Chemistry Reference
In-Depth Information
technique (3.5-4.5 GPa, 900-1050 C) was also successful in the preparation of the
cuprates belonging to the Ba
2
Ca
n
1
Cu
n
O
2n
(O,F)
2
(n
¼
2-5) homologues series [17-19].
However, at ambient pressure the high-temperature solid-state reaction does not lead to
fluorinated derivatives of the hole-doped complex cuprates, especially if the compound
contains alkali earth metal cations. For example, different reports on an increase of T
c
above 100 K for fluorinated YBa
2
Cu
3
O
y
F
x
(Y123), obtained by a solid-state reaction with
BaF
2
have never been reproduced. Moreover, thorough investigation of the products of
solid-state reaction aimed at fluorinated Y123 demonstrated that BaF
2
is always present in
the reaction mixture obtained by high temperature annealing at 800-950 C, independently
from the form in which fluorine was present in the initial reagents [20-23]. High-tem-
perature solid-state reaction cannot be used for the preparation of other HTSC cuprates,
such as Sr
2
CuO
2
F
2
, which decomposes at temperatures
480 C producing SrF
2
,Sr
2
CuO
3
and Sr
14
Cu
24
O
41
[24,25]. However, Sr
2
CuO
2
F
2
þ
can be prepared under much 'softer'
conditions by low-temperature treatment of the parent Sr
2
CuO
3
phase by various fluorinat-
ing agents: a mixture of 10 %F
2
and 90 %N
2
(15min, t
¼
210 C) [26], NH
4
F in a presence
of O
2
(6-8 h, t
¼
225
o
C) [25, 27, 28] or XeF
2
(40min-10h,t
¼
160 -350 C) [24].
The impossibility of using a high-temperature solid-state reaction for preparation
of fluorinated cuprates can be related to high lattice energies of their decomposition
products, namely fluorite-like fluorides of alkali-earth metals AF
2
and rare-earth oxy-
fluorides ROF. This can be reflected by high enthalpy of the reaction between Y123 and
gaseous fluorine:
YBa
2
Cu
3
O
6
þ
5
=
2F
2
!
YOF
þ
2BaF
2
þ
3CuO
þ
O
2
DH
o
¼
1518 kJ
Various fluorinating agents with either an oxidizing or a reducing character have been
used for low-temperature synthesis of complex copper oxyfluorides: gaseous fluorine
(pure [29-31] or diluted with other gases [26, 32-34]), NF
3
[35, 36], NH
4
F [31, 37-40],
NH
4
HF
2
[42-44], transition metal fluorides (ZnF
2
, CuF
2
, NiF
2
, AgF
2
[31, 45-49]), ClF
3
(here fluorine is incorporated along with chlorine) [50-53]. NH
4
F and NH
4
HF
2
are
nonoxidizing fluorinating agents, although they allow superconductors with formal copper
oxidation state
>þ
2 to be obtained in the presence of oxygen due to reactions [28]:
ð
2
þ Þ
NH
4
F
þð
3
þ
2
Þ=
2O
2
þ
Sr
2
CuO
3
!
Sr
2
CuO
2
F
2
þ
þð
2
þ Þ=
2N
2
þ
2
ð
2
þ Þ
H
2
O
ð
2
þ Þ
NH
4
F
þ =
4O
2
þ
Sr
2
CuO
3
!
Sr
2
CuO
2
F
2
þ
þð
2
þ Þ=
2H
2
O
þð
2
þ Þ
NH
3
:
Moreover, a mechanism was proposed that allows NH
4
F to be an oxidizing fluorinating
agent even in an oxygen-free atmosphere. The reaction of the parent complex oxide with
NH
4
FproducesH
2
O as by-product. Releasedwater then decomposes the obtained fluorinated
oxide due to pyrohydrolysis. It is assumed that pyrohydrolysis leads to amorphous com-
pounds containing mostly Cu
þ
, so that the increase of formal copper valence in the super-
conducting oxyfluoride is explained by disproportionation: Cu(II) (initial oxide)
!
Cu(I)
(pyrohydrolysis products)
þ
Cu(II
þ
III) (superconducting oxyfluoride) [27]. The
target material is significantly contaminated by pyrohydrolysis products. Fluorides of metals,
which do not form thermally stable oxides, such as AgF
2
, can serve as oxidizing fluorinating
