Ag3Ni2O4-A new stage-2 intercalation compound of 2H-AgNiO2 and physical properties of 2H-AgNiO2 above ambient temperature

Article abstract of DOI:10.1016/j.jssc.2006.08.033

Ag₃Ni₂O₄ was obtained as single crystals from a mixture of 2H-AgNiO₂ and Ag₂O in oxygen high-pressure autoclaves (P6₃/mmc (no. 194), a = 2.9331 (6), c = 28.313 (9) A, Z = 2). It may be regarded as a stage-2 intercalation compound of the host 2H-AgNiO₂ and is the first staging compound constituted of alternating subvalent over(∞, 2) Ag₂⁺ and Ag⁺ sheets, inserted between NiO₂^- slabs. From a structural point of view, Ag₃Ni₂O₄ represents an intermediate between AgNiO₂ and the recently reported Ag₂NiO₂. The electronic structures of 2H-AgNiO₂ and Ag₃Ni₂O₄ have been investigated based on DFT band structure calculations. The high-temperature characteristics of the starting material 2H-AgNiO₂ were investigated. The inverse magnetic susceptibility, electrical resistivity and differential scanning calorimetry (DSC) show a phase transition in the temperature range of T = 320 - 365 K.

Full text of DOI:10.1016/j.jssc.2006.08.033

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Journal of Solid State Chemistry 180 (2007) 8–15
www.elsevier.com/locate/jssc
Ag Ni O —A new stage-2 intercalation compound of 2H–AgNiO and
3
2 4
2
physical properties of 2H–AgNiO above ambient temperature
2
Ã
¨
Timo Sorgel, Martin Jansen
Max-Planck-Institut f u¨ r Festk o¨ rperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany
Received 29 June 2006; received in revised form 29 August 2006; accepted 31 August 2006
Available online 23 September 2006
Abstract
Ag Ni O was obtained as single crystals from a mixture of 2H–AgNiO and Ag O in oxygen high-pressure autoclaves (P6 /mmc (no.
3
2
4
2
2
3
˚
1
94), a ¼ 2:9331ð6Þ, c ¼ 28:313ð9Þ A, Z ¼ 2). It may be regarded as a stage-2 intercalation compound of the host 2H–AgNiO and is the
2
2
þ
+
ꢀ
first staging compound constituted of alternating subvalent 1 Ag and Ag sheets, inserted between NiO
2
slabs. From a structural
2
point of view, Ag
2
3 2 4 2 2 2
Ni O represents an intermediate between AgNiO and the recently reported Ag NiO . The electronic structures of
H–AgNiO and Ag Ni O have been investigated based on DFT band structure calculations. The high-temperature characteristics of
2
2
3
4
2
the starting material 2H–AgNiO were investigated. The inverse magnetic susceptibility, electrical resistivity and differential scanning
calorimetry (DSC) show a phase transition in the temperature range of T ¼ 3202365 K.
r 2006 Elsevier Inc. All rights reserved.
Keywords: Layer compounds; Silver nickelates; Intercalation; Staging; Subvalency
1
. Introduction
absorption near edge structure spectroscopy and quantum
chemical calculations [5,6].
Ternary silver oxides are known to show a rich and
interesting structural chemistry which is associated with
extended clustering in the silver sublattice [1,2]. Since these
+
Ag substructures represent sections of the structure of
In this paper, we report on the synthesis of single crystals
of Ag Ni O , which may be regarded as a stage-2
intercalate of 2H–AgNiO [7].
3
2
4
2
elemental silver, showing the same interatomic separations,
it has been suggested that silver should be susceptible to
form subvalent compounds [2]. Indeed, a number of them
have been realized till now, showing various structural
motives and oxidation states of silver. Ag OsO contains
2. Experimental
Reacting 2H–AgNiO with Ag O has yielded Ag Ni O
4
2
2
3
2
as main product besides Ag NiO .
2
2
Ag Ni O crystals were found in experiments with
3
1
3
6
2
4
4
þ
silver atom centered icosahedra Ag13 with silver in the
subvalent state of +4/13 [3]. A two-dimensional arrange-
varying molar ratios, temperatures, oxygen pressures and
mineralizers. Here we report on a synthesis which leads to
an optimal proportion of Ag Ni O in the product batch.
2
þ
ment in the form of 1 Ag layers, which can again be seen
2
as a section of elemental silver metal, was found previously
3
2
4
Ag O was precipitated from an aqueous AgNO (p.a.,
2 3
in the binary fluoride Ag F [4]. Recently, a first ternary
2
Roth) solution by dropwise adding of an aqueous KOH
(p.a., Roth) solution. The precipitate was washed with
distilled water and dried overnight at 373 K. 2H–AgNiO2
was synthesized as reported elsewhere [7]. About 396 mg of
compound, Ag NiO , was reported to also be constituted
2
2
2
þ
of 1 Ag layers, coexisting with nickel in the oxidation
2
state +3, as proven by the magnetic susceptibility, X-ray
2H–AgNiO and 116 mg Ag O were thoroughly ground
2 2
and transferred into a gold ampoule. About 1 mL of a 5 M
KOH solution was added as a mineralizer. The ampoule
was heated in an oxygen high-pressure autoclave at a rate
Ã
Corresponding author. Fax: +49 711 689 1502.
E-mail address: m.jansen@fkf.mpg.de (M. Jansen).
0022-4596/$ - see front matter r 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2006.08.033

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T. S o¨ rgel, M. Jansen / Journal of Solid State Chemistry 180 (2007) 8–15
9
of 100 K/h to 823 K where it was kept for 70 h under an
Table 1
3 2 4
Experimental details on the X-ray single crystal investigation of Ag Ni O
oxygen pressure of 130 MPa. Then it was cooled to room
temperature at a rate of ꢀ100 K/h and the products were
washed with distilled water.
Formula weight (g/mol)
Temperature (K)
504.99
298(2)
Powder X-ray data were recorded on a Stoe Stadi P
diffractometer (Stoe & Cie, Darmstadt, Germany). An
˚
Wave length (A
)
0.71073
0.15 ꢁ 0.15 ꢁ 0.10
3
Crystal size (mm )
˚
Crystal system; space group
(no.)
Lattice parameters (A)
3
Hexagonal; P6 /mmc (194)
external standard (Si, a ¼ 5; 43088ð4Þ A) was applied.
Single crystal X-ray data were recorded on a SMART-
APEX CCD diffractometer (Bruker AXS Inc., Madison,
USA) with graphite-monochromated MoKa radiation. The
reflection intensities were integrated with the SAINT [8]
subprogram in the SMART [9] software package. An
empirical absorption correction (SADABS [10]) was
applied. The crystal structure was solved by direct methods
and refined by full-matrix least squares using the
SHELXTL software package [11].
˚
a ¼ 2:9331ð6Þ
c ¼ 28:313ð9Þ
210.94(9); 2
7.951; 22.328
3
˚
Volume (A
); Z
3
–1
r
X-ray (g/cm ), m (mm )
Diffractometer
Bruker AXS Smart APEX, MoKa
radiation, graphite monochromator,
o-scans
1.44–34.91
2y range (1)
h k l range
No. of measured reflections;
ꢀ4php4, ꢀ4pkp4, ꢀ44plp44
2800; (R(int) ¼ 0.0298)
R
int
Magnetic susceptibilities were determined with a Quan-
tum Design SQUID magnetometer MPMS type 7.0
No. of independent
reflections
Completeness to
237
(
Quantum Design, San Diego, USA) in the temperature
92.5%
range 5 KoTo400 K. A powder sample (mE120 mg) was
placed into a high-purity quartz glass tube (Suprasil,
Heraeus, Hanau, Germany) which was subsequently evac-
uated and flooded with helium gas under reduced pressure.
The molar susceptibility was corrected for the diamagnetic
contribution of the closed shell electrons [12].
theta ¼ 34.911
a
Absorption correction
Data/restraints/parameters
Multiscan
237/0/16
1.353
2
Goodness-of-fit on F
Final R-values (I42s(I))
R-values (all data)
R
R
1
¼ 5.17%, wR
2
2
¼ 10.41%
¼ 11.74%
1
¼ 5.60%, wR
3
)
˚
DFmax; DFmin (e/A
3.560; –3.811
The resistivity was measured as a function of tempera-
ture employing the van der Pauw method [13]. Four Pt
wires served as Ohmic electrical contacts and were pressed
together with the powder (diameter: 6 mm, thickness:
E1 mm).
Differential scanning calorimetry (DSC) measurements
were performed on a DSC 404 unit (Netzsch, Selb,
Germany) in aluminum crucibles at a heating rate of
a
SADABS: V. 2.10 G. M. Sheldrick, Bruker AXS Inc., Madison, USA
2003).
(
Table 2
Atomic coordinates and equivalent isotropic displacement parameters (in
2
˚
A
) for Ag Ni O
4
3
2
Atom
Position
x
y
z
U(eq)
1
0 K/min.
DFT calculations were performed on the basis of the
Ag1
Ag2
Ni1
O1
2c
4f
4f
4f
4e
1/3
1/3
1/3
1/3
0
2/3
2/3
2/3
2/3
0
1/4
0.013(1)
0.016(1)
0.011(1)
0.015(3)
0.015(3)
experimental structures of 2H–AgNiO [7] and Ag Ni O
4
0.0402(1)
0.8580(1)
0.1774(5)
0.8949(5)
2
3
2
(
this work). The TB-LMTO-ASA program [14] (Tight
Binding Linear Muffin Tin Orbital calculations within the
Atomic Sphere Approximation) was used choosing the
scalar relativistic option and the Local Density Approx-
imation (LDA) functional of v. Barth and Hedin [15] to
describe the exchange and correlation interaction. The radii
of the atomic spheres and additional empty spheres to
minimize the overlap of the atomic spheres [16] were
determined by applying the standard procedures of the
program. Due to the downfolding process, for oxygen only
the 2p states were considered in the starting configuration
of the calculation.
O2
U(eq) is defined as one third of the trace of the orthogonalized U tensor.
ij
Table 3
Anisotropic displacement parameters (in A ) for Ag
2
˚
3
Ni
2
O
4
, in the form
2
2
2
exp(ꢀ2p (h a* U11+y+2hka*b*U12))
Atom
U
11
U
22
U
33
U
23
U
13
U
12
Ag1
Ag2
Ni1
O1
0.014(1)
0.017(1)
0.011(1)
0.018(4)
0.014(4)
0.014(1)
0.017(1)
0.011(1)
0.018(4)
0.014(4)
0.011(1)
0.013(1)
0.012(1)
0.011(5)
0.018(6)
0
0
0
0
0
0
0
0
0
0
0.007(1)
0.009(1)
0.005(1)
0.009(2)
0.007(2)
The Hartree–Fock calculations were performed with the
program Crystal [17] and the basis sets and pseudo
potentials were taken from the work of Wedig et al. [18].
O2
3
. Results
structure details and selected bond lengths and angles are
given in Tables 1–4 [19].
According to
a
single crystal structure analysis,
Ag Ni O is hexagonal (P6 /mmc). Experimental crystal
In the product mixture, which was investigated by X-ray
powder diffraction, Ag Ni O , 2H–AgNiO and Ag NiO
2
3
2
4
3
3
2
4
2
2

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T. S o¨ rgel, M. Jansen / Journal of Solid State Chemistry 180 (2007) 8–15
10
Table 4
3 2 4
Selected bond lengths and angles for Ag Ni O
˚
Connected atoms
Distance/angle (A/1)
Ag1–O1 (2x)
Ag2–O2 (3x)
2.06(1)
2.50(1)
Ag1–Ag1 (6x)
Ag2–Ag2 (3x)
Ag2–Ag2 (6x)
2.9331(6)
2.837(2)
2.9331(6)
Ni1–O1 (3x)
Ni1–O2 (3x)
1.967(6)
1.990(8)
Ni1–Ni1 (6x)
2.9331(6)
180.0
O1–Ag1–O1 (1x)
O1–Ni1–O(1) (3x)
O1–Ni1–O(2) (6x)
O1–Ni1–O(2) (3x)
O2–Ni1–O(2) (3x)
96.4(5)
84.3(4)
178.9(5)
95.0(5)
Ni1–O1–Ni1 (3x)
Ni1–O2–Ni1 (3x)
96.4(5)
95.0(5)
Ni1–O1–Ag1 (3x)
Ni1–O2–Ag2 (6x)
Ni1–O2–Ag2 (3x)
120.6(4)
95.6(2)
164.3(6)
Ag2–O2–Ag2 (3x)
71.9(4)
Ag2–Ag2–Ag2 (3x)
Ag2–Ag2–Ag2 (6x)
Ag2–Ag2–Ag2 (3x)
Ag2–Ag2–Ag2 (6x)
Ag2–Ag2–Ag2 (6x)
Ag2–Ag2–Ag2 (6x)
Ag2–Ag2–Ag2 (3x)
58.87(3)
60.0
62.26(6)
90.0
120.0
121.13(3)
180.0
were found. Additional weak Bragg reflections could be
indexed with a unit cell, corresponding to the metrics of a
potential 2H–Ag NiO
2
(a ¼ b ¼ 8; 798ð5Þ and c ¼
2
˚
1
6; 135ð5Þ A, corresponding to a ¼ 3a(3R–Ag NiO ) and
2
2
3 2 4
Fig. 1. Structure of Ag Ni O with unit cell.
c ¼ (2/3)c(3R–Ag NiO )).
2
2
ꢀ
The dominating structural feature of Ag Ni O is NiO
2
3
2
4
ꢀ
layers which are stacked along the c-axis alternating with
+
The NiO2 layers in Ag Ni O differ from those in
3
2
4
+
Ag -single and Ag -double layers (Fig. 1). Thus the new
2H–AgNiO [7] and Ag NiO [5] (at T4260 K), as the
2
2
2
2
þ1
þ0:5
þ3 ꢀ2
oxide can be written as Ag Ag Ni O ,
2 4
or
þ3 ꢀ2
O , respectively. It is an inter-
2
Ni–O distances are no longer equal. This is due to the
different coordinations of the oxide ions, which are
tetrahedral (O1Ni Ag1) and octahedral (O2Ni Ag2 ),
2
þ1
þ3 ꢀ2
þ0:5
Ag Ni
O
ꢁ Ag Ni
2
2
mediate between AgNiO and Ag NiO , yet the polytype is
2
2
2
3
3
3
of 2H instead of 3R type [20]. This intermediate character
is supported by the bond distances and angles, which are
close to those in 2H–AgNiO and Ag NiO . The bond
respectively. However, also in Ag Ni O there is obviously
3 2 4
a spin-1/2 triangular lattice like in AgNiO and Ag NiO ,
2
2
2
which is responsible for a variety of phase transitions
driven by orbital and spin ordering in these compounds.
Ag NiO undergoes a Jahn–Teller phase transition at
2
2
2
distances in the Ag1-single layers are slightly shortened as
compared to 2H–AgNiO (0.1%), while they are elongated
2
2
2
by 0.3% and 0.5%, respectively, in the Ag2-double layers
as compared to Ag NiO . As the silver single and double
layers are commensurate, the Ag1–Ag1 and Ag2–Ag2
T ¼ 260 K [18], while for 2H–AgNiO no structural phase
S
2
transition was noticed in the investigated range 5–400 K
[21,22]. Instead, an electronically driven phase transition
occurs in the range of T ¼ 320–365 K, which can be
derived from susceptibility and electrical resistivity
measurements. The inverse magnetic susceptibility of
2
2
distances in the a–b-plane are all equal and amount to
2
+
.9331(6) A. The distance between the Ag2 layers in
˚
˚
Ag Ni O is in the nanometer range (c=2 ¼ 14:16 A), in
3
2
4
contrast to the respective distance in Ag NiO
2
2H–AgNiO (Fig. 2) reveals, apart from the antiferromag-
2
2
˚
(
c=3 ¼ 8:03 A).
netic phase transition at T ¼ 22 K which has been

ARTICLE IN PRESS
T. S o¨ rgel, M. Jansen / Journal of Solid State Chemistry 180 (2007) 8–15
11
Fig. 2. Inverse magnetic susceptibility of 2H–AgNiO
2
up to T ¼ 400 K.
discussed elsewhere [7,21,22], two Curie–Weiss regions in
the range of 22 KoTo320 K. The data points were fitted
independently for the intervals 40–120 and 209–299 K. This
has led to magnetic moments of meff ¼ 1.81 m , and 1.96 m ,
B
B
and Weiss constants of Y ¼ ꢀ107, and ꢀ149 K, respec-
P
tively. In the range of 320 KoTo365 K, there is a smooth
step into a third high-temperature Curie–Weiss region,
with meff ¼ 1.96 m and Y ¼ ꢀ161 K (fit of data points in
B
P
the range 370–400 K).
The resistivity measurement above room temperature
shows a step-like decrease between two metallic regimes in
the range 325 KoTo365 K. The conductivity improves by
about 5%.
Fig. 3 shows the resistivity normalized to r(310 K),
which shows a decrease of the resistivity by 5% upon the
phase transition. The resistivity below room temperature
was published elsewhere [7].
2
Fig. 3. Resistivity r of 2H–AgNiO , relative to r(310 K).
DSC measurements show a reversible phase transition
with small peaks at T ¼ 358 K (heating experiment) and
T ¼ 354 K (cooling experiment).
polarized calculations are sufficient to deduce the essential
features of the band structures.
The band structures were calculated for 2H–AgNiO and
2
Ag Ni O on DFT basis. They were performed without
3
The band structure of 2H–AgNiO2 in the fatband
presentation is shown for Ag-4p, Ag-5s and -5p, Ni-3d
and O-2p states in Fig. 4. Ni-3d and O-2p states dominate
at the Fermi level, which can be seen from the partial DOS
in Fig. 5. Yet, also Ag-5s, -5p and -4d states contribute to
the highest occupied bands. The integrated partial DOS
(IDOS) shows that the Ag-4d states are not fully occupied
and more than half an electron is promoted into Ag-5s and
-5p states (Table 5). In contrast to the band structure of
2
4
spin polarization, as spin polarized calculations did not
converge. As is known for spin polarized calculations
which were performed for 3R–AgNiO and Ag NiO on a
2
2
2
mixed Hartree–Fock and DFT basis, the results are very
similar to the ones obtained from non-spin polarized
calculations with respect to band structure characteristics
and density of states (DOS) [18]. Therefore it was
concluded that in the family of silver nickelates, non-spin

ARTICLE IN PRESS
T. S o¨ rgel, M. Jansen / Journal of Solid State Chemistry 180 (2007) 8–15
12
2
Fig. 4. Band structure of 2H–AgNiO with fatbands of (a) Ag-5s and -5p, (b) Ag-4d, (c) Ni-3d and (d) O-2p states.
2
Fig. 5. Partial DOS and IDOS of the Ag-5s and -5p, Ni-3d and O-2p states in 2H–AgNiO .

ARTICLE IN PRESS
T. S o¨ rgel, M. Jansen / Journal of Solid State Chemistry 180 (2007) 8–15
13
3R-AgNiO [21,23], Ag-4d states occupy bands very close
to the Fermi level.
Fig. 8). Yet, the states of Ni-3d and O-2p still dominate the
Fermi level. Like in 2H–AgNiO , the Ag1-4d states
2
2
For Ag Ni O the band structure is also shown in
4
contribute to bands right below the Fermi level, whereas
the Ag2-4d states correspond more to the Ag-4d states
found in Ag NiO and contribute to bands of lower
energies. From the partial IDOS, it is evident that Ag2 is
subvalent in contrast to Ag1, as there is more than one
electron in addition in Ag2 states (see Table 6).
3
2
fatband representation for the Ag-5s and -5p, Ag-4d
Fig. 6), Ni-3d and O-2p states (Fig. 7). The band structure
is more complex than the one of 2H–AgNiO , and it shows
(
2
2
2
the characteristics of both the band structures of
2
H–AgNiO and Ag NiO [21]. The 5s and 5p states of
2 2 2
+
Ag2, which make up the Ag2 layers, contribute more
pronouncedly to the states at the Fermi level than those of
+
Ag1, which constitutes the single Ag layers (see DOS in
4. Discussion
In 2H–AgNiO , the changes in the course of the inverse
2
Table 5
magnetic susceptibility and the electrical resistivity at
T4320 K might be due to spin and orbital ordering. There
are a number of degenerate states so that a change in the
ordering pattern may easily occur upon temperature
change. The range of the transition between the second
and third Curie–Weiss region corresponds well with
the temperature region where dr/dT is negative. In
2
Integrated DOS for Ag, Ni, O, E and E1 in 2H–AgNiO (E and
E1 ¼ empty spheres)
Atom
IDOS(s) (states/cell) IDOS(p)
states/cell)
IDOS(d)
(states/cell)
(
Ag
Ni
O
0.41
0.39
0.07
0.25
0.16
0.29
0.54
3.91
0.25
0.09
9.18
8.30
0.06
0.12
—
3
R–AgNiO , a negative slope in dr/dT is reported for
2
E
E1
T4300 K [24], yet there is no report on a change into a
further region with dr/dT40 at higher temperatures.
3 2 4
Fig. 6. Band structure of Ag Ni O with fatbands for (a) Ag1-5s and -5p, (b) Ag1-4d, (c) Ag2-5s and -5p and (d) Ag2-4d states.

ARTICLE IN PRESS
T. S o¨ rgel, M. Jansen / Journal of Solid State Chemistry 180 (2007) 8–15
14
3 2 4
Fig. 7. Band structure of Ag Ni O with fatbands for (a) Ni-3d, (b) O1-2p and (c) O2-2p states.
3 2 4
Fig. 8. Partial DOS and IDOS of the Ag1-5s and -5p, Ag1-4d, Ag2-5s and -5p, Ag2-4d, Ni1-3d, O1-2p and O2-2p states in Ag Ni O .

ARTICLE IN PRESS
T. S o¨ rgel, M. Jansen / Journal of Solid State Chemistry 180 (2007) 8–15
15
Table 6
Acknowledgments
3 2 4
Integrated DOS for Ag1, Ag2, Ni1, O1, O2, E, E1 and E2 in Ag Ni O (E,
E1 and E2 ¼ empty spheres)
We thank Dr. U. Wedig for the Hartree–Fock calcula-
tion and discussion, Dr. J. Nuss for the single crystal X-ray
data collection, E. Brucher for the magnetic susceptibility
¨
Atom
IDOS(s) (states/cell) IDOS(p)
states/cell)
IDOS(d)
(states/cell)
(
measurement and G. Siegle for the electrical resistivity
measurement.
Ag1
Ag2
Ni1
O1
O2
E
0.39
0.62
0.37
0.07
0.09
0.26
0.17
0.19
0.26
0.58
0.52
3.91
3.99
0.26
0.10
0.15
9.08
9.68
8.28
0.06
0.05
0.13
—
References
E1
E2
—
[
[
1] M. Jansen, J. Less-Common Met. 76 (1980) 285.
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[
Since Ag Ni O is only obtained if 2H–AgNiO is used
2
3
2
4
as an educt, of which the stacking sequence of Ni- and O-
layers only slightly differs from the one in Ag Ni O , we
suggest that Ag Ni O forms in a topotactic-like reaction.
The sum of activation energies for this intercalation
process is lower, since apart from widening of the structure,
less rearrangement of the Ni- and O layers is needed as
compared to 3R–AgNiO2.
The observations can also be rationalized in terms of
thermodynamics. The free enthalpy DG of the reaction is
likely to be less negative upon staging, as compared
to a total intercalation process. DG of the reaction
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[
[
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[
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[
[
¨
8] SAINT Version 6.45, Bruker AXS Inc., Madison, USA, 2003.
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000.
12] A. Weiss, H. Witte, Magnetochemie, Verlag Chemie, Weinheim,
973.
13] L.J. Van der Pauw, Philips Res. Rep. 13 (1958) 1.
2
2
3
R–AgNiO +Ag-Ag NiO was estimated from open cell
2 2 2
[
[
voltage measurements to be Eꢀ5 kJ/mol at T ¼ 500 K
21]. Upon staging, the system reduces its free enthalpy by a
1
[
smaller extent. Assuming an intercalation of 3R–AgNiO2,
[14] O.K. Andersen, O. Jepsen, Tight Binding Linearized Muffin Tin
Orbital Method Within the Atomic Sphere Approximation, Version
upon which a hypothetical stage-3 compound is formed,
4
15] U. Von Barth, L. Hedin, J. Phys. C 5 (1972) 1629.
.7, Stuttgart, Germany, 1996.
this would be one third, which might explain why such a
stage-3 phase was not observed, so far. For the intercala-
tion of 2H–AgNiO , this is approximately one half. From
[
[
16] O.K. Andersen, A.V. Postnikov, S.Y. Savrasov, The muffin-tin
orbital point of view, in: W.H. Butler, P.H. Dederichs, A. Gonis, R.L.
Weaver (Eds.), Applications of Multiple Scattering Theory to
Materials Science, vol. 253, Materials Research Symposium Proceed-
ings, Pittsburgh, 1992, pp. 37–70.
2
open cell voltage measurements on a cell 2H–AgNiO /AgI/
2
Ag, the free enthalpy was derived as DGEꢀ2.5 kJ/mol [21].
To summarize, it can be concluded that silver nickelates
might be accessible in a variety of compositions, which are
all very close regarding their free enthalpies of formation.
It is possible that, due to different activation energies, the
reaction paths chosen still depend on the structural details
of the starting material. Due to their structural similarity,
Ag Ni O was only obtained when starting from
[
17] V.R. Saunders, R. Dovesi, C. Roetti, M. Causa, N.M. Harrison, R.
Orlando, C.M. Zicovich-Wilson, CRYSTAL98, CRYSTAL98 User’s
Manual, University of Torino, Torino, 1998.
[
[
18] U. Wedig, P. Adler, J. Nuss, H. Modrow, M. Jansen, Solid State Sci.
8 (2006) 753.
19] Further details of the crystal structure investigation can be obtained
from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-
Leopoldshafen, Germany (fax: (49) 7247-808-666; e-mail: crysda-
ta@fiz-karlsruhe.de) on quoting the depository number CSD-416735.
3
2
4
2
H–AgNiO , whereas Ag NiO is the only product when
2 2 2
starting from 3R–AgNiO2.
[20] L.S. Ramsdell, Am. Miner. 32 (1947) 64.
[21] T. Sorgel, Dissertation, University of Stuttgart, 2006; The Publication
3
+
Ag Ni O contains spin-1/2 Ni
3
ions on a triangular
ꢀ
¨
2
4
Server of the University of Stuttgart, available through /http://
elib.uni-stuttgart.de/opus/S, to be published.
¨
22] E. Wawrzynska, E. M. Wheeler, R. Coldea, T. Sorgel, M. Jansen, in
lattice, yet due to the different environment of the NiO
2
3
sheets, Ni finds itself, unlike as in AgNiO and Ag NiO ,
+
2
2
2
[
[
[
in a distorted coordination with oxygen. This may lead
again to a different physical behavior. Concerning the
silver sublattice, Ag Ni O is an intermediate between
preparation.
23] R. Seshadri, C. Felser, K. Thieme, W. Tremel, Chem. Mater. 10
(1998) 2189.
3
2
4
24] Y.J. Shin, J.P. Doumerc, P. Dordor, C. Delmas, M. Pouchard, P.
Hagenmuller, J. Solid State Chem. 107 (1993) 303.
AgNiO2 and Ag NiO , showing Ag(I) and subvalent
2
2
Ag(+0.5) in one single compound.