1254
CHARKIN et al.
Table 4. Bond lengths (Å) in the structures of
and [
Ꮽ
][
Ꮿ
], for example, PbBi3WO8Cl
PbBiO2Cl +
Pb0.4Bi3.1La0.5Nb0.6W0.4O8Cl
(2)
and
Bi2WO6. In this case, the latter act as complex modules
capable of forming structures of higher hierarchy.
When complex structures can decompose into simpler
ones, the criterion of the geometric compatibility of
layers (firstꢀorder modules) and/or their combinations
(secondꢀorder modules [7]) becomes very strict. For
example, PbBiO2Cl (unit cell parameter
reacts with Bi2WO6 (the tetragonal subcell parameter
a0 = 3.863 Å) to form PbBi3WO8Cl ( 0 = 3.894 , but
the reaction of PbBiO2Br ( = 3.988 with Bi2WO6
proceeds with very low yield. As expected, the reaction
of PbBiO2I ( = 4.053 with Bi2WO6 does not occur
Bi3.5La0.5Nb0.6W0.4Cs0.6O8Cl2 (3)
Bond
2
3 [14]
Bi1–O1
Bi1–Cl
Bi2–O1
Bi2–O2
Nb–O2
Nb–O3
×
×
×
×
×
×
4
4
4
4
2
4
2.228(4)
3.351(4)
2.409(5)
2.851(4)
1.93(1)
2.322(8)
3.175(4)
2.295(4)
2.91(3)
a = 3.956 Å)
a
Å)
1.69(3)
a
Å)
1.939(1)
1.926(1)
a
Å)
at all in the temperature range studied (750–800°C).
Conversely, if the chemical composition is such
that the decomposition into neutral fragments is
(3), which we studied earlier [14], with the same comꢀ
position of the perovskite layer and the same content
of the introduced La3+ (Table 3). As is seen, structure
impossible and any [Ꮽ][Ꮾ] or [Ꮽ][Ꮿ] sequence bears
3
is characterized by almost regular (Nb,W)O6 octaheꢀ
an electrostatic charge (for example, in Bi4NbO8Br
[Bi2O2]2+[NbO4]3–[Bi2O2]2+[Br]–), the structure is staꢀ
ble even at significant distortions of one or several layꢀ
ers. For example, in the Bi4Nb0.6W0.4Cs0.6O8Cl2 strucꢀ
ture in layers of the CsCl type, the CsCl8 eightꢀvertex
polyhedra are elongated from regular cubes with an
edge of 4.115 Å (as in CsCl per se) to square prisms
dra and the distances between the apical oxygen atoms
and Nb(W) and Bi(Pb) are within common ranges. It
is precisely these atoms in the parent orthorhombic
structures that most strongly deviate from the “ideal”
positions; therefore, in refinement of the oxohalide
structures in the framework of the tetragonal symmeꢀ
try, the positions of these atoms are determined with a
with the base length a0 = 3.876 Å. The introduction of
minimal accuracy. Structure
2 is noticeably closer to
Pb2+ into this structure makes possible its decomposiꢀ
tion into electroneutral fragments Bi2WO6 and
Pb0.6Bi1.4Cs0.6O2Cl2, the distortion of CsCl type layers in
ideal than [14]. It is pertinent to draw an analogy
3
with the related structure of Pb2Bi3Nb2O11Cl, in which
the fluorite layers also contain noticeable amounts of
Pb2+ [23]: despite the existence of noticeable second
harmonic generation of laser radiation and the maniꢀ
festation of the orthorhombic structures in electron
diffraction patterns, it has been possible to solve the
structure (according to neutron diffraction data) only
for a centrosymmetric tetragonal subcell. It is likely
that, in the structures of Aurivillius phases and their
more complicated derivatives, the activity of the Pb2+
lone pair is rather poorly pronounced.
the latter being somewhat smaller (a = 3.909 Å, Fig. 3).
The lack of solubility of Ln3+ in the PbBi3WO8Cl
structure cannot be explained by size mismatch
between Ln3+ ions and O8 antiprisms. In our opinion,
the reason is that the introduction of Ln3+ entails the
change in the Aurivillius part of the structure, as in
Bi2WO6, but the highꢀtemperature
α
ꢀ(Bi,Ln)2WO6
form stabilized by lanthanide ions is incompatible with
PbBiO2Cl in the lattice parameters (Fig. 4). At the
same time, the Pb0.4Bi3.6 – xLaxNb0.6W0.4O8Cl solid
solution is quite stable since its decomposition into
electroneutral fragments is hindered. In addition, it is
likely that the presence of NbV suppresses the formaꢀ
Compatibility Criteria in Multilayer Structures
As emphasized in reviews [7, 24] dealing with the
application of the modular approach to design of layꢀ
ered structures, the probability of the existence of new
compounds is estimated on the basis of four critetia:
electroneutrality and geometric and chemical comꢀ
patibility of layers; separate factors, which cannot be
generalized, constitute the fourth “specific” criterion.
Our studies show that, after some modification, these
criteria deduced for the simplest twoꢀlayer structures
are quite applicable to more complicate multilayer
structures.
tion of ꢀ(Bi,Ln)2WO6. This is indirectly confirmed by
α
negative results of pilot experiments to search for
(Bi,Ln)2 – xThxW1 – xNbxO6 solid solutions. In this case,
we are more likely dealing with the manifestation of
the chemical compatibility criterion.
In our experiments, the chemical compatibility criꢀ
terion is most clearly traced in all structures in the
absence of Ae2+ cations, especially Sr2+, which is close
in radius to Pb2+ and La3+. It is likely that, in systems
containing W (and, evidently, Mo), formation of alkaꢀ
line earth tungstates is thermodynamically most favorꢀ
able. An attempt to obtain CaBi3WO8Cl from secondꢀ
order modules CaBiO2Cl and Bi2WO6 led to the forꢀ
mation of a mixture of CaWO4, Bi24O31Cl10, and
A specific feature of complicated layered structures
of the [
Ꮽ][Ꮾ][Ꮽ][Ꮿ] type (in our case, [
Ꮽ] is the fluꢀ
orite layer, [
Ꮾ] is the perovskite layer, and [Ꮿ] is the
ꢀBi2O3 stabilized with WVI (and presumably Ca2+). At
the same time, in the systems that do not contain Mo
and W, formation of phases with structure in which
halide or metal halide layer) is that, for some variants
of chemical composition, the complicated structure
δ
can decompose into electroneutral fragments [Ꮽ][Ꮾ]
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010