A reinvestigation of quaternary layered bismuth oxyhalides of the Sillen X1 type

Article abstract of DOI:10.1016/S0022-4596(03)00297-4

A family of layered bismuth oxyhalides, L^I_0.5Bi _1.5O₂X and L^IIBiO₂X has been reinvestigated. Formation of X1-type Sillen compounds has been established for L^I=Li, Na, L^II=Ca, Sr, Ba, and X=Cl, Br, I, but the details of their crystal structures are different. While all L ^I_0.5Bi_1.5O₂X, CaBiO₂Br, and CaBiO₂I adopt the disordered tetragonal Nd₂O ₂Te structure, all compounds of L^II=Sr and Ba are orthorhombic and isostructural to PbSbO₂Cl, due to L/Bi cation ordering. Crystal structures have been determined for CaBiO₂I, SrBiO₂Br, SrBiO₂I, and BaBiO₂I. We discuss the factors which determine the occurrence and type of cation ordering in the quaternary bismuth and antimony X1-type oxyhalides. We also predict that more isostructural compounds can be prepared with antimony.

Full text of DOI:10.1016/S0022-4596(03)00297-4

ARTICLE IN PRESS
Journal of Solid State Chemistry 175 (2003) 316–321
A reinvestigation of quaternary layered bismuth oxyhalides
of the Sillen X1 type
D.O. Charkin,a P.S. Berdonosov,b V.A. Dolgikh,b and P. Lightfootc,ꢀ
a Department of Materials Sciences, Moscow State University, 119992 Moscow GSP-2, Russia
b Department of Chemistry, Moscow State University, 119992 Moscow GSP-2, Russia
c School of Chemistry, St. Andrews University, Purdie Building, North Haugh, Fife KY16 9ST, UK
Received 3 March 2003; received in revised form 15 May 2003; accepted 17 May 2003
Abstract
A family of layered bismuth oxyhalides, LI0.5Bi1.5O2X and LIIBiO2X has been reinvestigated. Formation of X1-type Sillen
compounds has been established for LI=Li, Na, LII=Ca, Sr, Ba, and X=Cl, Br, I, but the details of their crystal structures are
different. While all LI0.5Bi1.5O2X, CaBiO2Br, and CaBiO2I adopt the disordered tetragonal Nd2O2Te structure, all compounds of
LII=Sr and Ba are orthorhombic and isostructural to PbSbO2Cl, due to L/Bi cation ordering. Crystal structures have been
determined for CaBiO2I, SrBiO2Br, SrBiO2I, and BaBiO2I. We discuss the factors which determine the occurrence and type of
cation ordering in the quaternary bismuth and antimony X1-type oxyhalides. We also predict that more isostructural compounds
can be prepared with antimony.
r 2003 Elsevier Inc. All rights reserved.
Keywords: Bismuth oxyhalides; Crystal structures; Cation ordering; Sillen phases
1. Introduction
recently as truly tetragonal (Fig. 1a) [13]. This structure
was also reported for Bi2O2Se [14] and Nd2O2Te [15].
The structures of the oxychlorides with divalent
elements, LIIBiO2Cl (LII=Cd, Ca, Sr, Ba) demonstrate
two different versions of complete ordering of the LII
and BiIII cations contributing to the fluorite layers. The
smallest Ca2+ [9] and Cd2+ [10] give a monoclinic
structure (Fig. 1b) while the larger Sr2+ [9] and Ba2+ [8]
result in an orthorhombic superstructure (Fig. 1c) found
earlier for the related BaSbO2Cl [16], PbSbO2Cl
[7,17,18], and PbBiO2Cl [4]. The lead compounds are
peculiar. PbBiO2Cl crystallizes in the orthorhombic
structure as the natural mineral perite [4]; however,
synthetic perite is always tetragonal [6]. PbSbO2Cl may
be converted from orthorhombic to tetragonal form by
treatment with boiling water [17]; more recent investiga-
tion has shown that stoichiometric PbSbO2Cl and
PbSbO2Br are orthorhombic but a shift of composition
to PbSbO2.125X0.75 makes the compounds tetragonal [7].
Stoichiometric PbSbO2I is also tetragonal, akin to
PbBiO2Br and PbBiO2I [7]. Though both stoichiometry
and size of the contributing ions are likely to affect the
structural details of these X1-type Bi and Sb oxyhalides,
neither the oxybromides nor oxyiodides of bismuth and
The first members of the Sillen layered family of
bismuth oxyhalides were reported as early as 1939 [1,2];
the family now comprises oxyhalides of Bi with Li, Na,
Ca, Sr, Ba, Pb, and Cd [3]. Their crystal structures can
be regarded as ordered alternation of fluorite-derived
[M2O2] slices (where M=Bi and its substituents listed
above) and single or double halide ion sheets. These
layer sequences give rise to ideal tetragonal structures
with aB3.9 A (i.e., O2/2a(fluorite)) and c varying from
6 to 50 A dependent on structural complexity. Despite
numerous investigations in this area (dedicated both to
their crystal chemistry [3–10] and catalytic activity in
oxidative coupling of methane [11]) many structural
details are as yet unclear. The compounds with the
simplest ‘‘X1’’ structure, which corresponds to alterna-
tion of [M2O2]+ slices and [X]ꢁ halide ion sheets, were
initially described as tetragonal, i.e., adopting the ideal
structure (which is an anti-type to ThCr2Si2 [12]).
However, as yet only Na0.5Bi1.5O2Cl has been reported
ꢀCorresponding author. Fax: +44-1334-463808.
E-mail address: pl@st-and.ac.uk (P. Lightfoot).
0022-4596/03/$ - see front matter r 2003 Elsevier Inc. All rights reserved.
doi:10.1016/S0022-4596(03)00297-4

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D.O. Charkin et al. / Journal of Solid State Chemistry 175 (2003) 316–321
317
a
2. Experimental
a
c
CaBiO2X and SrBiO2X (X=Br, I) were prepared
directly from LO and BiOX in alumina crucibles sealed
into evacuated silica ampoules by annealing the
corresponding 1:1 mixtures at 750ꢂC (Ca) or 800ꢂC
(Sr) for 5 days. CaO and SrO were obtained by calcining
the carbonates at 1100ꢂC for 2 days. The barium
compounds were prepared in a two-step process. First,
a 1:1 mixture of BaCO3 and Bi2O3 was annealed in
dynamic vacuum at B800ꢂC for 8 h. The resulting dark
red precursor was mixed with anhydrous BaX2 (pre-
pared from BaX2 ꢃ 2H2O at 250ꢂC in dynamic vacuum)
and annealed under the same conditions as for SrBiO2X.
In the search for SnBiO2X (X=Cl, Br, I), SnO and
BiOX (1:1) were annealed in evacuated silica tubes
at 700ꢂC for 7 days. The precursors were prepared
according to [22]. For synthesis of CdBiO2X (X=
Br, I), CdO and BiOX were annealed at the same
conditions as given for CdBiO2Cl [10].
c
(a)
(b)
b
c
a
Na0.5Bi1.5O2X were obtained directly from 1:1:1
mixtures of NaX (NaI ꢃ 2H2O was dehydrated under
the same conditions as BaX2), BiOX and Bi2O3 in
evacuated silica tubes at 650ꢂC for 10 days. The
potassium, copper and silver-containing samples were
treated the same way, but for the d-metals, the annealing
temperature was lower (400–450ꢂC). The lithium com-
pounds were synthesized from LiBiO2 and BiOX in
alumina crucibles sealed into silica ampoules and
annealed at 650ꢂC. LiBiO2 in its turn was prepared by
melting Li2CO3 and Bi2O3 in an alumina crucible under
dynamic vacuum for 30 min.
(c)
Fig. 1. Crystal structures of the three variants of the Sillen X1
structure type. (a) Tetragonal form (Nd2O2Te-type), (b) monoclinic
form (CaBiO2Cl-type), (c) orthorhombic form (PbSbO2Cl-type). Note
the differing cation ordering schemes for (b) and (c).
alkali and alkaline earth metals, to the best of our
knowledge, have been studied in detail since 1941.
Moreover, we could not find any reference to existence
of CaBiO2X (X=Br and I1) and SrBiO2I; BaBiO2I
was mentioned briefly in Ref. [19], but no data were
presented for this compound.
The phase purity of all samples was monitored by
X-ray powder diffraction using a Guinier camera (FR-
552, Enraf-Nonius, CuKa1 radiation). The reflection
positions on the Guinier films were determined with an
accuracy of 0.005 mm, converted into interplanar spa-
cings, and cell parameters of the obtained compounds
were refined using the whole pattern by least-squares fits
using a home-made program.
Upon preparation of BaBiO2Br as a precursor for
more complex bismuth oxyhalides [20], we have noticed
that it is orthorhombic, similar to BaBiO2Cl [7] and not
tetragonal as reported originally by Sillen [2]. A question
arose whether other X1 compounds described originally
as tetragonal may actually have lower symmetry.
Representatives of this family might also exist among
oxyhalides of bismuth and some other L cations. Hence,
the whole series of LI0.5Bi1.5O2X and LIIBiO2X oxyha-
lides (X=Br, I) has been reinvestigated, excluding
LIIBiO2Cl and Pb compounds (which have been studied
recently [6–10]). Preliminary results of our investigation
have been reported in Ref. [21]. Besides the known
L=Li, Na, Ca, Cd, Sr, and Ba, we also considered
L=SnII, K, CuI, and Ag.
3. Results
The X-ray diffraction data indicated formation
of X1-type compounds only for LI=Li and Na and
LII=Ca, Sr, and Ba with all three halogens. The
compounds of Li, Na were found to be tetragonal,
in fair agreement with Sillen’s original reports [1,2];
CaBiO2Br and CaBiO2I are also tetragonal. On the
contrary, all compounds of Sr and Ba appeared to be
orthorhombic (PbSbO2Cl type). No new example of the
monoclinic CaBiO2Cl structure type [9] has been found.
The cell parameters of the compounds which were
obtained single phase are listed in Table 1. A small
amount of BiOI was found in the Li0.5Bi1.5O2I sample;
1 There is a misprint in Ref. [3]: the cell parameters attributed to
CaBiO2I actually belong to CdBiO2I [2].

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318
Table 1
Lattice parameters for the X1 bismuth oxyhalides
(a) Structure type Nd2O2Te—SG I4=mmm; Z ¼ 2
Compound
a (A)
c (A)
Compound
a (A)
c (A)
Li0.5Bi1.5O2Cl
Li0.5Bi1.5O2Br
Li0.5Bi1.5O2Ia
Na0.5Bi1.5O2Cl
3.8457(5)
3.8789(5)
3.935(2)
3.880(1)
12.060(3)
12.486(2)
13.236(4)
12.136(4)
Na0.5Bi1.5O2Br
Na0.5Bi1.5O2I
CaBiO2Br
3.9205(3)
3.9987(5)
3.9617(5)
4.0222(2)
12.542(2)
13.322(3)
12.584(3)
13.239(1)
CaBiO2I
(b) Structure type PbSbO2Cl—SG Cmcm, Z ¼ 4
Compound
a (A)
b (A)
c (A)
SrBiO2Br
SrBiO2I
5.768(2)
5.776(3)
5.968(2)
6.052(2)
12.806(4)
13.509(8)
13.162(6)
14.031(6)
5.645(2)
5.863(2)
5.748(2)
5.825(3)
BaBiO2Br
BaBiO2I
a BiOI is present as an impurity phase.
the composition of the target phase was therefore
attributed by analogy with the isostructural compounds.
Despite numerous efforts, CdBiO2Br and CdBiO2I
could not be prepared single-phase; admixtures of X3-
type phases (which seem to be more thermally stable
compared to X1) were always present. However, powder
patterns of X1-richest samples indicated truly tetragonal
symmetry akin to the corresponding calcium com-
pounds (no superstructure reflections or splitting of
the strongest peaks characteristic for ordered structures
have been detected).
The samples aimed at SnBiO2X appeared black, and
X-ray diffraction data indicated presence of SnO2,
Bi2Sn2O7 and Bi which means a redox reaction had
actually proceeded. The search for K0.5Bi1.5O2X yielded
mixtures of KX and solid solutions based on Bi24O31X10
(X=Cl, Br) or Bi7O9I3. Diffraction patterns of samples
with Cu and Ag yielded mixtures of unknown phases
with X=Cl and I. For X=Br, the patterns appeared
similar to each other and could be, with moderate
success, indexed on the basis of I-centered tetragonal
cells with aB3.9 A and cB22 A indicating the existence
of a novel, possibly layered bismuth oxyhalide family.
Our attempts to prepare tetragonal LIIBiO2.125X0.75
with LII=Sr and Ba by analogy with PbSbO2.125X0.75
also proved unsuccessful, resulting in mixtures of
orthorhombic LIIBiO2X and unknown phases.
excluded from the refinement. Atomic coordinates of
tetragonal PbBiO2I [6] and orthorhombic BaBiO2Cl [8]
served as initial parameters. The GSAS package soft-
ware [23] was used for all calculations. Two typical
Rietveld refinement plots, one for tetragonal CaBiO2I
and the other for orthorhombic SrBiO2I, are presented
in Figs. 2a and b. The atomic coordinates and selected
bond distances are given in Tables 3 and 4, respectively.
There are just minor differences in bond distances
between isotypic CaBiO2I and PbBiO2I [6]. The (L,Bi)–
O bond is 0.08 A shorter but (L,Bi)–I distance 0.02 A
longer in CaBiO2I compared to PbBiO2I. It is curious
to note that in the orthorhombic SrBiO2Cl–SrBiO2Br–
SrBiO2I series, the Sr–X, Bi–X, and Bi–O distances
elongate as expected but not Sr–O which shortens
slightly when passing from X=Cl to Br and I. This
shortening is even more pronounced comparing the
structures of BaBiO2Cl and BaBiO2I (0.11 A). When
passing from SrBiO2I to BaBiO2I, both Bi–O and Bi–I
distances elongate (by B0.04 and B0.15 A, respec-
tively). The latter value, together with the Bi–I distances
themselves indicates very little or no bonding between
Bi and I, at least, in BaBiO2I.
5. Discussion
Our investigations have added several new represen-
tatives to the X1 bismuth oxyhalide family. Besides
L=Pb, these structures form only with L=Li, Na, Ca,
Cd, Sr and Ba, in fair agreement with Sillen’s results
[2,24]. These cations readily adopt the same coordina-
tion environment as Bi3+, and difference between their
radii and that of Bi3+ does not exceed 0.25 A [25]. If
the difference in ionic radii is larger or the coordination
preferences are different, the X1 structure is unlikely to
occur, which is the case for L=K, Cu, or Ag. Sn2+
meets both these requirements but is perhaps a too
4. Crystal structures
Structural details have been determined for tetragonal
CaBiO2I and orthorhombic SrBiO2Br, SrBiO2I, and
BaBiO2I. The experimental conditions are gathered
in Table 2. The powder data sets were collected on a
Stoe STADI/P diffractometer using monochromatized
CuKa1 radiation. Some samples were attached to the
holder with vaseline oil; in such cases, two small 2W
regions containing the strongest oil reflections have been

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319
Table 2
Experimental conditions for the powder diffraction experiments
Compound
CaBiO2I
SrBiO2Br
408.5
SrBiO2I
BaBiO2I
Formula weight
Crystal system
Space group
Cell parameters:
a (A)
408.0
Tetragonal
I4/mmm (# 139)
455.5
Orthorhombic
Cmcm (# 63)
505.1
4.01943(4)
4.01943(4)
13.2144(1)
213.490(4)
2
5.7676(1)
12.8246(4)
5.6460(1)
417.62(2)
5.8723(1)
13.5344(2)
5.7363(1)
455.91(2)
4
6.0492(1)
14.0224(2)
5.8212(1)
493.78(1)
b (A)
c (A)
V (A3)
Z
Calculated density
Diffractometer
Radiation
6.346
6.497
6.636
6.796
STOE STADI/P
CuKa1 (l ¼ 1:54059 A)
2W range (deg)
Number of reflections
Number of data points
Number of free parameters
Analyzing package
RðF2Þ
5–100
48
12–120
198
5338a
27
5–100
151
10–100
199
9499
18
9499
23
9919
25
GSAS
0.0952
0.1011
0.1376
2.29
0.0748
0.0880
0.1200
2.61
0.0858
0.0994
0.1349
2.26
0.0930
0.0545
0.0770
6.75
Rp
Rwp
w2
a The 2W step size is 0.02ꢂ, for others 0.01ꢂ.
strong reducing agent to neighbor Bi3+ in the same
structure.
The occurrence and type of cation ordering in the
X1-type bismuth (and perhaps antimony) oxyhalides
depends on at least three parameters: the overall
stoichiometry of the fluorite layers, the relative size
of the ions contributing to the structure, and their
electronic structure. The ordering occurs only in the case
of a L:Bi (or L:Sb) ratio of 1:1. As soon as
the interlayer charge balance requires non-integer
stoichiometry of the fluorite layers (0.5:1.5 in case of
LI), only the ideal Nd2O2Te structure is realized. This is
consistent with structural data for bismuth oxyhalides
with related structures, where Bi3+ in the [Bi2ꢁxLxO2]
layers is substituted by lower-valent L cations so that
x is non-integer: the overwhelming majority of these
compounds crystallize in the highest-symmetrical tetra-
gonal structures [20,25–27].
The [LMO2]X stoichiometry of the X1 compounds
(L=Ca, Cd, Sr, Pb, Ba; M=Sb, Bi) may actually lead
to all three structures shown in Fig. 1. The ordered
PbSbO2Cl structure seems to be realized when the L2+
cations are noticeably larger than M3+, and the
Xꢁ anion is, on the contrary not much larger than
both L2+ and M3+, i.e., chloride. If 1=2ðRðL2þÞ þ
RðM3þÞÞ{RðXꢁÞ; the disordered Nd2O2Te structure
becomes more favorable, which is best demonstrated
by the PbSbO2X and CaBiO2X series, where both
oxyiodides and CaBiO2Br adopt the Nd2O2Te structure.
The other, as yet very rare, version of cation-ordered
X1 structure, adopted hitherto only by CaBiO2Cl and
CdBiO2Cl, was assumed to occur due to the small size
Fig. 2. Final Rietveld plots for (a) CaBiO2I, (b) SrBiO2I.

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320
Table 3
Atomic coordinates for the new bismuth oxyhalides
(a) Nd2O2Te type (CaBiO2I)
Atom
Site
z
U ꢄ 100
Ca0.5Bi0.5
4e (0,0,z)
4d (0,1/2,1/4)
2a (0,0,0)
0.3290(2)
2.0(1)
2.6(4)
3.0(1)
O
I
—
—
(b) PbSbO2Cl type
Compound
SrBiO2Br
SrBiO2I
BaBiO2I
Atom
Site
xðyÞ
U ꢄ 100
xðyÞ
U ꢄ 100
xðyÞ
U ꢄ 100
L
4c (0,y;1/4)
4c (0,y;1/4)
8e (x;0,0)
0.3943(4)
0.0796(2)
0.247(2)
0.9(1)
1.8(1)
2.0(6)
2.3(2)
0.4050(3)
0.0739(2)
0.247(1)
1.8(1)
2.3(1)
2.6(4)
3.1(1)
0.3971(2)
0.0689(2)
0.251(2)
1.8(1)
1.6(1)
3.2(4)
2.6(1)
Bi
O
X
4c (0,y;1/4)
0.7456(4)
0.7445(2)
0.7480(2)
Table 4
Selected bond distances for the new bismuth oxyhalides
others are disordered; this most likely occurs due to
the same configuration of Pb2+ and Bi3+ (6s2). It would
be interesting to compare the PbSbO2X with their
strontium analogs, as yet unknown. A systematic study
of related antimony (and possible arsenic) oxyhalides
could give a deeper insight into crystal chemistry of X1
type oxyhalides. Some of these investigations are now
in progress.
Compound CaBiO2I SrBiO2Br SrBiO2I
BaBiO2I
L–O
L–X
2.265(1) ꢄ 4 2.442(9) ꢄ 4 2.432(5) ꢄ 4 2.543(6) ꢄ 4
3.631(2) ꢄ 4 3.345(4) ꢄ 2 3.511(3) ꢄ 2 3.552(3) ꢄ 2
3.458(4) ꢄ 2 3.652(3) ꢄ 2 3.677(2) ꢄ 2
Bi–O
Bi–X
2.265(1) ꢄ 4 2.249(9) ꢄ 4 2.272(5) ꢄ 4 2.315(6) ꢄ 4
3.631(2) ꢄ 4 3.605(3) ꢄ 2 3.735(2) ꢄ 2 3.881(2) ꢄ 2
3.584(3) ꢄ 2 3.776(2) ꢄ 2 3.931(3) ꢄ 2
Acknowledgments
This work was carried out as a part of Russian
Foundation of Basic Researches Project No. 01-03-
38006a.
of Ca2+, which is only seven-coordinated, contrary to
eight-fold coordination in both other X1 structures [9],
coupled with the smaller size of Cl, leading to only
six-coordination of the anion. In contrast, Ca2+ is
eight-coordinated both in CaBiO2Br and in CaBiO2I
involving larger halide ions.2
References
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