Suppression of modulations in fluorinated Bi-2201 phases

Article abstract of DOI:10.1006/jssc.2000.9020

Bi₂Sr_1.6La_0.4CuO_6.33 and Bi₂Sr_1.2La_0.8CuO_6.46 are fluorinated using XeF₂ as a fluorinating agent and studied by X-ray powder diffraction and high-resolution electron microscopy. The highly fluorinated Bi-2201 phase undergoes a structural transformation that is accompanied by the formation of an I-centered tetragonal unit cell, the suppression of the √2a_{p x} √2a_p superstructure, the disappearance of the (BiO)₂ modulation, and an elongation of the c parameter of about 1.65-1.77 A. The modification of the Bi₂O₂ block into a Bi₂O_2-δF_2+δ block with filled tetrahedral interstitial positions is responsible for the observed structural changes and eliminates the mismatch between the (BiO) rock salt layers and the perovskite (CuO₂) layers. The structure of the fluorinated material is compared with the structures of other intercalated Bi-based cuprates and phases composing Bi₂(O,F)₄ blocks in the structure.

Full text of DOI:10.1006/jssc.2000.9020

Journal of Solid State Chemistry 156, 445}451 (2001)
doi:10.1006/jssc.2000.9020, available online at http://www.idealibrary.com on
Suppression of Modulations in Fluorinated Bi-2201 Phases
J. Hadermann, N. R. Khasanova, and G. Van Tendeloo
EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium
and
A. M. Abakumov, M. G. Rozova, A. M. Alekseeva, and E. V. Antipov
Department of Chemistry, Moscow State University, 119899 Moscow, Russia
Received August 16, 2000; in revised form October 12, 2000; accepted October 27, 2000
observed by di!erent di!raction methods. The inserted
oxygen provides holes to the conduction band contributing
Bi2Sr1.6La0.4CuO6.33 and Bi2Sr1.2La0.8CuO6.46 are 6uorinated
using XeF2 as a 6uorinating agent and studied by X-ray powder
thereby to the superconductivity. For compounds of a
di4raction and high-resolution electron microscopy. The highly
given cation composition the amount of extra oxygen, the
6uorinated Bi-2201 phase undergoes a structural transformation
d value, can be varied in a limited range depending on the
that is accompanied by the formation of an I-centered tetragonal
synthesis conditions (3, 4). These parameters appear
unit cell, the suppression of the (2ap;(2ap superstructure, the
disappearance of the (BiO)2 modulation, and an elongation of the
c parameter of about 1.65+1.77 A. The modi5cation of the Bi2O2
block into a Bi2O2؊dF2؉d block with 5lled tetrahedral interstitial
to be crucial for the phase stability and for the super-
conducting properties of the Bi-2201 phase. Monophasic
samples with
a
partial substitution of Bi for Sr,
are not superconducting, but supercon-
positions is responsible for the observed structural changes and Bi Sr CuO
ꢀ>V ꢀ\V
ꢃ>B
eliminates the mismatch between the (BiO) rock salt layers and
the perovskite (CuO2) layers. The structure of the 6uorinated
material is compared with the structures of other intercalated
Bi-based cuprates and phases composing Bi2(O,F)4 blocks in the
structure. ( 2001 Academic Press
ductivity can be induced by rare earth doping on the Sr site.
The parabolic-like compositional dependence of ¹ with
#
a maximum of 33 K at x"0.4 was observed in
a Bi Sr La CuO
solid solution (4). On further La
ꢀ
ꢀ\V
V
ꢃ>B
doping ¹ decreases and for Bi Sr La CuO
the
#
ꢀ
ꢁꢄꢅ ꢆꢄꢇ
ꢃ>B
Key Words: 6uorination; Bi-2201; Bi2Sr1.6La0.4CuO6.33؊xF2؉x
Bi2Sr1.2La0.8CuO6.46 ; Bi2Sr1.6La0.4CuO6.33
;
superconductivity disappears. The suppression of supercon-
ductivity at a higher La content was assigned to the reduc-
tion of the Cu oxidation state upon heterovalent
substitution, and attempts to oxidize further by oxygen
treatments were unsuccessful.
.
1. INTRODUCTION
Fluorination was successfully explored for oxidizing
Among the di!erent layered cuprates the bismuth di!erent cuprates, and several new superconducting com-
cuprates, Bi Sr Ca Cu O
, exhibit a particular pounds were prepared (5, 6). In contrast to the anion
ꢀ
ꢀ
L\ꢁ L ꢀL>ꢂ>B
ꢀ\
behavior. These oxides, which consist of an intergrowth exchange of F\ for O , which reduces the copper oxida-
structure, di!er from the corresponding thallium bilayer tion state, #uorine intercalation on the interstitial site leads
cuprates by the presence of a strong modulation. The in- to the oxidation of the conducting (CuO ) planes and, in
ꢀ
commensurate modulation is believed to be caused by several cases, to structural transformations (5, 6, 17). It is of
a mismatch between the (BiO) rock salt type layer and the interest to examine the possibility of further oxidation of the
perovskite (CuO ) layers. The mismatch is reduced by Bi-based cuprates by #uorination and to study its e!ect on
ꢀ
the insertion of extra oxygen in the (BiO) layer and by ¹ . Considering the structural simplicity of the Bi-2201
#
the cooperative displacements of the Bi and O atoms from phase and the strong e!ect of slight hole doping on its
their ideal positions within this plane which enables the properties (since all holes correspond to one (CuO ) plane)
ꢀ
appropriate Bi coordination to occur (1, 2). However, this the #uorination of the Bi-2201 phase was attempted, and
rearrangement of the Bi O
block a!ects the rest of the two compounds, the superconducting Bi Sr La CuO
ꢁꢄꢃ ꢆꢄꢂ ꢃꢄꢅꢅ
, were
ꢀ ꢀ>B
ꢀ
structure through the corrugation of the adjacent layers and the non superconducting Bi Sr La CuO
and leads to the appearance of strong satellite re#ections selected for this purpose.
ꢀ
ꢁꢄꢀ ꢆꢄꢈ
ꢃꢄꢂꢃ
445
0022-4596/01 $35.00
Copyright ( 2001 by Academic Press
All rights of reproduction in any form reserved.

446
HADERMANN ET AL.
2. EXPERIMENTAL
Electron di!raction (ED) and high resolution electron
microscopy (HREM) were performed with a JEOL 4000EX
The initial compounds, Bi Sr La CuO
ꢁꢄꢃ ꢆꢄꢂ ꢃꢄꢅꢅ
ꢀ
and microscope. The image simulations were made using
Bi Sr La CuO , were prepared by a ceramic method the MacTempas software. The electron di!raction X-ray
ꢁꢄꢀ ꢆꢄꢈ ꢃꢄꢂꢃ
using Bi O , SrCO , CuO, and La O as starting materials. (EDX) analysis and electron di!raction were performed
ꢀ
ꢀ ꢅ
ꢅ ꢀ ꢅ
Stoichiometric amounts of these materials were mixed, using a Philips CM20 microscope with a LINK-2000
pressed into pellets, and heated at 720, 800, 850, and 8703C attachment. The presence of #uorine in the samples was
with subsequent regrinding for 100 h. The X-ray di!raction checked qualitatively since an exact quantitative determina-
patterns are indexed in the Amaa space group with cell tion of the #uorine content was di$cult due to the low
parameters a"5.398(2) A, b"5.368(1) A, c"24.349(4) A Z number.
and a"5.439(4) A, b"5.422(2) A, c"24.00(1) A for the
"rst
and
second
compound,
respectively.
3. RESULTS
Bi Sr La CuO
demonstrates
a
superconducting
ꢀ
ꢁꢄꢃ ꢆꢄꢂ
ꢃꢄꢅꢅ
#
transition with a ¹ of 27 K and no superconductivity is
The #uorination conditions and the cell parameters of the
detected for Bi Sr La CuO
ꢁꢄꢀ ꢆꢄꢈ ꢃꢄꢂꢃ
as well as the oxygen content, determined by iodometric particularly for Bi Sr La CuO
. The observed parameters obtained compounds are given in Table 1. In several cases,
ꢀ
, the #uorination re-
sults in a partial decomposition of the initial compounds
ꢀ
ꢁꢄꢀ ꢆꢄꢈ
ꢃꢄꢂꢃ
titration, agree with the previously published data (1, 4).
The initial compounds (0.3 g) were mixed with XeF
(provided by the Laboratory of Inorganic Synthesis of the and BiF detected by X-ray di!raction. Due to the presence
with the release of badly crystallized LaOF, SrF , BiOF,
ꢀ
ꢀ
ꢅ
Institute of Applied Chemical Physics &&Kurchatovskii In- of broad re#ections corresponding to the admixture phases
stitut,'' Moscow, Russia) in molar ratios of 1:1, 1:1.5, and the characterization of these samples by X-ray di!raction is
1:3 and ground in an agate mortar. All the operations with di$cult and they were not investigated further.
XeF were carried out in a glove box "lled with dried N .
The mixtures were then placed in a Ni crucible and sealed in ence of the magnetic susceptibility was measured and no
For all the #uorinated samples the temperature depend-
ꢀ
ꢀ
an N -"lled copper tube. Di!erent #uorinated samples were superconducting transition down to 12 K was detected in
ꢀ
prepared by annealing at 150 and 2003C for 15 h and at any of them.
3003C for 40 and 80 h and subsequently furnace cooling to
room temperature.
The X-ray patterns of Bi Sr La CuO
ꢁꢄꢃ ꢆꢄꢂ ꢃꢄꢅꢅ
at 150 and 2003C (samples 2 and 3) indicate the presence of
#uorinated
ꢀ
The phase composition of the samples and the lattice two phases in both samples. While the "rst phase (A) is
parameters of the compounds were determined by X-ray identi"ed as similar to the initial compound with slightly
di!raction using a focusing Guinier camera FR-552 (CuKa
radiation, germanium internal standard). The formal copper formation occurs for the second phase (B). Its lattice
modi"ed cell parameters, a signi"cant structural trans-
ꢁ
valence of the pure samples was determined by iodometric parameters are determined as a+b+3.8 A and c+26 A,
titration.
indicating a considerable expansion along the c axis (about
The ac susceptibility measurements were carried out in 1.6}1.8 A) with a simultaneous suppression of the (2
the temperature range of 12}100 K at an external "eld superstructure in the a}b plane. A signi"cant amount of the
amplitude of 1 Oe and a frequency of 27 Hz.
B phase is found in both samples.
TABLE 1
Fluorination Conditions and Cell Parameters for the Initial and Fluorinated Compounds
No.
Fluorination conditions
a(A)
b(A)
c(A)
<
!ꢀ
1
2
Bi Sr La CuO
1:1 XeF ; 15 h at 1503C
5.398(2)
5.419(1)
3.858(3)
5.418(1)
3.837(1)
5.407(1)
5.410(2)
5.430(4)
5.432(1)
5.430(1)
5.368(1)
5.381(1)
3.832(3)
5.3802(9)
24.349(4)
24.370(9)
25.99(3)
24.406(6)
26.18(2)
24.200(8)
24.26(1)
24.00(1)
24.07(1)
24.15(1)
2.25
ꢀ
ꢁꢄꢃ ꢆꢄꢂ
ꢀ
ꢃꢄꢅꢅ
A
B
A
B
3
1:1 XeF ; 15 h at 2003C
ꢀ
4
5
6
7
8
1:1.5 XeF ; 40 h at 3003C
5.407(1)
5.410(2)
5.422(2)
5.432(1)
5.430(1)
2.16
ꢀ
1:3 XeF ; 80 h at 3003C
*
*
ꢀ
Bi Sr La CuO
2.12
2.05
ꢀ
ꢁꢄꢀ ꢆꢄꢈ
ꢃꢄꢂꢃ
1:1.5 XeF ; 40 h at 3003C
ꢀ
1:1 XeF ; 15 h at 1503C
ꢀ
*Decomposition products were detected on X-ray di!raction patterns.

SUPPRESSION OF MODULATIONS IN FLUORINATED Bi-2201
447
For both initial compounds the #uorination at 3003C for
40 h produces almost pure samples (Nos. 4 and 7). Their
X-ray patterns are similar to those of the starting materials
and are indexed in the Amaa space group, with slightly
modi"ed cell parameters. It should be pointed out that in
several cases (samples 4, 5, 7, and 8) a pseudotetragonal unit
cell was considered, but the actual symmetry of these phases
is orthorhombic due to the modulation, the presence of
which was con"rmed by electron microscopy and is de-
scribed later. The iodometric titration reveals the reduction
of the copper oxidation state for both sample 4 (from
< "#2.25 to #2.16) and sample 7 (from < "#2.12
#ꢀ
#ꢀ
to #2.05). The #uorination at 3003C for a longer time
(80 h) results in the decomposition of Bi Sr La CuO
while for Bi Sr La CuO
small changes in the cell parameters and to the appearance
of some decomposition products.
,
ꢀ
ꢁꢄꢀ ꢆꢄꢈ
ꢃꢄꢂꢃ
(sample 5) it leads only to
ꢀ
ꢁꢄꢃ ꢆꢄꢂ
ꢃꢄꢅꢅ
The structure characterization of the #uorinated com-
pounds by X-ray di!raction is complicated because in
several cases the samples contain two phases, and the over-
lapping of some re#ections due to the similar subcells is
unavoidable. The presence of the satellites caused by the
modulation adds even more di$culties to this investigation.
To resolve these problems, the samples 2}4 and 7 were
studied by electron microscopy. Moreover HREM can pro-
vide information about the structure modulation which is
di$cult to elucidate from X-ray di!raction only.
FIG. 2. Parts of the EDX spectrums of the A phase (A) and the B phase
(B) (sample 2), showing the F(Ka) peak in the low energy range.
The transmission electron microscopy (TEM) study of
samples 2 and 3 con"rms the presence of two di!erent
phases (A and B) in these samples. The ED patterns taken
*
from the A phase are similar to those of the initial com- resents a [010] ED pattern of the A phase together with the
pound: the basic spots compatible with the Amaa space corresponding HREM image where the corrugation of the
group are accompanied by satellite re#ections. Figure 1 rep- layers caused by the modulation can be seen. The modula-
*
*
tion vector q"0.23a #0.7c , estimated from the ED pat-
terns, is rather close to that of the initial compound. The
EDX analysis reveals a small amount of #uorine in the
crystallites of the A phase (Fig. 2A), con"rming thereby that
the small changes in the unit cell parameters found by X-ray
di!raction are due to the #uorination.
The ED patterns of the B phase (Fig. 3) show that it has
a unit cell with a+b+3.8 A and c+26.0 A, which agrees
well with the X-ray di!raction data. No satellite re#ections
and no additional re#ections that might correspond to
a (2a superstructure are observed on any of ED patterns.
ꢁ
The reconstruction of the reciprocal lattice using sets of ED
patterns from several crystals reveals the extinction condi-
tions hkl:h#k#l"2n, compatible with a body-centered
space group. Thus, for the #uorinated material of the
B phase, the space group I4/mmm can be chosen for the
tetragonal phase (sample 2) and Immm for the orthorhombic
one (sample 3). The transformation relations between the
*
*
initial and the new unit cell are given by [110] "[100] ,
'
*
*
[110] "[010] , where I denotes the new, I-centered unit
cell. The EDX analysis does not reveal any changes in the
FIG. 1. [110] HREM image for the A phase of BI Sr La Cu(O,F)
ꢁꢄꢃ ꢆꢄꢂ
(sample 2). The corresponding ED pattern is shown as an inset.
'
ꢀ
V

448
HADERMANN ET AL.
FIG. 3. Electron di!raction patterns taken along the main zones of the B phase (sample 2).
cation composition of the B phase, but, contrary to the phases are observed. Their ED patterns are rather similar to
A phase, a large amount of #uorine is detected in the the corresponding patterns of the initial compounds, but
structure (Fig. 2B).
small amounts of #uorine detected by the EDX analysis
Phase B is unstable and decomposes under an intense suggest that the changes in the cell parameters are caused by
electron beam. This does not a!ect the recording of the the #uorination.
di!raction patterns, but severely hinders the acquisition of
HREM images. To avoid these problems we attempted
a low-dose technique that suggests the use of the spread
beam with long exposure times. The HREM images of the
4. DISCUSSION
Previous studies demonstrated that the #uorination of
B phase taken in this way con"rm the results of the recipro- cuprates involves di!erent reactions. The oxidation of the
cal space investigation. The HREM image along the [110]
(CuO ) planes results from the intercalation of #uorine into
'
ꢀ
direction shown in Fig. 4 presents a modulation-free layered a structure that contains either anion vacancies or intersti-
crystal structure with the positions of the (BiO) layers in- ces suitable for the accommodation of #uorine. Another
dicated by arrows. According to the image simulation the Bi competing reaction taking place during the #uorination is
atoms correspond to the dark spots in these rows. From the an anion exchange, i.e. the replacement of oxygen atoms by
HREM image along [100] given in Fig. 5 one can see that #uorine. It may lead either to the partial reduction of Cu if
'
ꢀ\
\
the staggered con"guration of the (BiO) layers is main- one O ion is replaced by one F ion (a reaction of type I)
ꢀ
ꢀ\
\
tained.
or, if one O is replaced by two F , to the increase of the
From the TEM investigation of samples 4, 5, and 7 no overall amount of anions in the structure without changing
remarkable changes in the structures of the #uorinated the Cu oxidation state (a reaction of type II). The inner walls
of the Cu tube, which is used as a reaction vessel, adsorb the
released oxygen. Usually both kinds of anion exchange
reactions occur simultaneously at high temperatures
(350}5003C), while at low temperatures (150}3003C) the
insertion process dominates (5, 17).
FIG. 4. HREM image of the B phase taken along [110] . The inset
shows the corresponding calculated image (*f"!115 nm, t"9 nm). The (BiO) layers are indicated by arrows and con"rm the staggered con"gura-
arrows indicate the (BiO) layers.
FIG. 5. [100]I HREM image of the B phase (sample 2). Positions of the
'
tion of these layers.

SUPPRESSION OF MODULATIONS IN FLUORINATED Bi-2201
449
The #uorination of the Bi-2201 phase appears to follow arrangement of this structural block, while the rest of the
the anion exchange reaction, even if the #uorination temper- structure (the other interlayer distances) is comparable to
ature is rather low (150}2003C). All the #uorinated mater- the initial one. The proposed positions of #uorine, oxygen,
ials exhibit unit cells that are expanded in the a}b plane in and Bi in the transformed block are quite similar to those in
comparison with the cell dimensions for the initial non- the Aurivillius phase, Bi NbO F, and in BiOF, where the Bi
ꢀ
ꢉ
#uorinated phases, thus re#ecting the reduction of the Cu coordination is a square antiprism with four Bi}F distances
atoms due to reaction I. Indeed, the decrease in the formal of 2.29 A and four Bi}O distances of 2.71 A (12, 14). There-
Cu valence is supported for samples 4 and 7 by the results of fore, the Bi O
block transforms into a Bi O
F
ꢀ ꢀ>V
ꢀ ꢀ\V ꢀ>V
the iodometric titration. The low formal Cu valence can also block (if all anion positions are occupied, but most probably
provide an explanation for the absence of superconductivity some of them are vacant) with the coordination described
in the #uorinated Bi-2201 phases. Another possible explana- above. The comparison of the interlayer distances in the
tion for the absence of superconductivity could be that Bi is initial and the #uorinated Bi-2201 with those observed for
oxidized rather than copper, and superconductivity is elimi- the Bi O F or Bi O blocks in the Aurivillius-type phases
ꢀ ꢀ ꢀ
ꢀ ꢂ
nated due to charge redistribution. For the highly #uor- (14, 15) allows us to unravel the structural reason for the
inated B phase the signi"cant modi"cation of the structure unit cell expansion along the c axis upon the "lling of
can be attributed to the insertion of a considerable amount the interstitial positions by extra anions. As shown in Fig. 6,
of #uorine by reaction II. A possible model for this trans- the distance between the two Bi layers above and below the
formation is drawn on the basis of the ED patterns and the interstitial positions decreases from 3.18}3.20 A in the non-
HREM images of the B phase and the structure considera- #uorinated Bi-2201 to 2.49}2.56 A in the Aurivillius phases
tion of di!erent Bi-containing compounds.
(14, 15) due to the electrostatic interaction between the inter-
ꢅ>
As a possible location for the introduced #uorine the stitial anions and the Bi cations. On the other hand, the
position between the double (BiO) layers was considered. repulsion between the interstitial anions and the oxygen
The bonding between these two layers is weak, and this atoms in the (BiO) layers increases the distance between the
position is subjected to various intercalations (I , Br , oxygen atoms. In the initial Bi-2201 structure the (BiO)
ꢀ
ꢀ
HgBr , LiI ) (7}11). Iodine incorporation into the structure layers are nearly #at and the distance along the c axis
ꢀ
ꢅ
of the Bi-2212 and Bi-2223 phases leads to an expansion between the Bi3# and the oxygen atoms in this layer is in the
along the c axis and changes the rock salt alternation of the range of 0.24}0.26 A. This distance increases to 0.92}1.07 A
(BiO) layers. The separation of the two (BiO) layers by the in the Aurivillius phases, overpowering the decrease in the
iodine layer results in the disappearance of the staggered separation between the Bi layers and resulting in the overall
con"guration of the (Bi O ) block, i.e., the loss of the rela- expansion of the whole block with 0.7}1 A (from
ꢀ ꢀ
tive shift of the (BiO) layers and the formation of a primitive
unit cell. No serious e!ect on the modulation is observed
due to the weak interaction between the (BiO) and the
I layers, and only a small decrease in ¹ is detected for the
#
intercalated compounds (8).
For the B phase, #uorine insertion also results in a re-
markable expansion along the c axis; however, the rock salt
con"guration of the (BiO) layers is maintained as shown by
the I-centered space group and the HREM evidence. The
disappearance of the modulations and of the (2a ;(2a
ꢁ
ꢁ
superstructure indicates that the inserted #uorine signi"-
cantly modi"es the Bi O
block arrangement, eliminating
ꢀ ꢀ>B
the mismatch between the blocks. Fluorine is proposed to
be accommodated between the (BiO) layers in the inter-
stitial #uorite type positions tetrahedrally coordinated by
ꢅ>
four Bi cations. To model this transformation we con-
sidered the structures of several compounds in which Bi is
coordinated by O and F, such as BiOF (12), BiF (13), and
ꢅ
Bi NbO F (14). The structure model comprising Bi cations
ꢀ
ꢉ
in a capped square antiprism with four short Bi}F and "ve
longer Bi}O distances (Fig. 7) was built using the cell para-
meters and the Immm space group derived from the ED
patterns. In this coordination behavior the inserted #uorine
FIG. 6. Comparison of the interlayer distances in the Bi-2201 and
forms a strong bond to Bi, which is accompanied by a re- Aurivillius phases (the structure of Bi NbO F is shown as an example).
ꢀ
ꢉ

450
HADERMANN ET AL.
3.66}3.72 A to 4.32}4.7 A). The thus expected value of the could be explained by the increased coordination number of
increase in the c parameter should be 1.4}2 A, which agrees the Bi atoms, followed by the elongation of the interatomic
well with the experimentally observed value of 1.65}1.77 A. distances as well as by the repulsion forces between the
The structure proposed above (Fig. 7) is used for the anions. As a result, the mismatch between the (BiO) rock
image simulations (Fig. 4) and gives a good agreement with salt type layer and the perovskite (CuO ) layers is no longer
ꢀ
the experimental images. However, this structure should be existent, leading to the suppression of the modulations in
regarded as only a rough model because the low scattering the structure of the highly #uorinated Bi-2201 phases.
ꢀ\
\
factors of O and F and the small di!erences between
In contrast to the case of #uorination, the intercalation of
them make it di$cult to examine the distribution of oxygen iodine has a di!erent e!ect on the structure of the Bi-
and #uorine between the di!erent anion positions or to containing cuprates, which could be explained by the di!er-
determine their occupancies. The same arrangement of Bi ences in their size and electronegativity. The smaller #uorine
and F was recently reported for the Bi-2223 compound (r(F\)"1.19 A) is located between the double (BiO) layers
#uorinated by NH HF (16). In this case the #uorination in the tetrahedra of four Bi atoms and forms strong bonds
ꢂ
ꢀ
also produced a mixture of two phases, and the structure of modifying this block. However, the larger size of
\
\
the #uorinated compound, Bi Sr Cu O F , was re"ned by
I
(r(I )"2.20 A) requires the formation of a polyhedron
ꢀ
ꢀ
ꢅ ꢈ ꢂ
the Rietveld method on the basis of neutron di!raction data. with a larger coordination number and with longer in-
In the new compound #uorine was found to replace oxygen teratomic Bi}I distances. In the phase with the ideal
in the (BiO) layer in a 2:1 ratio, leading to a similar expan- IBi Sr Ca Cu O composition [8], iodine is located in
ꢀ
ꢀ
ꢀ
ꢅ V
sion (about 1.8 A) along the c axis, but the disappearance of a square prism of Bi atoms with Bi}I separations of
neither the (2a ;(2a superstructure nor the modulation 4.07}4.22 A. These distances are even longer than the Cs}I
ꢁ
ꢁ
was mentioned.
separation in the CsI structure (ICs cubes with
ꢈ
The #uorination of the Bi-2201 phases leads not only to d(Cs}I)"3.96 A) showing that the Bi}I bond has a weak
an increase in the thickness of the Bi O
F
block but van der Waals interaction and does not signi"cantly in#u-
ꢀ ꢀ\V ꢀ>V
also to the expansion of this block in the a}b plane. This ence the structure or the dimensions of the (BiO) layers.
ACKNOWLEDGMENTS
This work was performed in the framework of IUAP-PAI 4/10 of the
Belgian government and was partially supported by INTAS-99-1136 and
Volkswagen Grant I/75849.
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