Structural and magnetic properties of Cu-Ni-Cr spinel oxides

Article abstract of DOI:10.1016/j.physb.2006.05.181

The compounds CuCr₂O₄ and NiCr₂O₄ crystallize at room temperature in a tetragonal distorted spinel structure, s.g. I4₁/amd, with axes ratio c/a<1 and >1, respectively. The distortion is caused by the Jahn-Teller ions Cu²⁺ and Ni²⁺ which flatten or elongate their surrounding oxygen tetrahedron. CuCr₂O₄ and NiCr₂O₄ form a complete solid solution series Cu_1-xNi_xCr₂O₄ where for 0.825a temperature-dependent crystallographic phase transition from cubic to tetragonal symmetry between 865 K (CuCr₂O₄) and 310 K (NiCr₂O₄). The phase Cu_0.15Ni_0.85Cr₂O₄ undergoes a second crystallographic transition to orthorhombic symmetry, space group Fddd, at T=300 K. The neutron diffraction experiments as well as SQUID measurements reveal magnetic ordering of the ions between 150 and 50 K which partially occurs as a two-step mechanism.

Full text of DOI:10.1016/j.physb.2006.05.181

ARTICLE IN PRESS
Physica B 385–386 (2006) 196–198
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Structural and magnetic properties of Cu–Ni–Cr spinel oxides
Ã
M. Tovara, , R. Torabia, C. Welkerb, F. Fleischerc
aHahn-Meitner-Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany
bUniversity of Cape Town, South Africa
cMax-Planck-Institut fu¨r Mikrostrukturphysik, Halle, Germany
Abstract
The compounds CuCr2O4 and NiCr2O4 crystallize at room temperature in a tetragonal distorted spinel structure, s.g. I41/amd, with
axes ratio c/ao1 and 41, respectively. The distortion is caused by the Jahn–Teller ions Cu2+ and Ni2+ which flatten or elongate their
surrounding oxygen tetrahedron. CuCr2O4 and NiCr2O4 form a complete solid solution series Cu1ꢀxNixCr2O4 where for
0.825oxo0.875 members with orthorhombic symmetry were found. Using neutron powder diffraction and thermal analysis methods
several members of the solid solution series were investigated. On cooling, all samples showed a temperature-dependent crystallographic
phase transition from cubic to tetragonal symmetry between 865 K (CuCr2O4) and 310 K (NiCr2O4). The phase Cu0.15Ni0.85Cr2O4
undergoes a second crystallographic transition to orthorhombic symmetry, space group Fddd, at T ¼ 300 K. The neutron diffraction
experiments as well as SQUID measurements reveal magnetic ordering of the ions between 150 and 50 K which partially occurs as a two-
step mechanism.
r 2006 Elsevier B.V. All rights reserved.
Keywords: Jahn–Teller; Rietveld analysis; Spinel; Phase transition; Magnetism
1. Theoretical background
solid solution series Cu1ꢀxNixCr2O4 several interesting
crystallographic phenomena can be observed which are
topic of this work.
Oxide systems are well suitable systems for systematic
studies of macroscopic and microscopic structure distor-
tions due to the Jahn–Teller effect. The Jahn–Teller effect
means that an ion being in an energetic degenerated ground
state (which holds for several transition metal ions) reduces
its local symmetry to overcome the degeneration. The
direction of the distortion can partially be predicted
using orbital schematic considerations [2]. For example
the introduction of Cu2+ ion (d9 system) in a tetrahedral
(oxygen) environment leads to a flattening of the tetra-
hedron, while the tetrahedron is elongated in case of Ni2+
(d8 system) (Fig. 1) [1,3].
Thus introducing Cu2+ or Ni2+ into the tetrahedral site
of a cubic chromium spinel ([T2+]Cr2O4) leads to strong
local tetrahedral distortion which further causes a macro-
scopic and temperature-dependent distortion of the
cubic spinel structure. Since the direction of the structure
distortion caused by these two ions are oppositional, in the
2. Experimental
Synthesis of the samples was done by precursor method
where the educts Cu(II) acetate, Ni(II) acetate and Cr(III)
nitrate were mixed in the appropriate amounts. Urea was
added to support complete combustion of the educts. The
resulting nanosized powder was annealed for 24 h at
1100 K to give the desired product.
It turned out that the synthesis of pure samples became
more difficult with increasing Ni content. Thus, samples
with x40.6, even after longer annealing time still showed
some impurities of Cr2O3 which was taken into Rietveld
calculations as an additional phase. However, samples with
x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.85 could be
synthesized as pure phases, no significant additional
reflections could be detected in the X-ray diffractograms.
Using differential scanning calorimetry (DSC) as well as
low-temperature X-ray diffraction (Guinier method) all
Ã
Corresponding author. Tel.: +49 30 8062 2768.
E-mail address: tovar@hmi.de (M. Tovar).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2006.05.181

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M. Tovar et al. / Physica B 385–386 (2006) 196–198
197
Fig. 1. Jahn–Teller effect on CuO4 and NiO4 tetrahedron.
samples were investigated for crystallographic phase
transitions. To avoid high and irregular background due
to incoherent scattering of Cr3+ and Ni2+ ions when
exposed to Cu-anode X-rays and in order to investigate the
complete required temperature range with high precision
and low thermal gradient, all crystallographic information
were extracted from neutron powder diffraction data
collected at the E9 neutron powder diffractometer at
BENSC, HMI, Berlin. Additional magnetic information
were gained by SQUID magnetometry.
Fig. 2. Phase diagram of the system CuCr2O4–NiCr2O4.
3. Results and discussion
The phase diagram was established and revised since
only incomplete or theoretical data could be found in the
literature. DSC as well as low-temperature X-ray and
neutron diffraction experiments revealed three different
phase regions: cubic, tetragonal and orthorhombic (Fig. 2).
As can be seen, the experimentally determined phase
boundary between the tetragonal and the orthorhombic
phase deviates notedly from the reported calculated curve
[4] (dashed lines in Fig. 2). All samples with xo0.7 and
40.9 exhibit tetragonal symmetry down to 2 K, only
samples with x ¼ 0.8, 0.85 and 0.9 showed orthorhombic
symmetry on cooling below room temperature. Thus the
orthorhombic region is much smaller than was expected.
Room temperature neutron powder diffraction data (Fig.
3) were used for Rietveld analysis of the crystal structures. In
the picture a crossover of the reflection at sample x ¼ 0.85 is
marked. This crossing is due to changing of the c/a ratio of
the tetragonal spinels. While due to Jahn–Teller flattening
CuCr2O4 shows axes ratio c/ao1, NiCr2O4 shows c/a41
due to Jahn–Teller elongation. The crossing point is close to
the end member NiCr2O4 since the Jahn–Teller effect for
Ni2+ is much weaker than for Cu2+. The observations are in
good agreement with further investigations.
Fig. 3. Neutron powder diffractograms of the members of the solid
solution series Cu1ꢀxNixCr2O4.
The influence of the Jahn–Teller effect on the individual
tetrahedra can be better seen and estimated by looking to
the inner dimensions of the tetrahedra. In Fig. 4 distances
Cu/Ni–O and angles O–Cu/Ni–O are plotted as function of
composition. In a flattened tetrahedron (CuO4) there are—
in respect to the ideal tetrahedron angle 109.4 1—four
smaller angles and two larger ones, while in an elongated
Fig. 4. Bond distances and angles in the Cu/NiO4 tetrahedra.

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198
Fig. 5. Lattice parameters of Cu0.15Ni0.85Cr2O4 as function of tempera-
ture.
Fig. 6. SQUID measurements of the members of the solid solution series
Cu1ꢀxNixCr2O4.
tetrahedron (NiO4) it is vice versa. Hence obviously not
only the lattice parameters crossover but also the inner
tetrahedral angles. They start for CuO4 at 104.5 1 (4x) and
122.0 1 (2x) and end for NiO4 at 111.0 1 (4x) and 106.5 1 (2x)
while the distances Cu/Ni–O remain quite constant at
4. Conclusion
The system Cu1ꢀxNixCr2O4 is strongly influenced by the
Jahn–Teller effect which gives rise to symmetry reduction
from cubic to tetragonally and orthogonally distorted
spinel structure. The latter shows highly anisotropic
thermal expansion which can be ascribed to the opposi-
˚
about 1.96570.005 A.
As can be seen from the phase diagram (Fig. 2) the phase
Cu0.15Ni0.85Cr2O4 undergoes two crystallographic transi-
tions: cubic to tetragonal at 350 K and tetragonal to
orthorhombic at 300 K. Temperature-dependent neutron
powder diffraction was carried out to study the crystal
structure as a function of temperature and to determine the
still unknown low-temperature crystal structure (which has
to be discussed elsewhere). From Fig. 5 it can be nicely seen
that the respective phase transition is of first-order
character due to jumps in the lattice parameters from the
particular low-temperature to the high-temperature phase.
At temperature below 50 K all investigated samples
showed magnetic ordering (see also Ref. [5]) in the neutron
diffractograms. SQUID measurement revealed that the
transition temperature strongly depends on the Cu/Ni ratio
(Fig. 6) and that the transition of Cu0.15Ni0.85Cr2O4 occurs
in two steps. Additional neutron powder diffraction
investigations with finer temperature intervals confirmed
this observation.
tional behaviour of Cu2+ and Ni2+
.
At low temperatures all compounds show anti-ferro-
magnetic ordering. TN decreases and the magnetisation
increases with increasing Ni content. This corresponds to
the fact that Ni2+ has two unpaired electrons whereas
Cu2+ has only one. The observed two-step mechanism may
have its origin in different ordering temperature of the ion’s
sublattices.
References
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[2] J.D. Dunitz, E. Orgel, J. Phys. Chem. Solids 3 (1957) 20.
[3] M. Tovar, PhD Thesis, Universitat Heidelberg, 2000.
¨
[4] A. Wold, et al., Appl. Phys. 34.4 (1963) 1085.
[5] B.L. Dubey, N. Nath, B.N. Tiwari, A. Tripathi, Bull. Mater. Sci. 5.2
(1983) 153.