
solutions shows a positive deviation from those of the mechanical mixture of the
end-members, suggesting negative mixing enthalpy. The formation enthalpies of
the spinel solid solutions from their constituent oxides plus oxygen and from the
elements were also determined.
KEYWORDS
calorimetry, Fe
3
O
4
-CuFe
2
O
4
, Mn
3
O
4
-CuMn
2
O
4
, spinel solid solutions, thermodynamics of formation
and mixing
1 | INTRODUCTION
From a fundamental viewpoint as well as in terms of applica-
tions, solid solutions of transition-metal oxides are important
because of their great diversity in physical and chemical
properties.
1–3
There is a revival of interest in compounds
containing copper, manganese, and iron oxides with spinel
structures. For example, Cu-Mn oxide-based catalysts have
been proposed for the removal of air pollutants like carbon
monoxide and nitrous oxides from exhaust gas.
4
Cu
x
Mn
3x
O
4
spinel has high catalytic performance for steam
reforming of methanol, and also exhibits high electrical con-
ductivity.
4,5
Similarly, spinel ferrites have received recent
interest because of their potential application in magnetic
and electronic materials, sensors, and catalysts.
6–8
The crys-
tal chemistry of copper-manganese oxides is complicated
because of the properties of the Jahn-Teller ions Cu
2+
and
Mn
3+
, which produce lattice distortions, reducing the sym-
metry of the spinels from cubic to tetragonal. CuMn
2
O
4
spi-
nel is found to be cubic or tetragonal depending on the
temperature of quenching.
9
Even though in CuMn
2
O
4
, Mn
3+
and Cu
2+
ions are expected to produce a tetragonal distortion,
the symmetry of CuMn
2
O
4
, at least as revealed by normal
laboratory X-ray diffraction, remains cubic.
9–11
Miyahara
attributed this anomalous behavior as arising from two oppos-
ing effects, namely a distortion with c/a>1 caused by the
Mn
3+
ions at octahedral sites compensated by an opposing
distortion with c/a<1 due to Cu
2+
ions at the tetrahedral
sites.
12
Investigations on Cu
x
Mn
3x
O
4
spinels revealed that
Cu
2+
has a lower octahedral site preference than Mn
3+
and
thus manganese has a greater tendency to occupy octahedral
sites.
10,13,14
Spinel ferrites, Cu
x
Fe
3x
O
4
, normally have a
cubic structure,
3,4
however it has been suggested that copper
ferrite can exist in tetragonal form when annealed below
950°C.
15
CuFe
2
O
4
has been described as an inverse spinel,
the Cu
2+
ions occupying the octahedral sites.
16,17
However,
later it was recognized that a small fraction of Cu
2+
(not
exceeding 20%) can occupy the tetrahedral sites.
18
As the synthesis, processing, and applications of these
spinels depend on their temperature range of stability and
thermodynamic properties, numerous phase equilibria and
thermodynamic studies have been carried out on them. The
Received: 10 October 2016
|
Accepted: 25 January 2017
DOI: 10.1111/jace.14813
J Am Ceram Soc. 2017;1–9. wileyonlinelibrary.com/journal/jace © 2017 The American Ceramic Society | 1