Chemistry Reference
In-Depth Information
are displaced from their ideal positions, most probably due to their interaction with
interstitial anions [136]. In the LaHo
0.75
Sr
0.25
CuO
3.9
R
*-phase, fluorinated with XeF
2
,
inhomogeneous fluorine distribution results in two co-existing phases [137]. In the orthor-
hombic phase with relatively low fluorine content the structure of the fluorite-type blocks
is not affected, whereas the NaCl-type blocks acquire a structure similar to that in
La
2
CuO
3.6
F
0.8
. The phase with a larger fluorine content has a monoclinic unit cell with
a
m
b
m
5.5
˚
a
t
p
2, c
m
12.84
˚
c
t
,
91.4 (
t
belongs to the tetragonal subcell).
In this phase, as in the case of the T'-phase, larger c
m
value in comparison with
c
t
¼
12.553
˚
reflects that extra anions partially occupy the apical positions of copper
coordination environment. The anions are equally distributed among the tetrahedral and
octahedral interstices of the initially different (La
0.75
Sr
0.25
O)(OLa
0.75
Sr
0.25
) (NaCl-type)
and (La
0.25
Ho
0.75
)O
2
(La
0.25
Ho
0.75
) (fluorite-type) blocks. Thus, at higher fluorination
degree, the structural changes occur in both types of blocks. Vacant and occupied tetra-
hedral sites in these blocks alternate in a 'chess-board' manner. The monoclinic distortion
arises from cooperative displacement of the R- and A-cations due to interaction with
ordered anions in the tetrahedral interstices.
From the results of structural investigations on the T, T' and T* phases several common
trends can be derived [138]. Insertion of fluorine atoms does not directly affect the (CuO
2
)
planes, but it can cause distortions due to cooperative tilting of the Cu coordination
polyhedra. Fluorine atoms enter into the (R,A)
2
O
2
blocks. Fluorine enters more easily
into the NaCl-type blocks, whereas fluorination causes substantial structural changes in the
fluorite-type blocks. At a high degree of fluorination the anion rearrangement occurs in
both blocks resulting in the structures where short fragments of the NaCl and fluorite
structures alternate along the direction perpendicular to the (CuO
2
) planes, as well as in the
plane parallel to the (CuO
2
) planes. Such structures can be considered as intermediate
between the structures of the T and T' phases and are to some degree similar to the T*
structures. Completing copper coordination environment to an octahedron is at the origin
of the significant increase of the c-parameter in the fluorinated T' and T* phases.
In the (R,A)
2
O
2
blocks, where R and A are the rare-earth and alkali-earth cations,
respectively, the bonding between the cations and neighbouring anions is mostly ionic.
Thus, in the NaCl-type (R,A)
2
O
2
blocks the bonds within the ((R,A)O) layers are as strong
as the bonds linking the neighbouring ((R,A)O) layers. The NaCl-like Bi
2
O
2
blocks are
also present in the structures of Bi-based layered cuprates Bi
2
Sr
2
Ca
n
1
Cu
n
O
2n
þ
4
þ
.
Structural modulations are intrinsic in the (BiO) layers, associated with a mismatch
between the (BiO) layers and perovskite-like fragment. This mismatch is reduced via
strong shifts of atoms from their ideal sites and insertion of additional oxygen anions in the
bismuth-containing layers [139]. In contrast to ionic type of interactions in the (R,A)
2
O
2
blocks, only weak Van der Waals interaction occurs between two adjacent (BiO) layers.
Due to a weak link between the (BiO) layers, different halogen-containing species can be
intercalated into the Bi-based layered cuprates (I
2
,Br
2
, HgBr
2
, LiI
3
) [140-144]. The iodine
atoms in the n
¼
2 Bi2212 and n
¼
3 Bi2223 structures form an intermediate layer between
the (BiO) and (OBi) layers, resulting in significantly increasing separation between these
layers. The structural modulations of the (BiO) layers remain because weak interaction
with the iodine atoms does not significantly affect the atomic arrangement in these layers.
On the other hand, the Bi-F bond is more ionic in comparison with other halogens and
fluorine insertion can result in substantial changes in the structure of the (BiO) layers. In
