Chemistry Reference
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aspects of the structures, like the octahedral coordination of both Li and Fe atoms and
the tetrahedral coordination of the P atoms, remain unaltered in the HP phase and
provide new examples of compounds, which undergo the olivine
spinel transition,
where the coordination number of cations does not increase with pressure.
As it is well known, these coordination features, together with the interatomic
distances, defining the cation-centred coordination polyhedra, are the unique struc-
tural data commonly used to study, describe and even “understand” crystal struc-
tures. It should be outlined that in many instances, the term “description” is
inadequately used as an equivalent of “understanding”. Following this tendency,
the olivine-like structures are described as distorted
hcp
arrays of O atoms, with one
half of the octahedral voids occupied by the big cations (e.g. Mg, Li, Fe) and one
fourth of the tetrahedral voids occupied by the small cations (Si, P, B, etc.). In the
case of ternary oxides, like Mg
2
[SiO
4
] (forsterite,
!
nma), all the Mg atoms occupy
octahedral voids but are splitted into two distinct Wyckoff positions. The Si atoms
in forsterite, also located at 4
c
, occupy tetrahedral holes.
When triphylite, LiFe[PO
4
], is compared with forsterite (Mg
2
[SiO
4
]), one sees
that the Li and Fe atoms, both occupying octahedral voids, are separated in the two
Wyckoff positions 4
a
(0, 0, 0) and 4
c
(
x
,¼,
z
), respectively. This separation,
irrelevant in forsterite, becomes enigmatic in triphylite, as well as in other iso-
structural compounds such as CaMg[SiO
4
] and sinhalite AlMg[BO
4
]. If we assume
the tendency of both Li and Fe atoms to occupy octahedral voids, one might expect
a random distribution of both atomic species, indistinctly, in the two Wyckoff
positions, just as if they were the Mg atoms in Mg
2
[SiO
4
].
These features are indicative that the old crystal-chemical model, based mainly on
closed packed arrays of anions, where cations are contemplated as “isolated charged
spheres”, is unable to give a satisfactory explanation of the principles governing crystal
structures. It is also generally admitted that the derived rule, which attributes to the
ionic size effects, a main role in determining the structure, has failed in explaining even
most simple structures. The main conclusion is that the classical model can not account
for the separation of both types of cations (Li and Fe) in triphylite.
Alternative approaches accounting for unexpected structural features have
been reported by O'Keeffe and Hyde [
5
]. These authors realized that the oxygen
subarrays showed a great irregularity (the O-O distances in FeLiPO
4
range from
2.40 to 3.74
˚
), and also that, in a great amount of oxides, the cation arrays adopted
structures of either elements or simple alloys. In view of these coincidences, the
authors proposed to describe the structures as oxygen-stuffed alloys [
5
].
In the case of the olivine-like compounds (Mg
2
[SiO
4
] and FeLi[PO
4
]), the Mg
2
Si
(FeLiP) subarrays are in fact orthorhombic distortions of the hexagonal Ni
2
In type.
Ni
2
In type (
P
nma) which is rather
related to a stuffed NiAs-type structure. These features are in agreement with the
array in olivine, not as Ni
2
In type, but as a 4,5,9-
T
11 coordinated three-periodic net,
following the
P
6
3
/mmc), but it is an orthorhombic distortion (
P
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