Local Coordination of Low-Spin Ni3+ Probes in Trigonal LiAl yCo1-yO2 Monitored by HF-EPR

Article abstract of DOI:10.1021/jp0376119

The local coordination of low-spin Ni³⁺ probes in trigonal LiAl_yCo_1-yO₂ was studied using high-frequency EPR spectroscopy. This technique allows distinguishing between two types of Ni³⁺ ions: Ni³⁺ ions in trigonal crystal field ( ²E_g ground state) and Ni³⁺ ions in tetragonal crystal field (²A_1g ground state). When a Ni³⁺ ion is in a uniform Co environment, the orbitally degenerate state for Ni ³⁺ is preserved and the EPR spectrum of the ground vibronic doublet state can be observed (intermediate Jahn-Teller effect). The value of g ₁ decreases as the mean M - O bond is contracted. The local tetragonal distortion can be observed for Ni³⁺ ions located in a mixed Co_6-yAl_y environment. Along the progressive replacement of Co by Al, the strength of the crystal field for Ni³⁺ increases gradually and the extent of the tetragonal distortion displays a tendency to increase. The maximum effect of Jahn-Teller distortion is achieved when the Ni³⁺ ion falls in a pure Al environment.

Full text of DOI:10.1021/jp0376119

J. Phys. Chem. B 2004, 108, 4053-4057
4053
Local Coordination of Low-Spin Ni3+ Probes in Trigonal LiAlyCo1-yO2 Monitored by
HF-EPR
Radostina Stoyanova* and Ekaterina Zhecheva
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Ricardo Alca´ntara and Jose´ L. Tirado
Laboratorio de Qu´ımica Inorga´nica, UniVersidad de Co´rdoba, Edificio C-3, Primera Planta,
Campus de Rabanales, 14071 Co´rdoba, Spain
ReceiVed: NoVember 26, 2003
The local coordination of low-spin Ni3+ probes in trigonal LiAlyCo1-yO2 was studied using high-frequency
EPR spectroscopy. This technique allows distinguishing between two types of Ni3+ ions: Ni3+ ions in trigonal
crystal field (2Eg ground state) and Ni3+ ions in tetragonal crystal field (2A1g ground state). When a Ni3+ ion
is in a uniform Co environment, the orbitally degenerate state for Ni3+ is preserved and the EPR spectrum of
the ground vibronic doublet state can be observed (intermediate Jahn-Teller effect). The value of g1 decreases
as the mean M-O bond is contracted. The local tetragonal distortion can be observed for Ni3+ ions located
in a mixed Co6-yAly environment. Along the progressive replacement of Co by Al, the strength of the crystal
field for Ni3+ increases gradually and the extent of the tetragonal distortion displays a tendency to increase.
The maximum effect of Jahn-Teller distortion is achieved when the Ni3+ ion falls in a pure Al environment.
1. Introduction
netic character of LiAlyCo1-yO2 permits exploring the effect of
the nontransition metal neighbors on the electronic structure of
The layered solid solutions of LiCoO2 with R-LiAlO2 are of
interest as cathode materials for lithium ion batteries since they
are able to deintercalate/intercalate lithium reversibly at a
potential higher than 4 V.1,2 The reversible electrochemical
extraction of Li takes place concomitantly with the reversible
oxidation of Co3+ to Co4+. Irrespective of the fact that the Al
dopant does not participate in the electrochemical reaction, it
has been found that the progressive replacement of Co by Al
leads to a smooth increase of the potential where Li is
extracted.1-4
transition metal ions in the honeycomb layer using Ni3+ spin
probes. Due to the high sensitivity of the EPR spectroscopy in
the X-band, even small amounts of paramagnetic Ni3+ ions
replacing Co3+ in the diamagnetic LiAlyCo1-yO2 can be detected
by EPR (less than 0.05%, which is the common impurity content
for cobalt salts without special purification).2 In the CoO2 layers,
the Ni3+ spin probes adopt the low-spin configuration with a
double degenerate ground state, which is Jahn-Teller unstable
in a trigonal symmetry. Therefore, the EPR spectrum of Ni3+
is dominated by the Jahn-Teller effect.9 When Al substitutes
for Co in LiCoO2, a strong change in the EPR spectrum of Ni3+
spin probes has been detected. For LiAlyCo1-yO2 obtained by a
solid-state reaction, the EPR spectrum of Ni3+ displays several
signals with tetragonal symmetry.2 However, the low resolution
of the EPR spectrum of Ni3+ recorded at 9.23 GHz (X-band)
makes their interpretation difficult.2 The requirement to obtain
EPR spectra with a better resolution necessitates using high-
frequency and high-field EPR spectroscopy (HF-EPR). This
technique has been shown to provide invaluable information
on the electronic structure of transition metal complexes.10-12
These materials have also been shown to be interesting for
NMR and EPR spectroscopic studies.2,5 Using 27Al MAS NMR
spectroscopy, the novel aspect of the 27Al NMR chemical shift
has been found for these materials. The 27Al MAS NMR spectra
collected by using a high spinning speed in a high magnetic
field (11.7 T) display a regular chemical shift from 62.5 to 20
ppm with an increment of 7 ppm along the substitution of Al
for Co.5 The observed changes in the 27Al NMR chemical shift
have been attributed to the Co nucleus located in the second
metal shell. Moreover, the distribution of the components in
the complex profile of the NMR spectrum has been shown to
obey the statistical distribution of the Co/Al nucleus. This effect
is specific for the Co nucleus only. For example, the Ga nucleus
characterized by a high quadrupolar magnetic moment and also
located in the second coordination sphere has no significant
effect on the chemical shift of 27Al.6
1
For systems with S ) /2, HF-EPR allows resolving small g
tensor anisotropy,12 whereas for systems with integer spin, the
higher field used permits observation of the EPR spectrum of
a system denoted as “EPR-silent” in conventional X-band
spectroscopy.13 Recently, HF-EPR technique has been success-
fully applied to study in more details the electronic structure of
Ni3+ in tetragonal La2Ni0.5Li0.5O4 14 and trigonal LiGayCo1-yO2
solid solutions.6 Although the bonding properties of Ni(III)O6-
polyhedra in fluorides and oxides with perovskite- and K2NiF4-
type structures has been the subject of the intensive studies by
conventional EPR spectroscopy,15-20 little work has been done
on Ni3+ in layered oxides with trigonal crystal structures.
Nickel is found to replace Co3+ ions from the CoO2 layers,
thus forming LiNixCo1-xO2 solid solutions in the whole
concentration range: 0 e x e 1. For layered LiNi1-xCoxO2
7,8
compositions, it is well-known that Ni and Co adopt low-spin
configurations: (t2g)6(eg)1 and (t2g),6 respectively. The diamag-
* To whom correspondence should be addressed. E-mail: radstoy@
svr.igic.bas.bg.
10.1021/jp0376119 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/05/2004

4054 J. Phys. Chem. B, Vol. 108, No. 13, 2004
Stoyanova et al.
Figure 1. X-band (9.23 GHz) EPR spectra of Ni3+ spin probes in LiAlyCo1-yO2.
Here we extend the EPR study to the local coordination of
Ni3+ in trigonal LiAlyCo1-yO2 solid solutions by using high-
frequency EPR (HF-EPR). This technique permits more precise
differentiation between the Ni3+ ions having slightly different
g values. Solid solutions between LiCoO2 and LiAlO2, in which
Co and Al are statistically distributed within the layers as shown
by 27Al NMR,5 were prepared by the citrate precursor method.
The detection of absorption was performed with an bolometer.
The recording temperatures were varied from 5 to 300 K using
a variable temperature insert (Oxford Instruments).
3. Results and Discussions
Figure 1 summarizes the X-band EPR spectra of Ni3+ spin
probes in LiAlyCo1-yO2 solid solutions as a function of the
registration temperature. Above 300 K, the EPR spectrum of
Ni3+ spin probes in Al-substituted LiCoO2 consists of a single
line with Lorentzian shape and g factor insensitive toward the
Al content in LiCoO2 (g ) 2.142). The single Lorentzian line
with the same EPR parameters has already been detected in
the EPR spectrum of low-spin Ni3+ in LiCoO2. The dynamic
2. Experimental Section
Samples of LiAlyCo1-yO2 with 0 e y e 0.8 were prepared
by the citrate precursor method as described elsewhere.5 This
method consists of thermal decomposition of Li-Co-Al citrate
complexes. Lithium-cobalt-aluminum citric acid compositions
were obtained by dissolution of Li2CO3, CoCO3, and Al(NO3)3‚
9H2O in aqueous solutions of citric acid (0.1 M) at 80 °C. The
ratio between the components was as follows: Li:(Co + Al):
Cit ) 1:1:1 and Al:(Co + Al) ) y, 0 e y e 1.0. After
complexation, the solution obtained was cooled to room
temperature, then frozen instantly with liquid nitrogen, and dried
in a vacuum (20-30 mbars) at -20 °C with an Alpha-Crist
Freeze-Dryer. After drying, the solid residues were decomposed
at 450 °C with a heating rate of 1°/min and were further heated
at 800 °C for 24 h. The layered modification of LiAlO2 was
prepared by a solid-state reaction between boehmite (Al.O.OH
doped with Ni, Ni/Al ) 0.6%) and Li2CO3 at 600 °C for 30 h
in O2 atmosphere.
The lithium content of the samples was determined by atomic
absorption analysis. The total amount of cobalt was determined
complexometrically and by atomic absorption analysis. The
mean oxidation state of cobalt was determined by an iodometric
titration.
Electron paramagnetic resonance (EPR) measurements at 9.23
GHz (X-band) were carried out in a ERS 220/Q spectrometer
within the temperature range of 85-410 K. The g factors were
established with respect to a Mn2+/ZnS standard. The high-
frequency EPR spectra were recorded on a single-pass transmis-
sion EPR spectrometer built in the High-Magnetic Filed
Laboratory, Grenoble, France. The frequencies were changed
from 95 to 345 GHz using Gunn diodes and their multipliers.
2
Jahn-Teller effect for the Eg orbital state is thought to be
responsible for the appearance of the Lorentzian EPR line. On
cooling to 85 K, the EPR spectrum of Ni3+ in LiCoO2 remains
as a single Lorentzian line, whereas the EPR signal from Ni3+
in Al substituted oxides undergoes a strong change. The single
Lorentzian line is broken into several signals, the temperature
of splitting being dependent on the Al-to-Co ratio (Figure 1).
Below 230 K, a tetragonally asymmetric signal with g ) 2.1935
and g| ) 2.0535 is superimposed on the single Lorentzian line
for slightly doped LiAlyCo1-yO2, (0.05 e x < 0.25). In the EPR
notation, the tetragonal signal corresponds to a tetragonally
elongated “static” Jahn-Teller Ni3+ configuration. The g-tensor
of the tetragonal signal detected at 85 K displays a poor
dependence on the amount of Al substituted for Co. By
increasing the Al content, the anisotropic signal becomes more
complex. Below 270 K, a new anisotropic signal grows in
intensity in addition to the tetragonally asymmetric signal. At
85 K, the single symmetrical line still remains in the central
part of the EPR spectra of LiCo1-yAlyO2 and its intensity
decreases with the Al content (Figure 1). Contrary to the high-
temperature Lorentzian signal, the low-temperature symmetrical
signal displays a dependence of the g factor on the Al content:
the g factor decreases along the Al substitution. The overlapping
of the signal with tetragonal symmetry with the single Lorentzian
line has been observed at the EPR spectrum of Ni3+ spin probes
in Ga substituted LiCoO2, i.e., LiGayCo1-yO2 matrix.6 However,

Coordination of Low-Spin Ni3+ Probes
J. Phys. Chem. B, Vol. 108, No. 13, 2004 4055
Figure 2. EPR spectra recorded at 9.23 and 115 GHz of Ni3+ spin
probes in LiAlyCo1-yO2, Ni/(Al + Co) ) 0.006. For the sake of
comparison, thin lines correspond to the spectra of undoped oxides.
Figure 3. EPR spectra at 5 K of Ni3+ spin probes in LiAlyCo1-yO2,
Ni/(Al + Co) ) 0.006. EPR spectra were recorded at 115 GHz. Thin
lines correspond to the simulated signal with tetragonal symmetry. The
signal due to Ni3+ in trigonal crystal field is indicated in the
experimental spectrum of LiCoO2.
the important difference between the EPR spectra of Ni3+ in
both LiAlyCo1-yO2 and LiGayCo1-yO2 matrixes is manifested
in the low-magnetic field region. For a Ga-rich composition,
X-band EPR spectra exhibit a new signal with g ≈ 5 ascribable
to high-spin Ni3+ located near defect sites.6 Since this signal is
not detected for Al substituted LiCoO2, one may conclude that
these defects are specific for the Ga-substituted oxides.
Below 80 K, a further change in the EPR profile of Ni3+
takes place. To improve the resolution of the EPR spectrum,
HF-EPR was undertaken. However, due to the lower sensitivity
of the HF-EPR technique (single-pass technique) as compared
to the conventional X-band EPR spectrometer, a higher spin
concentration is needed. That is why the oxides have additionally
been doped with Ni with a concentration of Ni/(Co + Al) )
0.006. In this case, the signal in the X-band EPR spectrum is
broadened, but the EPR profile is preserved (Figure 2).
Moreover, the crystal structure parameters of Ni doped
LiAlyCo1-yO2 remain the same.
leading to a new set of vibronic basis states whose vibrational
part represents a distortion along one of the cubic 4-fold axes.
The transition from intermediate to quasistatic Jahn-Teller for
Ni3+ substituted for Co3+ in the CoO2 layers is associated with
the increased role of the random lattice strains due to lattice
inperfections.22,23
When Al substitutes for Co, there is a strong change in the
EPR profile of Ni3+ spin probes (Figure 3). For slightly
substituted oxides, the spectrum exhibits a tetragonal symmetry
with a g tensor depending on the amount of Al substituted for
Co (Table 1). This behavior is typical of the static Jahn-Teller
effect. By increasing the Al amount, the perpendicular complex
envelope is split into several components, whereas the changes
in the parallel envelope are less obvious. For the sample with
50% of Al, the perpendicular envelope can be presented as a
sum of the perpendicular components of signals characteristic
of the two end compositions, LiAl0.1Co0.9O2 and LiAl0.8Co0.2O2.
For the end LiAlO2 composition, the EPR spectrum consists of
a tetragonal symmetry signal with a small anisotropy in the
perpendicular region. By increasing the recording temperature,
the perpendicular components are broadened and, finally,
coalesce into one nearly symmetrical signal due to the dynamic
Jahn-Teller effect (Figure 4, Figure 1). There is a decrease in
the value of g when the Al content increases (Table 1). In all
cases, g is greater than g|, indicating that the Ni3+ ions are in
a tetragonally elongated octahedron with an 2A1g ground state.
Irrespective of the fact that the LiCoO2 host provides a trigonally
distorted site, the observation of Ni3+ in tetragonal symmetry
can be understood in terms of local tetragonal distortion not
affecting the collective trigonal symmetry. Moreover, the local
tetragonal distortion of the crystal site in a trigonal symmetry
host has also been found for NiO6 in trigonal LiNiO2 based on
Ni K-edge EXAFS analysis.24
At 5 K, Figure 3 compares the EPR spectra of Ni3+ in
LiAlyCo1-yO2 recorded at 115 GHz. The EPR spectrum of Ni3+
in pure LiCoO2 consists of an anisotropic doublet and a nearly
symmetric single line in the central part. According to our
previous X- and Q-band EPR studies on Ni doped LiCoO2, the
anisotropic doublets originate from the ground vibronic doublet
state, whereas the single line can be assigned to an excited
vibronic singlet and/or relaxation averaged singlet.9 This feature
2
is typical for Eg ions with a weak to moderate Jahn-Teller
coupling (intermediate Jahn-Teller effect).21,22 The correspond-
ing g factors are
g+ ) g1 + qg2 f(Rj)
g- ) g1 - qg2 f(Rj)
(1)
(2)
Here, q is the vibronic reduction factor, f(Rj) is the function of
the mean angle, Rj, between the vectors of the random strains
and the external magnetic field, and g2 ) g1 - 2.0023 is the
contribution of the orbital magnetic moment to the overall
magnetic moment.
In addition, a signal with a tetragonal symmetry can also be
resolved in the EPR spectrum of Ni3+ ions in LiCoO2 (Figure
3). This feature is characteristic of the “quasistatic” Jahn-Teller
effect: random strengths permit coupling of the excited A1-
(A2) vibronic levels to the ground vibronic doublet state, thus
The values of g| and g can be expressed by25
g| ) ge + 2(ê/δ)2
(3)
(4)
g ) ge + 3ê(1.38/E3 + 0.62/E4) + 2(ê/δ)2
The deviation of g| from the g factor of the free electron (ge)

4056 J. Phys. Chem. B, Vol. 108, No. 13, 2004
Stoyanova et al.
TABLE 1: g Tensor for Low-Spin Ni3+ in LiAlyCo1-yO2 Hosta
Ni3+ in tetragonal crystal field
Ni3+ in trigonal crystal field
matrix
g
g|
g1
qg2
rav, Å
rax/req
LiCoO2
LiCo0.9Al0.1O2
2.1759
2.1967
2.1868
2.2002
2.1924
2.1856
2.1768
2.1844
2.1721
2.0739
2.0394
2.0394
2.0405
2.0405
2.0396
2.0396
2.0391
2.0391
2.0379
2.1415
2.1393
0.0333
0.0437
1.932
1.931
LiCo0.5Al0.5O2
2.1344
0.0439
1.927
LiCo0.2Al0.8O2
LiAlO2
2.1280
0.0422
1.910
1.926
g1)2.1700
g2)2.1610
2.253
[19 ]
La2Li0.5Ni0.5O4
NaNiO2[ 21]
LaSrAl0.90Ni0.10O4
2.015
2.030
2.043
1.95
1.99
1.92
1.20
1.12
1.04
2.283
[20]
I - 2.212
II - 2.253
III: g1 ) 2.344
g2 ) 2.22
2.227
2.043
g3 ) 2.043
[19 ]
LaSrGa0.93Ni0.07O4
2.044
2.044
1.99
1.07
2.250
Ni3+ in cubic MgO [ 28]
2.1685
0.06
2.106
a The mean M-O bond length, the extent of tetragonal distortion expressed by the ratio between two long and four short bond lengths (rax/req).
Figure 4. Temperature variation of the EPR spectra of Ni3+ in LiAl0.5Co0.5O2 (left) and LiAl0.8Co0.2O2 (right). EPR spectra were recorded at 115
and 285 GHz, respectively.
results from the doublet-quartet energy separation, δ, whereas
the doublet-quartet energy separation and the ligand-field
energies of the transitions 2A f 2aT2, 2bT2, contribute to the g
value. In terms of Jahn-Teller stabilization, the low-spin
configuration (doublet state) can lower its energy with respect
to the high-spin configuration (quartet state) by increasing the
tetragonal elongation. This means that the g| value provides
information on the extent of the tetragonal distortion. Table 1
gives the g tensor values of Ni3+ in a tetragonal-symmetry host
(e.g., LaLi0.5Ni0.5O4, LaSrAl1-xNixO4, and LaSrGa1-xNixO4 with
the K2NiF4-type structure and the monoclinic layered structure
NaNiO2),19,20,26,27 where the extent of tetragonal distortion of
the Ni(III)O6 octahedron is determined by diffraction methods.
(The extent of the tetragonal distortion is expressed by the ratio
of the axial and equatorial bonds: rax/req.). As the extent of the
tetragonal distortion increases, the g| value tends to ge. The
increase in the g value reveals the weakness in the crystal field
due to the increase in the Ni-O bond length.
studies of nearly stoichiometric Li1-δNi1+δO2 (δ ≈ 0.02), the
extent of local tetragonal distortion for the Ni(III)O octahedron
has been evaluated: rax/req ) 1.09.24 The close tetragonal
elongation estimated for Al- and Ga-substituted LiCoO2 and
Li1-δNi1+δO2 suggests that the Jahn-Teller effect for Ni3+ in
layered lithium transition metal oxides is limited by the rigidity
of the close packed structure. The decrease in g with increasing
Al content means that the Ni-O bond becomes more covalent,
i.e., the strength of the crystal field increases. The discrete
strengthening of the crystal field of Ni3+ can be associated with
the effect of the first metal neighbors in the layer. The metal
layers of trigonal LiMO2 are composed of edge sharing MO6
octahedra, where every Ni3+ ion in the layer has 6 metal
neighbors at a distance a (a, unit cell parameter). Figure 5 shows
the dependence of the g tensor of Ni3+ on the number of Al
neighbors in the first metal shell. The relation between the
electronic structure and the g tensor for low-spin Ni3+ (eq 1)
indicate that the strength of the crystal field of Ni3+ increases
gradually with the number of Al in the first metal sphere, but
the extent of the tetragonal distortion is limited by the host
matrix. The discrete changes in the strength of the crystal field
of Ni3+ along the Al substitution bears resemblance with the
regular chemical shift of the 27Al nucleus (from 62.5 to 20 ppm)
observed in the NMR spectra of the LiAlyCo1-yO2 solid
solution.5 The effect of the metal neighbors on both the EPR
The comparison of the g-tensor for Ni3+ in a tetragonal-
symmetry host with that for Ni3+ in a layered LiAlyCo1-yO2-
host shows that the extent of tetragonal distortion for the Ni3+O6
octahedron tends to increase slightly with the Al content and
reaches 1.12 > rax/req g 1.08 (Table 1). A similar tetragonal
elongation for the Ni3+O6 octahedron has been found for Ga
substituted LiCoO2: rax/req g 1.08.6 Using Ni K-edge EXAFS

Coordination of Low-Spin Ni3+ Probes
J. Phys. Chem. B, Vol. 108, No. 13, 2004 4057
state for Ni3+ is preserved and the EPR spectrum of the ground
vibronic doublet state can be resolved (intermediate Jahn-Teller
effect). The value of g1 decreases as the mean M-O bond is
contracted. The local tetragonal distortion can be observed for
Ni3+ ions located in mixed Co6-yAly environment. The strength
of the crystal field for Ni3+ increases gradually, and the extent
of the tetragonal distortion shows a tendency to increase along
the progressive replacement of Co by Al.
Acknowledgment. E.Zh. and R.S. are indebted to the
National Science Fund of Bulgaria (Contract No. Ch1304/2003)
for financial support. The authors acknowledge financial support
from MCyT (Contract MAT2002-00434) and Programa Ramo´n
y Cajal. R.S. is grateful to the EC for a grant within the “Access
to Research Infrastructure action of the Improving Human
Potential Program”-European Community having enabled her
to perform the high-frequency EPR measurements at a High
Magnetic Field Laboratory in Grenoble, France. The authors
are very grateful to Dr. Anne-Laure Barra (High Magnetic Field
Laboratory, Grenoble) for her help.
Figure 5. Values of the g tensor for Ni3+ in tetragonal crystal field
versus the number of Al included in the next metal [AlnCo6-n
]
environment.
spectra of Ni3+ and the 27Al NMR spectra is the most interesting
experimental finding, which deserves further theoretical descrip-
tion. It is worth mentioning that metal neighbors in oxides with
a K2NiF4-type structure have been shown to induce either
tetragonal elongation or tetragonal compression of the Ni(III)O6
octahedra.17,29 A NiO6 distortion in LaSrAl1-xNixO4 due to the
preferential coordination of Sr2+ or La3+ has also been proposed
to explain the doubling of the perpendicular component of the
g tensor with constant parallel component.20
References and Notes
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The second feature of the low-temperature HF-EPR spectra
of Ni3+ in Al substituted oxides is the appearance of the signal
due to moderate JT coupling (Figure 3). For slightly substituted
oxides, this signal is barely detected, whereas a well-resolved
signal is observed with the Al rich oxide. The doublet signal
characteristic of Ni3+ in LiCoO2 is not observed for Ni3+ in
layered LiAlO2. The value of g1 decreases as the Al content
increases. As in the case of pure LiCoO2, this signal can be
assigned to Ni3+ in a trigonally distorted crystal field, i.e., to
2
Ni3+ ions in Eg ground state. For comparison, Table 1 also
gives the EPR parameters for the ground vibronic state of Ni3+
dopants in a MgO matrix with a cubic symmetry.28 There is a
dependence of the value of g1 on the mean M-O bond length
determined by XRD technique.
The observation of two types of Ni3+ ions in a trigonal
LiAlyCo1-yO2 host can be rationalized if we take into account
the effect of the metal neighbors. When Co3+ ions surround
Ni3+ only in the metal layer, the local trigonal symmetry of the
host site is preserved and Ni3+ in Eg ground state is detected.
The progressive changes in the metal coordination sphere from
Co3+ to Al3+ along with the substitution are, on one hand,
responsible for the local tetragonal distortion of the host site
(19) Reinen, D.; Kesper, U.; Belder, D. J. Solid State Chem. 1995, 116,
355.
(20) Yablokov, Yu. V.; Ivanova, T. A. Coord. Chem. ReV. 1999, 190-
192, 1255.
and, on the other, contribute to the electronic structure of Ni3+
.
(21) Ham, F. S. Phys. ReV. 1968, 166, 307.
The first metal coordination sphere becomes important and the
nature of the atom in this position will lead to further
differentiation of Ni3+ in a tetragonal crystal field. The
maximum effect of Jahn-Teller stabilization is achieved when
Ni3+ is in pure Al surrounding.
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4. Conclusions
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The use of HF-EPR spectroscopy allows us to distinguish
between Ni3+ ions in 2Eg and 2A1g ground states. When the Ni3+
ion is in a uniform Co environement, the orbitally degenerate
2047.
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