Chemistry Reference
In-Depth Information
Ca results in the Sr
2
Nd
0.2
Ca
0.8
Cu
2
O
5
F compound with T
c
¼
85 K. It is assumed that in
these phases the fluorine anions are located in the NaCl-type blocks, forming the
(SrF)(FSr) or (SrO
0.5
F
0.5
)(F
0.5
O
0.5
Sr) layers.
The effect of fluorination on the structure and properties of the T'-phases was most
comprehensively studied with fluorine-containing derivatives of Nd
2
CuO
4
but the majority
of studies deal with the Nd
2
CuO
4
x
F
x
, where n-type superconductivity was found with
T
c
¼
27K for x
¼
0.3 [9]. As was discussed in Section 13.3.1, a replacement of oxygen with
an equivalent amount of fluorine in the fluorite-type Nd
2
O
2
slabs does not significantly alter
the crystal structure. Insertion of fluorine into the Nd
2
CuO
4
structure results in a new
compound with the idealized Nd
2
CuO
3
F
2
composition [131]. This compound was found
using transmission electron microscopy in the samples of Nd
2
CuO
4
, fluorinated with XeF
2
at
300-400 C. In the monoclinic structure of Nd
2
CuO
3
F
2
(a
12.96
˚
, b
5.5
˚
, c
5.8
˚
,
92) the anions in the tetrahedral interstices are arranged into chains running along the
c-axis and alternating along the b-axis with the chains of vacant interstices (Figure 13.15).
Some of the anions occupy the apical position in the coordination environment of the Cu
cations. A formation of the octahedral coordination environment for the Cu cations results in
an increase of the c-parameter from 12.167
˚
in the initial material to 12.96
˚
in the
fluorinated phase. The Cu(O,F)
6
octahedra are cooperatively tilted to maximize the distances
between apical anions and anions in the tetrahedral interstices. The copper oxidation state in
the fluorinated samples was not high enough to achieve p-type superconductivity.
a
c
b
Nd
O, F
Figure 13.15 The crystal structure of Nd
2
CuO
3
F
2
. The Cu cations are situated in octahedra.
The vacant tetrahedral interstices are shown as empty squares
A first stage of fluorination with XeF
2
of Nd
2
CuO
3.5
, an anion-deficient derivative of
Nd
2
CuO
4
, results in a superconductor with T
c
¼
6 - 11 K [132]. Nd
2
CuO
3.5
consists of
fluorite-type Nd
2
O
2
slabs and (CuO
1.5
N
0.5
) layers with Cu atoms in dumbbell and strongly
distorted square-planar coordinations [133], so that there are no conducting (CuO
2
) planes
in this structure (Figure 13.16). Fluorination at 200 C leads to an increase of the formal
