K. Đuriš, R. K. Kremer, M. Jansen
ARTICLE
estingly, evidence for long-range AFM ordering has not been The magnetic susceptibilities were determined in the temperature range
from 2 to 400 K, in discrete magnetic fields up to 7 T using a SQUID-
seen in heat capacity measurements.
Magnetometer (MPMS-XL, Quantum Design). The diamagnetic sus-
ceptibilities were determined using tabulated values.[ The heat ca-
25]
pacity of a pressed and sintered pellet sample (~ 20 mg) was measured
with the relaxation method (PPMS, Quantum Design) between 2 and
Starting materials for the preparation of the potassium cuprate were 250 K.
Experimental Section
potassium azide, potassium nitrate (Riedel-De Haen AG Seelze, Han-
nover, 99.5 %) and CuO prepared by heating of Cu(C
2
O
4
)·0.5H
2
O
Supporting Information (see footnote on the first page of this article):
X-ray powder data.
(
Alfa Aesar, 98 %) in a flow of oxygen at 623 K for 20 h. The potas-
sium azide was synthesised from aqueous HN and potassium carbon-
3
ate (Sigma Aldrich, 99 %). The starting compounds were dried under
–
3
vacuum (10 mbar) at 393 K overnight, mixed thoroughly in an agate
mortar in a ratio according to Equation (1), and placed under argon in
a closed steel container, provided with a silver inlay.
Results and Discussion
Synthesis, Crystal Structure
7
KN
3
+ 2 KNO
3
+ 6 CuO → 3 K
3
Cu
2
O
4
+ 23/2 N
2 4
Cu O were obtained apply- trate route, whereas the phase pure powders were obtained by
–1
2
(1)
Single crystals of K Cu O were prepared via the azide/ni-
3
2
4
Black needle shaped single crystals of K
3
ing the temperature schedule 298 → 533 K (100 K·h ), 533 → 673 classical solid state reaction. The TG–DTA measurements did
–
1
–1
K (5 K·h ), 673 → 923 K (600 K·h ), and subsequent annealing for
0 h at 923 K.
not show any chemical reaction or phase transition during
heating up to 923 K, where decomposition occurs. The crystal
structure of the title compound was determined from single
crystal data collected at 298 K, where it was found that
5
Hazards: The temperature control, as given above must be strictly
followed. Rapid heating or running the reaction in a tightly closed
container, could lead to dangerous explosion.
K Cu O crystallises orthorhombic with the space group
3
2
4
Cmcm (No.63).
Using the azide/nitrate route,[8] the products always contained potas-
sium cuprates(I) in addition to coarse crystalline K
come this difficulty in the synthesis of single phase K
an alternative route according to Equation (2) was applied, by using
Details and results of the structure refinement are given in
Table 1, Table 2, and Table 3, interatomic distances, coordina-
tion numbers (CN), effective coordination numbers (ECoN)
and mean fictive ionic radii (MEFIR) for K Cu O in Table 4.
3
Cu
2
O
4
. To over-
3
Cu
O powder,
2 4
, prepared according to[ and K
19]
3
2
4
KCuO O, which was prepared from
2
2
The collected powder diffraction pattern for K Cu O is shown
[
20]
3
2
4
purified potassium by oxidation with molecular oxygen.
O + KCuO + CuO → K Cu
Respective mixtures, including 2 mol-% excess of K
ferred to a gold ampoule, closed under argon atmosphere and treated used.
in Figure 1, and the results obtained from the Rietveld refine-
ment are given in Table 5. For the Rietveld refinement atomic
K
2
2
3
2
O
4
(2)
O, were trans- coordinates obtained from the single crystal analysis were
2
at 723 K for 30 h. The obtained black powder, being very sensitive to
air and moisture, was sealed and stored in glass ampoules under argon
and all further handling was made in an inert atmosphere of purified
argon.
According to the single-crystal structure analysis, K Cu O
3 2 4
and shows a
[26]
is isostructural to the Ni- and Pt-analogues,
fully charge-ordered copper partial structure. The most promi-
1
nent structural feature are buckled one-dimensional CuO rib-
∞
2
bon chains, which are build up from planar, edge-sharing CuO4
plaquettes (Figure 2a). From the analysis of the Cu–O bond
lengths, valence states of either +2 or +3 can be unambigu-
X-ray powder studies were performed with a D8-Advance diffractome-
ter (Bruker AXS, Karlsruhe, Germany) Cu-Kα1 radiation, provided
from a Ge(111)-monochromated primary beam (Johannson monochro-
mator). For refinement of the powder pattern, the program TOPAS[
was used.
21]
2+
ously assigned to each copper atom (d(Cu –O) = 1.939(3) Å,
3+
d(Cu –O) = 1.851(3) Å).
In K Cu O the O–Cu–O angles in the CuO square units
3
2
4
4
The single crystal diffraction data were collected with a smart APEX
I three circle single crystal diffractometer (Bruker AXS, Karlsruhe,
Germany) equipped with a CCD detector. Data collection and reduc-
deviate from the ideal 90° due to variation of oxidation states
II
III
of Cu and Cu , as well as due to the way, how these units
[22]
are connected. The chains extend along the c-axis. The dihe-
tion were carried out with the Smart software package.
Intensities
dral angle between two CuO square units is 155.7° (Fig-
4
were corrected for absorption effects applying a semi-empirical
method.[23] The structure was solved by direct methods and refined by
ure 2a). This is in contrast to the sodium cuprates
1
[24]
full-matrix least-squares fitting with SHELXTL.
(Na1+xCuO
2
), where the
∞
CuO
2
chains are nearly flat with dihe-
dral angles between two CuO square units approaching 180°.
4
Thermal analyses were carried out using a TG–DTA device (STA 409,
Netzsch, Selb, Germany) coupled with a quadrupole mass spectrometer
Different views of the crystal structure are given in Figure 2b.
The potassium ions occupy two crystallographically inde-
pendent sites with four- and sixfold coordination (Figure 2c).
(
QMG 421, Balzers). The samples were heated with a rate of 10
–
1
K·min in a corundum crucible, under dry argon.
1
K1 connects three CuO ribbons, with bonds to two oxygen
∞
2
atoms from each ribbon forming distorted prism (Figure 2c).
Electron paramagnetic Resonance (EPR) was measured with a Bruker
ER040XK microwave X-band spectrometer and a Bruker BE25 mag-
net equipped with a BH15 field controller, which was calibrated
against Diphenylpicrylhydrazyl.
K2 is equidistant with four oxygen atoms of two neighbouring
1
CuO ribbons, forming a pyramid where K2 is situated on the
∞
2
top of the pyramid.
1102
www.zaac.wiley-vch.de
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 1101–1107