Electrochemical synthesis of single-crystal (Ba0.05K0.95)BiO3·1/6H2O with the KSbO3 structure

Article abstract of DOI:10.1016/S0025-5408(98)00101-9

A new compound with a cubic structure, (Ba_0.05K_0.95)BiO₃·1/6H₂O (BKBO), was synthesized by electrochemical crystal growth using a rotating anode in molten KOH flux at 255 °C. The BKBO crystal had the KSbO₃ structure and belonged to space group Im3 (No. 204) with Z = 12. Refinement of X-ray diffraction data at room temperature gave a = 10.0204(2) angstroms, V = 1006.1(1) angstroms³, and R = 2.4% for 1465 unique reflections. Potassium, barium, and hydrated oxygen were found to occupy different sites in the tunnel structure. The electrical conductivity at room temperature was similar to the ionic conductor KBiO₃. BKBO decomposed to Bi₂O₃, K₂O, and BaO above 400 °C.

Full text of DOI:10.1016/S0025-5408(98)00101-9

Materials Research Bulletin, Vol. 33, No. 8, pp. 1195–1205, 1998
Copyright © 1998 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/98 $19.00 ϩ .00
PII S0025-5408(98)00101-9
ELECTROCHEMICAL SYNTHESIS OF SINGLE-CRYSTAL
(Ba0.05K0.95)BiO3⅐1/6H2O WITH THE KSbO3 STRUCTURE
D.-Y. Jung1, P. Han2, S.R. Wilson3, and D.A. Payne2*
1Department of Chemistry, SungKyunKwan University, Suwon 440–746, Korea
2Science and Technology Center for Superconductivity, Department of Materials Science
and Engineering, Fredrick Seitz Materials Research Laboratory, University of Illinois at
Urbana-Champaign, Illinois 61801 USA
3School of Chemical Sciences, University of Illinois at Urbana-Champaign,
Illinois 61801 USA
(Refereed)
(Received October 24, 1997; Accepted December 16, 1997)
ABSTRACT
A new compound with a cubic structure, (Ba0.05K0.95)BiO3⅐1/6H2O (BKBO),
was synthesized by electrochemical crystal growth using a rotating anode in
molten KOH flux at 255°C. The BKBO crystal had the KSbO3 structure and
belonged to space group Im3 (No. 204) with Z ϭ 12. Refinement of X-ray
diffraction data at room temperature gave a ϭ 10.0204(2) Å, V ϭ 1006.1(1)
Å3, and R ϭ 2.4% for 1465 unique reflections. Potassium, barium, and
hydrated oxygen were found to occupy different sites in the tunnel structure.
The electrical conductivity at room temperature was similar to the ionic
conductor KBiO3. BKBO decomposed to Bi2O3, K2O, and BaO above 400°C.
© 1998 Elsevier Science Ltd
KEYWORDS: A. oxides, B. crystal growth, C. X-ray diffraction, D. crystal
structure
INTRODUCTION
Mixed bismuth oxidation states have led to superconductivity in the perovskite structure for
Ba(Pb,Bi)O3 [1] and (Ba,K)BiO3 [2]. For the (Ba1ϪxKx)BiO3 crystalline solution series,
*To whom correspondence should be addressed.
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D.-Y. JUNG et al.
Vol. 33, No. 8
superconductivity has been observed in the range x ϭ 0.30–0.73 [3] with the highest Tc (38
K) at x ϭ 0.38 [4–7]. This is the highest known transition temperature for an oxide that does
not contain copper. In these bismuthates, the formal oxidation state of the bismuth ion is
between III and V. Since Bi(IV) is postulated to disproportionate into Bi(III) and Bi(V) for
BaBiO3, [8], it is important to synthesize and characterize compounds containing Bi(III) and
Bi(V). Trivalent bismuth is the most stable oxidation state for bismuth in an oxygen-
framework structure, and bismuth(V) has only been stabilized in sixfold coordination for a
few compounds: BaLaMBiO6 (M ϭ Mg,Ca,Sr) with the perovskite structure [9],
(Ba1.7K1.3)Bi2O7 in a Ruddlesden-Popper series [10], Sr2Bi2O7 of the weberite structure [1],
alkali metal/alkaline earth metal bismuth oxides such as NaBa3BiO6 [12], and KBiO3 with
the KSbO3 structure [13–18]. The oxidation state of Bi(V) is expected to be unstable and,
therefore, difficult to stabilize in an oxide lattice without the use of applied oxygen pressure.
Norton [19] and other groups [20–23] have shown that crystals of (Ba1ϪxKx)BiO3 with
superconducting properties can be grown by electrochemical methods [24–25]. Electrochem-
ical synthesis is a low-temperature approach to the stabilization of compounds that are
normally unstable at higher temperatures. The electrochemical potential is the driving force
for the reduced-temperature synthesis. Unusual oxidation states of Bi can be obtained when
reduced transition metals are used at the cathode and oxidized materials at the anode. In
addition, experimental conditions (e.g., deposition temperature, electric potential, and con-
centration of reactants) play an important role on the synthesis products obtained (e.g.,
perovskite (Ba1ϪxKx)BiO3; viz., the layered-structure (Ba1ϪxKx)3Bi2O7).
The most homogeneous crystals grown to date have been obtained by top-seeded flux
growth [5]. We have investigated the electrochemical synthesis of (Ba1ϪxKx)BiO3 with
increasing values of x and determined their electric properties. However, the end member
(x ϭ 1) KBiO3, unlike ANbO3 and ATaO3 (A ϭ alkaline metal) [26], did not have the
perovskite structure, but had an open KSbO3 cubic tunnel structure [27]. Several compounds
with the KSbO3 structure have been prepared and characterized for their properties [28,29].
In this paper we report the synthesis of a new compound in the BKBO system, which does
not have the perovskite structure. The discovery originated from a systematic study of K
incorporation into the perovskite structure at reduced temperatures by electrochemical syn-
thesis. The intent was to study the effect of electronic structure and oxidation states (e.g.,
Bi(III) and Bi(V)) on superconducting properties, for both under-doped and over-doped
compositions, and to extend the superconducting range to higher potassium contents than
previously obtained. Data are reported for the single-crystal structure, thermogravimetric
analysis (TGA), secondary ion mass spectrometry (SIMS), and electrical conductivity.
EXPERIMENTAL
Sample Preparation. Single crystals of BKBO were grown by an electrochemical method
from KOH flux in a Teflon container. Top-seeded flux growth was used with a rotating anode
in a three-electrode cell configuration [5]. The reactants, 1.5 g of Bi2O3 (Alfa, 99.999%), 1 g
of BaO (Alfa, 99.99%), and 250 g KOH (Alfa, 99.999%), were placed in the Teflon
container, which was positioned in a closed silica tube furnace equipped with flowing
nitrogen (5 mL/min.). The nitrogen was saturated with water to prevent dehydration of the
KOH flux. The reactants were equilibrated at 255°C for 1 h before crystal growth was started.
The electrodes in the electrochemical cell were bismuth-electroplated platinum wire (0.5
mm, 99.99%) i.e., for the anode, cathode, and reference electrodes. The working potential

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POTASSIUM BARIUM BISMUTHATE
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between the anode and the cathode was 0.85 V. These conditions corresponded to when the
current increased rapidly with increasing voltage. Current–voltage conditions were moni-
tored by a potentiostat (EG&G Princeton Applied Research, Potentiostat/Galvanostat model
273A). After electrochemical crystal growth, the electrodes were removed from the cell,
washed in deionized water to remove any residual flux, and then allowed to dry in air. BKBO
was deposited at the anode as red, well-faceted crystals.
Crystallographic Analysis. Preliminary X-ray data were obtained for (Ba0.05K0.95)BiO3
crystals by a powder diffraction (Rigaku Geigerflex) method using Cu K␣ radiation (␭ ϭ
1.5418 Å) monochromatized by graphite crystals. Analysis of the X-ray data indicated the
KSbO3 tunnel structure. The intensities of three representative reflections, measured over 60
min. of X-ray exposure time, remained constant, thus indicating the stability of the compound
in air and in the X-ray beam. A small red crystal of (Ba0.05K0.95)BiO3, 0.05 ϫ 0.05 ϫ 0.05
mm in size, was selected for further single crystal studies. X-ray data were obtained for
BKBO on a X-ray diffractometer (Siemens SMART-CCD), using Mo K␣ radiation (␭ ϭ
0.71073 Å) monochromatized by graphite crystals. Unit-cell parameters were calculated by
a least-squares analysis of 1465 reflections and gave a ϭ 10.0204(2) Å and V ϭ 1006.1(1)
Å3. The space group was determined to be Im3 (No. 204), assuming an ideal cubic KSbO3
structure for the BiO3 framework. Data were collected, using the ␻-scan method, to a
maximum 2␪ value of 57°. The data were corrected for Lorentz and polarization effects and
secondary extinction. Data were corrected empirically for absorption. All calculations were
performed through the use of Siemens crystallographic software [30]. The positions for
barium, potassium, and bismuth were determined by direct methods. Refinement was carried
2
out by a full-matrix least-squares refinement on Fo calculations. The final values of the
discrepancy factors were R ϭ 0.0242 and Rw ϭ 0.0531.
Physicochemical Characterization. Crystals were examined in a scanning electron micro-
scope (Hitachi S800) equipped for energy-dispersive spectroscopy (EDS). The stoichiometric
proportion of potassium, barium, and bismuth were determined for 30 representative crystals.
The results gave an average composition of (K0.95Ba0.05)Bi1.00. The oxygen stoichiometry
was determined by TGA. No impurities were detected by EDS.
Thermogravimetric data (TA Instruments, TGA 2950 system) were obtained at a heating
rate of 1°C/min. Flowing oxygen was used to prevent the formation of any carbonate phase.
Polycrystalline products were characterized by X-ray diffraction methods after TGA.
Electrical measurements were made using a Stanford SR350 lock-in amplifier at 15 Hz as
a function of temperature. The temperature was controlled between 280 and 660 K, using a
programmable controller (Eurotherm with S-type thermocouple). Crystal sections were cut in
the (111) direction and electroded with sputtered gold. The specimen size was approximately
2 ϫ 2 ϫ 0.5 mm.
RESULTS AND DISCUSSION
Synthesis. Figure 1 summarizes the reported [31] crystallographic data for (Ba1ϪxKx)BiO3
as a function of x. Superconductivity was determined for 0.3 Յ x Յ 0.72, with the highest
Tc at x ϭ 0.4 [6]. Below x ϭ 0.72, (Ba1ϪxKx)BiO3 had the perovskite structure with random
occupancy of the 12-fold A sites by barium and potassium. The phase diagram (Fig. 1) gives
the space groups as a function of temperature and K content. The end member (x ϭ 1),

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D.-Y. JUNG et al.
Vol. 33, No. 8
FIG. 1
Structures reported by neutron powder diffraction [31] and single crystal X-ray analysis in
the (Ba1ϪxKx)BiO3 system. Superconducting transition temperatures of the perovskite phase
are shown in the region 0.3 Յ x Յ 0.7 for the perovskite structure [6].
KBiO3 has the KSbO3 structure. As far as we know, no phase has been reported for 0.72 Ͻ
x Ͻ 1.0. We obtained a new compound, (Ba0.05K0.95)BiO3⅐1/6H2O, with the KSbO3 structure
for x Ն 0.95, but were unable to synthesize single-phase material for 0.72 Ͻ x Ͻ 0.95. Phase
separation was attributed to a solubility limit, where (Ba1ϪxKx)BiO3 dissociated into both the
perovskite and the KSbO3 structures between 0.72 Ͻ x Ͻ 0.95. The structures decomposed
above ca. 400°C, especially for high potassium contents (Fig. 1).
In this study, new crystals of BKBO [(Ba0.05K0.95)BiO3⅐1/6H2O] were grown by a rotating
anode electrochemical method at constant potential. Seed rotation improved the chemical
homogeneity of the crystals. Anodization gave high-oxidation states for bismuth at ambient
pressure. Otherwise, Bi(V) could only be obtained by high oxygen applied pressures. In our
method, red crystals of BKBO grew at the anode and could be isolated easily. The chemical
substitution level for x was found to be experimentally dependent on the solute concentration
in the flux, the electrochemical over-potential, the anode stirring rate, and the growth time.
For example, Figure 2 gives a scanning electron photomicrograph of crystals grown in the
range 0.72 Ͻ x Ͻ 0.95. The red-colored crystals had the KSbO3 structure, whereas the
blue-colored crystals had the perovskite structure. Seed crystals were selected for further
experimentation. Final BKBO crystals were grown on small red BKBO crystals deposited on
electroplated platinum wire without sacrificial reduction of ZnCl2 or CuCl2. The barium
content (x) was determined as a function of depth by EDS, using ion-beam sputtering, and
by SIMS.

Vol. 33, No. 8
POTASSIUM BARIUM BISMUTHATE
1199
FIG. 2
Scanning electron photomicrograph of different BKBO crystals deposited at the anode.
Crystallography. After refining the parameters corresponding to Bi (12e), O1 (24g), and O2
(12d) and examining the Fourier transform electron density maps, the positions for Ba, K,
and hydrated O3 were determined to be 8c (1/4 1/4 1/4), 16f (x x x), and 2a (0 0 0),
respectively. KBiO3⅐yH2O has been reported previously to have various degrees of hydration
(y) and, therefore, correspondingly different cell parameters, depending upon the method of
preparation [13–16]. Consequently, different potassium sites and waters of hydration have
been reported in previous accounts [17,18].
Tables 1 and 2 list the crystallographic parameters determined in this study. The number
in parentheses is the standard deviation for the last digit. Crystallographic data give a refined
formula of (Ba0.06K0.94)BiO3⅐1/6H2O, which is in acceptable agreement with the chemical
analysis (Ba0.05K0.95)BiO3⅐1/6H2O obtained by EDS. Anisotropic coefficients are summa-
rized in Table 3, and selected inter-atomic distances and bond angles are given in Table 4.
The BKBO structure, isostructural with KSbO3 [29], contains pairs of BiO6 octahedra that

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D.-Y. JUNG et al.
Vol. 33, No. 8
TABLE 1
Crystallographic Data for BKBO at
Room Temperature
Chemical formula
(Ba0.05K0.95)BiO3⅐1/6H2O
Formula weight (g/mole)
Crystal system
Space group
303.99
Cubic
Im3 (No. 204)
៮
Lattice parameters (Å)
a ϭ 10.0204(2)
V ϭ 1006.1(1)
12
Volume (Å3)
Z
␭ (Å)
0.71073
0.024
0.053
Ra
b
Rw
Goodness of fit
1.207
aR ϭ ¥ʈFo͉ Ϫ ͉Fcʈ/¥͉Fo͉.
2 2
2 2
2
bwR ϭ [¥[w(Fo2 Ϫ Fc ) ]/¥[w(Fo ) ]]1/2; w ϭ 1/[␴2(Fo )
2
ϩ (0.0062P)2 ϩ 49.27P], where P ϭ [max(Fo , ␾) ϩ
2
2(Fc )]/3.
share edges to form Bi2O10 dimers that share corners to form Bi–O linkages. The resulting
tunnel structure is illustrated in Figure 3.
The Bi–Bi distance in BKBO is 3.208 Å (Table 4). Assuming a Bi2O6 dimer with two
nondistorted octahedra and a Bi–O bond length of 2.118 Å, the calculated Bi–Bi distance
would be 3.00 Å, which is smaller than the experimentally measured value of 3.208 Å. Figure
4 is a schematic for the proposed dimer, where the Bi–Bi distance is increased due to
electrostatic repulsion. Table 4 indicates the Bi–O–Bi bond angles are 81.5° and 103.1°,
suggesting the electrostatic repulsion of metal cations.
Potassium ions are located at 16f sites in the tunnel. Although the electron densities at sites
16f, 8c, and 2a were assigned differently in previous investigations, for differently prepared
specimens of KBiO3 (or KBiO3(yH2O) [17,18], the most probable refinement of our data is
given in Table 2, based upon site occupancy. Tunnel oxygen (O3) was assigned to water,
based upon excess electron density. SIMS measurements indicated comparable counts for
TABLE 2
The Final Refined Atomic Positions and Estimated Crystallographic Parameters for
BKBO at Room Temperature
Coordinates
%
Atom
Wyckoff
x
y
z
Ueq
7(1)
3.8(1)
0.89(3)
1.6(2)
1.6(2)
3.0(7)
Occupancy
Ba
K
Bi
O1
O2
O3a
8c
16f
12e
24g
12d
2a
0.2500
0.1574(4)
0.5000
0.3384(8)
0.362(1)
0
0.2500
0.1574(4)
0.16006(5)
0.2884(8)
0
0.2500
0.1574(4)
8.84
70.64
100
100
100
0
0
0
0
0
100
aResults from hydrate water molecules.

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POTASSIUM BARIUM BISMUTHATE
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TABLE 3
Anisotropic Uij coefficients for BKBO
Atom
Position
U1
U22
U33
U12
U13
U23
Ba
K
Bi
O1
O2
O3
8c
16f
12e
24g
12d
2a
0.07(1)
0.07(1)
0.07(1)
0.05(1)
0.010(2)
0
0.003(3)
0
0
0.05(1)
0.010(2)
0
0
0
0
0.05(1)
0.010(2)
0
0
0
0
0.038(1)
0.0098(3)
0.012(4)
0.008(5)
0.030(8)
0.038(1)
0.0079(3)
0.010(4)
0.013(5)
0.030(8)
0.038(1)
0.0088(3)
0.025(4)
0.026(6)
0.030(8)
secondary hydrogen below 1000 Å. This hydrogen was attributed to crystalline water and
could be detected up to 3000 Å in depth, where the experiment was terminated.
Previous investigators have obtained KBiO3⅐yH2O from solution and oxidation of bismuth
ion [13,14]. Goodenough et al. [28] reported NaF stabilized the NaSbO3 structure. The
present results for BKBO [(Ba0.05K0.95)BiO3⅐1/6H2O] are consistent with those of previous
reports. On the other hand, high-pressure synthesis [16] yields the formation of anhydrous
KBiO3. Depending on whether the wet or dry method is chosen for preparation, KBiO3 may
or may not have water of hydration in the tunnel structure.
TGA. Figure 5 gives thermogravimetric data obtained for BKBO heated at 1°C/min. in
flowing oxygen. The measured weight loss of 5.9% at 400°C is comparable to the theoretical
value of 6.1% calculated from the proposed formula. X-ray diffraction indicated BKBO
decomposed to K2O, Bi2O3, and BaO after heat treatment. The products were hygroscopic.
Thermal decomposition of BKBO above 400°C is consistent with the need for reduced-
temperature synthesis. This can be achieved in a closed system by high-pressure synthesis in
oxygen or, in the present case, in an open system by electrochemical synthesis at 255°C [16].
Electrical Conductivity. Figure 6 gives the Arrehenius dependence of the low-frequency
conductivity (␴) indicating two activated processes, with a transition temperature around
130°C. The calculated (␴ ϭ ␴0exp(Ϫ⌬E/kT)) activation energies (⌬E) were 0.24 eV below
130°C and 0.35 eV above 130°C. These values are greater than the value reported for
TABLE 4
Selected Bond Distances (Å) and Angles (°)
Bond Distances
Bi™O1
Bi™O2
K™O1
K™O2
K™O3
Ba™O1
2.068(8) ϫ 2; 2.121(8) ϫ 2
2.118(7) ϫ 2
2.739(7) ϫ 3
3.030(8) ϫ 3
2.731(7)
K™K
3.153(8) ϫ 3; 3.216(14)
1.608(7)
3.778(2)
3.656(1) ϫ 4
3.208(1)
K™Ba
K™Bi
Ba™Bi
Bi™Bi
2.685(3) ϫ 6
Bond Angles
O1™Bi™O1
O1™Bi™O2
O2™Bi™O2
103.1(4), 89.7(1), 179.2(9)
87.7(3), 169.2(3)
81.5(5)

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D.-Y. JUNG et al.
Vol. 33, No. 8
FIG. 3
Illustration of the proposed (Ba0.05K0.95)BiO3⅐1/6H2O structure, indicating corner sharing of
the Bi2O10 clusters. The shaded circles represent potassium atoms located along the (111)
direction.
potassium ion conductivity (0.16 eV) in anhydrous KBiO3 [18]. The difference may be due
to blocking effects of barium and water of hydration in the tunnel structure. The room
temperature value of electrical conductivity for BKBO was 10Ϫ5S/cm, which is comparable
with that for KBiO3.
FIG. 4
Illustration of the octahedral dimer Bi2O10.

Vol. 33, No. 8
POTASSIUM BARIUM BISMUTHATE
1203
FIG. 5
TGA for (Ba0.05K0.95)BiO3⅐1/6H2O. The 5.9% weight loss at 500°C corresponds to the
decomposition reaction to BaO, K2O, Bi2O3, H2O, and O2.
FIG. 6
Temperature dependence of the electrical conductivity for (Ba0.05K0.95)BiO3⅐1/6H2O.

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D.-Y. JUNG et al.
Vol. 33, No. 8
CONCLUSION
Cubic crystals of hydrated BKBO, (Ba0.05K0.95)BiO3⅐1/6H2O, were grown by electrochem-
ical synthesis from KOH flux at 255°C. The new compound had the KSbO3 tunnel structure,
not the perovskite structure of the lower potassium content containing crystalline solutions.
Barium substitution for potassium was confirmed by XRD, EDS, and SIMS analyses. TGA
determined that (Ba0.05K0.95)BiO3⅐1/6H2O was thermally unstable above 400°C, with hydrate
water release from the lattice. The products were hygroscopic. Refinement of atomic posi-
tions gave the best fit for the site occupancy of potassium, barium, and hydrated oxygen at
different positions in the tunnel structure of KSbO3.
ACKNOWLEDGMENT
The research was supported by the National Science Foundations (DMR 91–20000) through
the Science and Technology Center for Superconductivity. We acknowledge the use of
facilities in the Center for Microanalysis of Materials.
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