Chemistry Reference
In-Depth Information
Fig. 2 The cation array of the
triphylite FeLi[PO
4
]
structure, after omitting the
O atoms. The FeLiP subarray
forms a Ni
2
In-like structure,
with the Li and Fe atoms
forming the trigonal prisms
which are centred by the PO
4
groups. Colour codes: Fe
(
green
), Li (
light grey
) and P
(
red-violet
)
a
b
Fig. 3 (a) Alternative description of a Ni
2
In type as a superstructure of the AlB
2
-type. The Li
atoms form the
hcp
3
6
layers whereas the P and Fe atoms compose the graphite-like layers. (b) The
same arrangement viewed along the
b
axis, almost perpendicular to the layers, to show the
planar nets, formed by the Ni(1) atoms, alternate with the graphite-like layers
formed by both Ni(2) and In atoms (Fig.
3a
). It should be outlined that in both
AlB
2
and Ni
2
In, the graphite-like layers (B and NiIn, respectively) are completely
regular, whereas in the olivine-like [FeLiP] subarray, the 6
3
planar nets show an
values of the three Fe-P contacts (3.18, 3.23 and 2.89
˚
).
If the atoms would be either C or Si atoms (Group 14 elements), this distortion
would lead to a loss of the aromatic character, consistent with the formation of one
double bond (2.89
˚
) and two single ones with distances of 3.18 and 3.23
˚
.It
should be recalled that this bonding scheme is the only way of maintaining the four
valences for each atom in a planar net. This argument, however, is difficult to
extrapolate to the mixed FeP nets, even if we consider them as a III-V compound,
like AlP for example, which normally adopt structures of the Group 14.
2.3 The FeLiP Subarray and NiAs·FeLi[PO
4
] and Li[ClO
4
]
A third description of the subarray derives from the NiAs-type structure. This
relationship was already used to rationalize the Ni
2
In structure in terms of the
