AgCuO2 - Synthesis, crystal structure, and structural relationships with CuO and AgIAgIIIO2

Article abstract of DOI:10.1006/jssc.2001.9255

The new silver cuprate AgCuO₂ was synthesized by applying a low-temperature route. AgCuO₂ precipitates as a microcrystalline black and thermally labile powder from concentrated aqueous solutions of copper and silver nitrate under alkaline and oxidizing conditions. Using the positional parameters of AgAgO₂ as a starting set, Rietveld refinements have been performed using X-ray and neutron powder data (P2₁/c, a = 5.8657(3), b = 2.8062(2), and c = 6.0770(3)A, β = 108.106(4)°, Z = 2, R_p = 6.61%, R_wp = 5.00% for joint refinement). The crystal structure consists of chains of ladder-like CuO₄ rectangles running along [010], interconnected by AgO₂ dumbbells. For physical and crystal chemical reasons the oxidation states + I for silver and + III for copper, respectively, have been assigned. Similar to AgAgO₂ and CuO the metal cations form a fcc structure with oxygen at positions slightly displaced from the centers of the octahedral vacancies. The small differences from the AgAgO₂ structure can be traced back to a different off-center position in the octahedral void.

Full text of DOI:10.1006/jssc.2001.9255

Journal of Solid State Chemistry 162, 220}224 (2001)
doi:10.1006/jssc.2001.9255, available online at http://www.idealibrary.com on
AgCuO dSynthesis, Crystal Structure, and Structural Relationships
2
I
III
with CuO and Ag Ag O
2
J. Curda, W. Klein, and M. Jansenꢀ
Max-Planck-Institut fu(r Festk o( rperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
Received February 15, 2001; in revised form May 16, 2001; accepted May 25, 2001; published online July 26, 2001
IN HONOR OF PROFESSOR PAUL HAGENMULLER ON THE OCCASION OF HIS 80TH BIRTHDAY
oxidation states can be unambiguously assigned to the
The new silver cuprate AgCuO
a low-temperature route. AgCuO
2
was synthesized by applying di!erent sites by the characteristic shapes of the respective
precipitates as a microcrystal- "rst coordination spheres, linear for silver(I) and square
2
line black and thermally labile powder from concentrated aque-
ous solutions of copper and silver nitrate under alkaline and
oxidizing conditions. Using the positional parameters of
planar for silver(III). This assignment is in agreement with
Ag'Ag'''O being diamagnetic. The marked discontinuity in
ꢁ
the group systematics, as revealed by comparing CuO and
AgAgO
formed using X-ray and neutron powder data (P2
a ؍ 5.8657(3), b ؍ 2.8062(2), and c ؍ 6.0770(3)A b ؍
08.106(4)3, Z ؍ 2, R ؍ 6.61%, Rwp ؍ 5.00% for joint re5ne-
ment). The crystal structure consists of chains of ladder-like
CuO rectangles running along [010], interconnected by AgO
dumbbells. For physical and crystal chemical reasons the oxida-
2
as a starting set, Rietveld re5nements have been per-
&
&AgO'', can be traced back to the second ionization poten-
1
/c,
tial, which among the coinage metals is highest for silver.
Because of the relevance of these features to the understand-
ing of superconductivity in copper oxides we have under-
taken investigations aiming at partial substitution of silver
for copper in CuO, and of copper for silver in AgO, respec-
>
,
1
p
4
2
tion states ؉ I for silver and ؉ III for copper, respectively, have tively. If in the desired ternary silver copper oxides dis-
been assigned. Similar to AgAgO
form a fcc structure with oxygen at positions slightly displaced
from the centers of the octahedral vacancies. The small di4er-
2
and CuO the metal cations ordered 2#/3# mixed valencies were achieved, one could
expect, as another attractive perspective, such phases to
show a transition to a superconducting state. However,
ences from the AgAgO structure can be traced back to a di4er-
2
Ag O , which thus far is the only silver oxide known to
ꢂ ꢃ
ent o4-center position in the octahedral void. ( 2001 Elsevier Science
Key Words: silver cuprate(III); silver oxide; copper oxide;
Rietveld re5nement.
contain divalent and tervalent silver at the same time, re-
mains paramagnetic and does not show a transition into
a superconducting state down to 3.8 K (5, 6).
Since the desired oxides will be thermally labile, it is
advisable to employ soft synthetic approaches (&&chimie
douce'').
INTRODUCTION
Within group 11 of the periodic table, the principle of
homology is violated in several respects. For instance, one
can take the monoxides, of which the copper representative
is rather stable (1), while AgO is thermally labile (2), and no
EXPERIMENTAL
Synthesis. Microcrystalline samples of AgCuO precipi-
ꢁ
gold monoxide exists at all. Most striking are the funda- tate from combined saturated aqueous solutions of 1.205 g
mentally distinct valence states of copper and silver in these (5 mmol) of AgNO (Merck, 99%) and 0.85 g (5 mmol) of
ꢂ
oxides. Divalent, thus paramagnetic, copper is localized on Cu(NO ) ) 3H O (Merck, 99%) by adding a solution of
ꢂ ꢁ
ꢁ
one independent crystallographic site and is coordinated by 3.0 g of KOH and 1.5 g of K S O in 150 ml of water at
ꢁ
ꢁ ꢄ
4
#2 oxygen atoms forming a heavily elongated octahed- 903C. The precipitate was "ltered o!, washed with deionized
ron, in accordance with the Jahn}Teller theorem (3). Quite water, and dried in air at 703C. Another means of access to
di!erently, Ag'Ag'''O shows charge disproportionation AgCuO is oxidation of an aqueous suspension of
and has to be regarded as a pseudo-ternary oxide (4). The Ag Cu O (7) under the same conditions.
ꢁ
ꢁ
ꢁ ꢂ
ꢁ
Structure determination. X-ray powder di!raction
data were recorded using a STOE-Stadi P di!ractometer
ꢀ
To whom correspondence should be addressed. Fax: #49-711-
6
891502. E-mail: martin@jansen.mpi-stuttgart.mpg.de.
2
20
0
(
022-4596/01 $35.00
2001 Elsevier Science
All rights reserved.

STUDY OF AgCuO
221
ꢁ
TABLE 1
Crystal Data and Structure Determination of AgCuO
2
Formula weight
203.413 g/mol
Space group
Unit cell dimensions
P2 /c (No. 14)
ꢀ
a"5.8657(3) A
>
b"2.8062(2) A
c"6.0770(3) A
b"108.106(4)3
95.075(9)A
>
>
Cell volume
Z
>
ꢂ
2
Calculated density
7.105 g/cmꢂ
Wave length (X-ray) (A
Data points (X-ray)
>
)
1.540598 (Ge monochromator)
8521
Data points (neutrons)
1151 (bank 1), 2079 (bank 2), 1909
(
bank 3)
E!ective 2h range (X-ray) (3) 10.00}95.00
E!ective d range (neutr.) (A 1.0}10.0 (bank 1), 0.5}4.0 (bank 2),
.4}2.7 (bank 3)
>
)
FIG. 1. Neutron di!raction pro"le "t for AgCuO with 5% CuO
bank 1, 28.753).
ꢁ
0
(
Re"ned parameters
R-values
24
Neutrons Neutrons Neutrons X-ray
Bank 1 Bank 2 Bank 3
Rꢁ
5.47%
2.75%
3.10%
6.62%;
5.00%
3.087
9.13%
3.44%
3.88%
10.37% 5.51%
Physical properties. The magnetization was recorded
using a Squid magnetometer (Quantum Design MPMS;
5}330 K). Measurements of the electrical conductivity of
$
R (20)
2.81%
2.86%
6.64%
9.00%
ꢀ
R
(20)
ꢁ
ꢀ
R ; R (simultaneous
ꢁꢀ
re"nement)
ꢀ
AgCuO were performed by the four-point probe method
ꢁ
(
Van-der-Pauw) on pressed pellets (diameter, 6 mm; thick-
sꢁ (20)
ness, 1 mm),
(
1
germanium monochromator, CuKa radiation, j"
RESULTS AND DISCUSSION
ꢀ
54.0598 pm, linear position-sensitive detector, Si as an
external standard).
Applying a low-temperature route, a new ternary
Neutron powder di!raction data were collected by the silver cuprate, AgCuO , has been synthesized as a micro-
ꢁ
time-of-#ight technique at the ISIS spallation source with crystalline, black powder (Fig. 6). According to EDX
the ROTAX di!ractometer. A vanadium crucible (dia- microprobe analyses the Ag/Cu ratio amounts to 1:0.99
meter"4 mm) was used as the sample container. The (average over about "ve spots per sample, several samples
detection was performed by using three He detector checked). AgCuO is insensitive to moisture and can be
ꢁ
banks, which were positioned at 2h"28.753 (bank 1), 72.853
(
bank 2), and 122.303 (bank 3).
Starting from the structural model of Ag'Ag'''O the
ꢁ
structure has been re"ned by pro"le analysis using the
Rietveld method [program GSAS (8)]. Essential data of the
structure re"nements are listed in Table 1. Measured and
calculated powder di!raction patterns and the respective
di!erence plots are shown in Figs. 1}4.
Chemical analysis. The cation ratio of AgCuO was
ꢁ
determined by energy-dispersive X-ray analysis on several
samples (Philips ESEM XL30 equiped with a W cathode
and an S-UTW-Si(Li) detector, EDAX PV9760/68). The
EDX spectra were recorded at an acceleration energy of
2
5 keV, and the Ag and Cu L-lines were used to calculate
the cation concentrations.
¹
hermal analysis. The thermal behavior was investigated
by DTA and TGA (Fig. 5) (STA 429, Netzsch, Selb,
Germany); the decomposition products were characterized
by X-ray powder di!raction.
FIG. 2. Neutron di!raction pro"le "t for AgCuO with 5% CuO
ꢁ
(
bank 2, 72.853). Re#ections caused by vanadium container are marked by
arrows.

2
22
CURDA, KLEIN, AND JANSEN
FIG. 5. Combined DTA and TG curves of AgCuO .
ꢁ
FIG. 3. Neutron di!raction pro"le "t for AgCuO with 5% CuO (bank
, 122.33). Re#ections caused by vanadium container are marked by
ꢁ
3
arrows.
AgCuO (Tables 2 and 3; Fig 6) contains silver in a linear
ꢁ
coordination by oxygen. The Ag}O bond length 2.178 A
s
stored undecomposed in air for months. However, it decom- and the O}Ag}O angle 1803 are as expected for monovalent
poses in an argon atmosphere to a mixture of Ag O and silver. Copper is surrounded by four oxygens in a rectangle
ꢁ
CuO within a few days. The thermal degradation, as re- planar fashion. As the bond lengths (1.871/1.873A
s
) are com-
corded by thermal analysis, proceeds in two steps at 2803C parable to those found in NaCuO (1.839 A
and 3103C, each accompanied by a loss of oxygen (Fig. 5). tion state of 3# can be assigned to copper. The CuO units
The intermediate forming at 2803C has been identi"ed as are linked via common edges in trans positions, thus form-
s
) (10), the oxida-
ꢁ
ꢃ
Ag Cu O (9) by X-ray powder di!raction. The "nal residue ing ladder-like linear chains. As a consequence of this type
ꢁ
ꢁ ꢂ
consists of Ag and CuO.
The product phase as obtained via precipitation from an
aqueous solution is of mediocre crystallinity, which is re#ec-
ted by the widths of the powder re#ections. Furthermore,
among others the sample prepared for the neutron data
collection contained small amounts of CuO (cf. Figs. 1}3).
Other samples were virtually free of alien phases. Neverthe-
less, the structural features could be determined unambigu-
ously by "tting X-ray and neutron powder pro"les.
FIG. 4. Rietveld re"nement patterns of AgCuO (X-ray).
FIG. 6. Crystal structure of AgCuO .
ꢁ
ꢁ

STUDY OF AgCuO
223
ꢁ
of connectivity, the O}O spacings at the connecting edges
are contracted to 2.461 A and the corresponding O}Cu}O
angles are reduced to 82.933, as compared to 2.806 A and
7.073 for the nonconnecting case. All ꢁ CuO chains are
TABLE 2
a
Positional and Isotropic (Ueq) Displacement Parameters
؊ 1 2
s
(10 pm ) for AgCuO
2
s
9
ꢅ
ꢁ
Atom
x
y
z
;%ꢂ
running along [010] and are interconnected by AgO
dumbbells.
ꢁ
Ag
Cu
O
0
0
0
0
5.7(7)
5.8(5)
15.6(6)
Although the structural features of AgCuO and those
ꢀ
ꢀ
ꢁ
reported for Ag'Ag'''O are closely related to each other, the
ꢁ
ꢁ
ꢁ
0.2969(4)
0.5040(3)
0.3561(4)
two oxides are not isostructural. Strong similarities in the
?
; "ꢀ (;ꢀꢀ#;ꢁꢁ#; ).
%ꢂ ꢂꢂ
lattice constants and the positional parameters of the metal
atoms in both oxides are obvious; however, under closer
inspection, signi"cant di!erences in the coordinates of oxy-
ꢂ
gen are revealed. These facts can be rationalized when and Ag'Cu'''O ) can be obtained by considerations in the
ꢁ
basing the interpretation and comparison of the respective sense of the Born}Haber cycle. The input data for the four
crystal structures on the partial structures of the metal couples M'Mꢀ'''/M''Mꢀ'' (M, Mꢀ"Ag, Cu) are identical for
atoms (11}13). These are equal and correspond to an all data concerning oxygen, within each couple and also
ordered variant of the fcc structure as is similarly realized in over di!erent combinations, while the atomization energies
the alloy AuCu (14). In either case oxygen occupies the of the metals involved are only identical within a given
slightly distorted octahedral sites. In AgCuO , the oxygen couple. When assuming that the lattice energies gained
ꢁ
atoms are shifted from the center of the octahedral Ag Cu
are comparable among all I/III and II/II combinations,
ꢂ
ꢂ
voids toward one face, such that the oxygen atoms are
receiving two short bonds from copper and one from silver.
A shift by a similar amount from the center of the octahedral
voids but in another direction results in cardinally di!erent
orientations of the AgO squares and AgO dumbbells in
TABLE 3
Selected Bond Lengths (A
>
) and Angles (3) for
ꢃ
ꢁ
I
III
AgCuO
2
, Ag Ag O (4), and CuO (18)
Ag'Ag'''O .
ꢁ
At this point, one is becoming aware of a surprisingly
general building principle, apparently common to all noble
metal oxides containing the metal atoms in linear and/or
square planar coordinations. The metal atoms in Cu O (15),
Ag O (16), Cu O (17), Ag Cu O (7), CuO (18), AgCuO ,
Ag'Ag'''O (4), or PdO (19) form variants of the fcc struc-
ture, which is found in the respective metals, too. Oxygen
partly occupies the voids in a way that linear or square
coordinations, as favored by the respective cations, result.
Atoms
Ag'Cu'''O
Ag'Ag'''O
Cu''O
ꢁ
ꢁ
Cu}O
1.8714(1) [2;]
1.8733(1) [2;]
2.7834(4) [2;]
2.1778(1) [2;]
2.7089(1) [2;]
*
1.95 [2;]
1.961 [2;]
2.781 [2;]
*
ꢁ
ꢁ
ꢃ ꢂ
ꢁ
ꢁ ꢂ
ꢁ
Ag'}O
Ag'''}O
Ag}Ag
2.161 [2;]
2.688 [2;]
2.903 [2;]
2.023 [2;]
2.028 [2;]
2.806 [2;]
3.255 [4;]
3.361 [2;]
3.408 [4;]
ꢁ
2
.7102(5) [2;]
*
*
*
While in PdO and the metal-rich compounds M O and
ꢁ
M O oxygen enters the tetrahedral sites, in the cases of the
ꢃ ꢂ
2.8062(2) [2;]
3.344(0) [4;]
MO/M O compositions octahedral voids are occupied,
ꢁ
ꢁ
establishing a relationship to the rock salt structure.
In agreement with the valence states attributed in terms of Ag'*Cu'''
structural arguments, Ag'Cu'''O is diamagnetic and
semiconducting with a band gap energy of about 0.2 eV. As
mentioned above, Ag Cu O has a face-centered cation
arrangement, too. But for transforming AgCuO into
Ag Cu O , a cation rearrangement is necessary, in addition
to the loss of oxygen. This fact might explain the di!erent O}O
decomposition products obtained at di!erent temperatures.
3
.484 [2;]
*
3.249(1) [4;]
3.504(1) [2;]
2.8062(2) [2;]
*
ꢁ
Cu}Cu
*
2.900 [4;]
3.083 [2;]
3.172 [2;]
3.344(0) [4;]
ꢁ
ꢁ ꢂ
3.421 [2;]
ꢁ
4.665 [2;]
2.625 [2;]
2.900 [4;]
ꢁ
ꢁ ꢂ
2.461(1)
.8062(2) [2;]
2.829 [2;]
2.900 [2;]
2
At room temperature binary oxides result from slow de-
composition, while at higher temperatures a rearrangement
Atoms
Ag'Cu'''O
Ag'Ag'''O
Cu''O
ꢁ
ꢁ
of the cations becomes possible and, accompanied by loss of O}Cu}O
82.926(5)3
97.074(5)3
1803
*
84.323
95.683
*
2
5 wt. % oxygen and reduction of Cu(III) to Cu(II),
O}Ag'}O
O}Ag'''*O
1803
88.583
1.423
Ag Cu O forms.
ꢁ
ꢁ ꢂ
*
*
A rough estimate of the plausibility of the charge distribu-
9
tions as thus far found experimentally (Cu''O, Ag'Ag'''O ,
ꢁ

2
24
CURDA, KLEIN, AND JANSEN
TABLE 4
combination of oxidation states as observed (Ag>/Cuꢂ>) is
Sums of Ionization Potentials for All Possible Charge Distribu- related to a higher sum of ionization potentials as compared
tions in MMꢀO
2
(M, M ꢀ ؍ Ag, Cu), and Their Di4erences (eV) to the opposite combination Cu>/Agꢂ>.
+Iꢀ(M'/M''')
+I (M''/M'')
*
ꢀ
ACKNOWLEDGMENTS
Cu/Cu
Ag/Cu
Cu/Ag
Ag/Ag
72.58
72.43
71.61
71.47
56.03
57.08
57.08
58.01
16.55
15.35
14.53
13.46
The authors thank E. Bruecher and G. Siegle from MPI Stuttgart for
magnetic and conductivity measurements, W. Kockelmann from ISIS for
recording neutron time-of-#ight spectra and R. Hagenmayer for assistance
in "tting of the neutron data.
REFERENCES
respectively, one can try and correlate the sums of the
ionization potentials (see Table 4) with the observations.
The di!erence in the sum of ionization potentials
1
2
3
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(
ꢀ ꢀ
smallest for the combination Ag/Ag, which is in agreement
4
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8
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ꢁ
one would expect, and there is no conclusive clue for predic-
ting the most probable charge distribution. Surprisingly, the
charge distribution according to Cu'Ag'''O which is fa-
ꢁ
vored when considering the ionization potentials only is not
found experimentally. This indicates on one hand that this
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9
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1
0. N. E. Brese, M. O'Keefe, R. B. von Dreele, and V. G. Young, Jr., J. Solid
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ꢁ
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2
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1
e.g., Ag'Cu'Cu'''O or Ag'Cu''Cu'''O .
ꢁ
ꢃ
ꢁ
ꢃ
1
4. E. Bjerkelund, W. B. Pearson, K. Selte, and A. Kjekshus, Acta Chem.
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CONCLUSION
1
1
1
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Ag Cu O exhibits charge ordering and has to be re-
ꢆꢇꢈ ꢆꢇꢈ
18. N. E. Brese, M. O'Keefe, B. L. Ramakrishna, and R. B. v. Dreele,
J. Solid State. Chem. 89, 184 (1990).
1
garded as a silver(I) oxocuprate(III). This assignment of
valence states is conclusively supported by characteristic
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