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along “c” of the lanthanum coordination polyhedra, which adopts the 10-fold
(30
1F) coordination, similar to that of Ba
3
La
2
(CO
3
)
5
F
2
, BaSm(CO
3
)
2
F, and
Ba
2
Ce(CO
3
)
2
F, for La or rare earth ions. The structure exhibits a 2d character with
infinite (LaO
6
)
2
F; double layers parallel to the (a,b) plane are separated by Na
1
ions.
The mineral-type structures obtained are PrCO
3
F (bastnaesite type), Ba
2
Ce
(CO
3
)
3
F (zhonghuacerite), BaSm(CO
3
)
2
F (huanghoite type), Ba
2
La
2
(CO
3
)
5
F
2
(cebaite type), and several new structures like Na
3
La
2
(CO
3
)
4
F exhibiting interesting
physical properties.
The 3d transitional metal fluorocarbonates could be synthesized at a slightly
lower temperature than the rare earth fluorocarbonates. The typical experimental
conditions are T
60
1
1
700
C and P
200 MPa. Several new compounds like BaMn
(CO
3
)F
2
, BaCu(CO
3
)F
2
, and KCu(CO
3
)F have been obtained. Under hydrothermal
conditions, the association of alkaline carbonates with 3d transition metal fluorides
leads, in most cases, to anionic exchange and to the formation of 3d transition
metal carbonates and alkaline fluorides. However, when the starting materials are
K
2
CO
3
and CuF
2
in 1:1 ratio, green crystals of KCu(CO
3
)F which can reach 4 mm
3
are obtained. Thus, the fluorocarbonates are being studied extensively, especially
by the French group (Leblanc and coworkers [18])[26
5
5
32].
The hydrothermal synthesis of fluorophosphates is slowly attracting the attention
of materials scientists because the magnetic effects have been investigated in fluorides
which present triangular cationic planes or K atom, lattices and where the coordination
MF6 polyhedra are mainly connected by vertices, and superexchange interactions
occur through 180
type M
M bridges, which frequently lead to antiferromagnet-
ism. Several phosphate minerals adopt the general formula M
2
PO
4
X, where M
2
1
5
F
3d
cation, Mg
2
1
,Ca
2
1
and X
2
5
F
2
,OH
2
; an arsenate or vanadate series also exist. The
occurrence of the triploidite substructure is said to be related to the ordered F
OH
substitution on the anionic sublattice. The coordination of the M
2
1
cations is octahe-
dral; in triploidite, the evolution of the M
F distances implies that half of the cations
M
2
1
adopt a fivefold coordination, consequently, the magnetic superexchange path-
way in triploidite results from that of
triplite by breaking one of
the three
M
2
1
M
2
1
magnetic bridges and strengthening the others
[33]
.
The majority of these fluorophosphates, particularly the oxyfluorinated prepared
under hydrothermal conditions, have been quite popular in recent years as micropo-
rous compounds belonging to the systems AF
F
2
H
1
,
a
M
2
O
3
a
P
2
O
5
a
amine (A
5
Al, Ga, 3d transition metal). The experiments are usually carried out at 217
C
under autogenous pressure, for 36
M
5
40 h. These compounds obtained under hydro-
thermal conditions show interesting crystal structures which are again difficult to
obtain by other conventional methods.
8.5 Oxyfluorinated Compounds
Oxyfluorinated compounds are being studied extensively in recent years owing
to
their
interesting
physical
properties. Several
phases
in
the
systems
