Chemistry Reference
In-Depth Information
A
CuO
2
(R,A)
2
O
2
CuO
2
A
n
=
2
n
=
3
Figure 13.14 The crystal structures of the n¼2 and n¼3 (R,A)
nþ1
Cu
n
O
2nþ2þ
Ruddlesden-
Popper type compounds
of the n
¼
2 members under fluorination differs from behaviour of La
2
CuO
4
. Treatment of
La
1.9
Sr
1.1
Cu
2
O
6.05
and La
1.9
Ca
1.1
Cu
2
O
5.95
was done with F
2
,NH
4
F and CuF
2
[31]. The
reaction with F
2
proceeds via fluorine insertion, while reaction with NH
4
F involves
fluorine substitution for oxygen. The reaction with CuF
2
at low temperature probably
occurs as both insertion and substitution, while at higher temperatures (
>
300 C)
the substitution process dominates. The fluorination results first in increasing a and c
parameters of the tetragonal unit cell. At this stage fluorine is supposed to enter into
tetrahedral interstices of the NaCl-like blocks. A larger degree of fluorination introduces
orthorhombic distortion into the Sr-containing structure with decreasing lattice parameters
in the ab plane and drastic increase of the c parameter from 20.048
˚
to 21.68
˚
. Such an
increase is related to the occupation of vacant anion positions in the (A
N
) layers by fluorine
atoms and formation of octahedral coordination for at least a part of the Cu atoms with
elongated Cu - (O,F) apical bonds due to Jahn-Teller effect. Such effect was not observed
in fluorinated La
1.9
Ca
1.1
Cu
2
O
5.95
, probably because of a smaller distance between the
(CuO
2
) planes due to smaller size of the Ca cations, which creates sterical difficulties
for fluorine insertion into the (A
N
) layers. Both fluorinated La
1.9
Sr
1.1
Cu
2
O
6.05
and
La
1.9
Ca
1.1
Cu
2
O
5.95
are not superconducting.
Superconducting n
¼
2-5 members of the Ruddlesden-Popper series were prepared
using the high-pressure, high-temperature technique. The Sr
2
CaCu
2
O
4.6
F
2
and
Sr
2
Ca
2
Cu
3
O
6.2
F
3.2
samples [15,16] demonstrate bulk superconductivity with T
c
¼
99 K
and 111 K, respectively, although they contain significant amount of admixture phases.
For higher members of this homologous series Sr
2
Ca
3
Cu
4
O
8
þ
F
2
and Sr
2
Ca
4
Cu
5
O
10
þ
F
2
T
c
was lower (40-95 K) [130]. For the Ba-based series Ba
2
Ca
n
1
Cu
n
O
2n
(O,F)
2
the highest
T
c
for n
¼
2, 3, 4 and 5 were 90, 120, 105 and 90 K, respectively [17,18]. The Sr
2
RCu
2
O
5
F
(R
¼
Y, La, Nd, Sm, Gd, Dy, Er and Yb) compounds are obtained under pressure and are
nonsuperconducting because of V
Cu
¼þ
2 [14]. Heterovalent replacement of Nd by
