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The structure of BaK0.03Bi0.97O3 by single-crystal X-ray diffraction
Physiea C 185-189 (1991) 697-698 North-Holland
THE STRUCTURE OF BaK0.03Bi0.9703 BY SINGLE-CRYSTAL X-RAY DIFFRACTION
C. CHAILLOUT 1, J. CHENAVAS 1, J. DURR 1, M. MAREZIOI, 2, L. SCHNEEMEYER 2 1. Laboratoire de Cristailographie, CNRS-UJF, 166X, 38042 Grenoble Cedex, France 2. AT&T Bell Laboratories, Murray Hill, N.J. 07974, USA
We determined by x-ray diffraction the structure of a slightly K-doped BaBiO3 single-crystal. The best refinements were obtained by describing the structure in the monoclinic 12/m space group similarly to the structure of the undoped compound. In this description the Bi cations occupy two crystallographically independent sites occupied by disproportionated Bi3+ and Bi5+ cations, respectively. "l~,ese cations are partially ordered. Suprisingly we found that for this low K concentration, the K atoms are located on one of the two Bi sites and are not, as expected, substituting for the Ba cations. 1. INTRODUCTION The structure of B a l - x K x B i O 3 has been determined for various x values by several groups from either single-crystal or powder neutron diffraction data. For low K content, two possible space groups have been reported : lmmm by L. Schneemeyer et al.1 and Ibmm by Pei et al.2 These space groups lead to quite different results since the Bi atoms occupy two different sites in the former and only one in the latter. We determined out the structure of a single-crystal with a small K concentration by xray diffraction. 2. EXPERIMENTAL The crystal used for the structure refinement was a small black twinned cube grown by the flux method 1. Chemical analysis (EDS) carried out on several crystals of the same batch showed that the crystals were homogeneous and had a composition such as Bal.00K0.07Bi0.94Ox. 3. STRUCTURE REFINEMENT TableI shows that refinements in space groups
lmmm (model 1) and Ibmm (model II) give rise to comparable R-factors. From the examination of the atomic positions, it is difficult to state which is the most probable model. Wc ~efined the structure in the 12/m space greup (as for the undoped BaBiO3 compound) which corresponds to the superimposition of model I and model II (table l). In this description the Bi cations occupy two crystallographically independent sites. Contrary to what has been reported before, we find that for this low x value, the K atoms are located on one of the two Bi sites and are not, as expected, substituting for the Ba atoms. 4. DISCUSSION Figure 1 gives a schematic representation of *1.~
.',.-.,-,.~,'~rrs~n~"
,"~f t h a ,
r~ot~h,aclr',=
,Mnro
tha,
P
axis. The four Bi-O distances perpendicular to the c axis are almost identical for both sites. Tnc octahedra are alternatively apicaUy-elongated and -contracted : this arrangement is different from that of BaBiO3 where the two octahedral sites are almost regular and alternatively large and small along the c axis.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B V. All rights reserved.
698
C. ChaiUout et a£ I The structure
of BaK~mBia~O3
Table I : Positional Parameters and Occupancy Factors after Refinement in the Immm, Ibmm, and I2/m Space groups Immm
Ibmm
I2/m
Ba
x y z p
1/2 0.5068(5) 0.501(1) 0 0 0 0.2472(3) 1/4 0.2514(7) 0.996(3) 1.00 0.994(2)
Bil
x y z p
0 0 0 0.977(3)
x y
i/2 1/2
1/2 1/2
Bi2
O1
02
Rw Ruw g2
0 0 0 0.989(2)
0 0 0 0.954(2)
z
0
0
p
1.00
0.993(4)
x y z p
0 0.01(3) 0 0 0.2663(6) 1/4 0.98(2) 0.98(2)
0.002(14) 0 0.265(1) 0.95(2)
x y z p
0.253(5) 0.271(2) 0 1.0
1/4 1/4 0.0216(6) 1.0
0.264(4) 0.234(3) -0.021(1) 1.0
4.43% 8.41% 1.34
4.37% 8.34% 1.29
3.87% 7.32% 1.35
If one calculates the formal valence of the two Bi sites by the Brown and Altermatt formula and constants3, one finds v(Bil) = 3.55+ and v(Bi2) = 4.45+ after the normalisation so that the average valence is 4+. These values are very close to those found for the crystals of BaBiO3 before annealing4 and indicate that the Bi cations are disproportionated and only partially ordered. The existence of the two sites and the disproportionation are in agreement with semiconducting properties of BaK0.03Bi0.9703. It should be pointed out that the presence of the K
~
°
(
6
)
2.04 (I) k
%
9•.2° (6)
2.30 (1) A
) Figure 1 : Schematic representation of the arrangement of the octahedra along the c axis.
cations on the Bil sites modifies slightly the cation oxygen distances and consequently the calculated valence of the Bi sites. REFERENCES 1. L.F. Schneemeyer, J.K. Thomas, T. Siegrist, B. Batlogg, L.W. Rupp, R.L. Opila, R.J. Cava, D.W. Murphy, Nature 335 (1988) 421. 2. S. Pei, J.D. Jorgensen, B. Dabrowski, D.G. Hinks, D.R. Richards, A.W. Mitchell, J.M. Newsam, S.K. Sinha, D. Vaknin, A.J. Jacobson, Phys. Rev. B41 (1990) 4126. 3. I.D. Brown, D. Altermatt, Acta Cryst. B41 (1985) 244. 4. C. Chaillout, A. Santoro, J.P. Remeika, A.S. Cooper, G.P. Espinosa, M. Marezio, Solid State Comm. 65 (1988) 1363.
THE STRUCTURE OF BaK0.03Bi0.9703 BY SINGLE-CRYSTAL X-RAY DIFFRACTION
C. CHAILLOUT 1, J. CHENAVAS 1, J. DURR 1, M. MAREZIOI, 2, L. SCHNEEMEYER 2 1. Laboratoire de Cristailographie, CNRS-UJF, 166X, 38042 Grenoble Cedex, France 2. AT&T Bell Laboratories, Murray Hill, N.J. 07974, USA
We determined by x-ray diffraction the structure of a slightly K-doped BaBiO3 single-crystal. The best refinements were obtained by describing the structure in the monoclinic 12/m space group similarly to the structure of the undoped compound. In this description the Bi cations occupy two crystallographically independent sites occupied by disproportionated Bi3+ and Bi5+ cations, respectively. "l~,ese cations are partially ordered. Suprisingly we found that for this low K concentration, the K atoms are located on one of the two Bi sites and are not, as expected, substituting for the Ba cations. 1. INTRODUCTION The structure of B a l - x K x B i O 3 has been determined for various x values by several groups from either single-crystal or powder neutron diffraction data. For low K content, two possible space groups have been reported : lmmm by L. Schneemeyer et al.1 and Ibmm by Pei et al.2 These space groups lead to quite different results since the Bi atoms occupy two different sites in the former and only one in the latter. We determined out the structure of a single-crystal with a small K concentration by xray diffraction. 2. EXPERIMENTAL The crystal used for the structure refinement was a small black twinned cube grown by the flux method 1. Chemical analysis (EDS) carried out on several crystals of the same batch showed that the crystals were homogeneous and had a composition such as Bal.00K0.07Bi0.94Ox. 3. STRUCTURE REFINEMENT TableI shows that refinements in space groups
lmmm (model 1) and Ibmm (model II) give rise to comparable R-factors. From the examination of the atomic positions, it is difficult to state which is the most probable model. Wc ~efined the structure in the 12/m space greup (as for the undoped BaBiO3 compound) which corresponds to the superimposition of model I and model II (table l). In this description the Bi cations occupy two crystallographically independent sites. Contrary to what has been reported before, we find that for this low x value, the K atoms are located on one of the two Bi sites and are not, as expected, substituting for the Ba atoms. 4. DISCUSSION Figure 1 gives a schematic representation of *1.~
.',.-.,-,.~,'~rrs~n~"
,"~f t h a ,
r~ot~h,aclr',=
,Mnro
tha,
P
axis. The four Bi-O distances perpendicular to the c axis are almost identical for both sites. Tnc octahedra are alternatively apicaUy-elongated and -contracted : this arrangement is different from that of BaBiO3 where the two octahedral sites are almost regular and alternatively large and small along the c axis.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B V. All rights reserved.
698
C. ChaiUout et a£ I The structure
of BaK~mBia~O3
Table I : Positional Parameters and Occupancy Factors after Refinement in the Immm, Ibmm, and I2/m Space groups Immm
Ibmm
I2/m
Ba
x y z p
1/2 0.5068(5) 0.501(1) 0 0 0 0.2472(3) 1/4 0.2514(7) 0.996(3) 1.00 0.994(2)
Bil
x y z p
0 0 0 0.977(3)
x y
i/2 1/2
1/2 1/2
Bi2
O1
02
Rw Ruw g2
0 0 0 0.989(2)
0 0 0 0.954(2)
z
0
0
p
1.00
0.993(4)
x y z p
0 0.01(3) 0 0 0.2663(6) 1/4 0.98(2) 0.98(2)
0.002(14) 0 0.265(1) 0.95(2)
x y z p
0.253(5) 0.271(2) 0 1.0
1/4 1/4 0.0216(6) 1.0
0.264(4) 0.234(3) -0.021(1) 1.0
4.43% 8.41% 1.34
4.37% 8.34% 1.29
3.87% 7.32% 1.35
If one calculates the formal valence of the two Bi sites by the Brown and Altermatt formula and constants3, one finds v(Bil) = 3.55+ and v(Bi2) = 4.45+ after the normalisation so that the average valence is 4+. These values are very close to those found for the crystals of BaBiO3 before annealing4 and indicate that the Bi cations are disproportionated and only partially ordered. The existence of the two sites and the disproportionation are in agreement with semiconducting properties of BaK0.03Bi0.9703. It should be pointed out that the presence of the K
~
°
(
6
)
2.04 (I) k
%
9•.2° (6)
2.30 (1) A
) Figure 1 : Schematic representation of the arrangement of the octahedra along the c axis.
cations on the Bil sites modifies slightly the cation oxygen distances and consequently the calculated valence of the Bi sites. REFERENCES 1. L.F. Schneemeyer, J.K. Thomas, T. Siegrist, B. Batlogg, L.W. Rupp, R.L. Opila, R.J. Cava, D.W. Murphy, Nature 335 (1988) 421. 2. S. Pei, J.D. Jorgensen, B. Dabrowski, D.G. Hinks, D.R. Richards, A.W. Mitchell, J.M. Newsam, S.K. Sinha, D. Vaknin, A.J. Jacobson, Phys. Rev. B41 (1990) 4126. 3. I.D. Brown, D. Altermatt, Acta Cryst. B41 (1985) 244. 4. C. Chaillout, A. Santoro, J.P. Remeika, A.S. Cooper, G.P. Espinosa, M. Marezio, Solid State Comm. 65 (1988) 1363.