FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 69

in some compounds such as the hexagonal I-Na 2 SO 4 phase and the HT-Ca 2 [SiO 4 ]
(also 6 3 /mmc). The reader can find additional information in [ 9 ] .

At this point, it is worth mentioning that the O'Keeffe and Hyde's approach had
only a descriptive purpose and that, when applied, new questions arise, i.e. why is
the Ni 2 In-type structure that precisely adopted by the cation arrays of olivines? Of
course, this is a new way of formulating the question asked above, that is, why
cations occupy these special voids? Attempts for answering these questions have
been advanced in the case of Na 2 [SO 4 ][ 4 , 10 , 11 ].
The findings of O'Keeffe and Hyde [ 5 ] were, in fact, a generalization of an
unexpected structural relationship, found earlier by Wondratschek, Merker and
Schubert [ 12 ] , between the cation array [Ca 5 P 3 ] of fluoroapatite (Ca 5 P 3 O 12 F) and
the Mn 5 Si 3 alloy. At that time, this similarity was considered as an important
matter, as deduced from the authors words (translated from German): “It is proba-
ble that this is not a casual coincidence but the strong analogies must obey a General
Principle”. Unfortunately, this important observation was ignored by inorganic
chemists during more than 20 years.
Several articles have been published in the last few years aiming to deep in the
understanding of this phenomenon [ 4 , 11 , 13 - 15 ] . In most cases, the structural
similarities have been established from direct observation of the structures. For this
reason, in recent papers, Blatov and co-workers [ 6 - 9 ] have provided a great number
of examples substantiating the model of cation arrays by means of automatized
computer crystallochemical analysis that allows for a systematic crystal-chemical
analysis of complete sets of structures. This computer-aided comparison has given
support to most of the already established relationships between alloys and cation
arrays, leading so to many other so far unknown relationships. Even if they provide
new insights of great interest in crystal chemistry, it should be emphasized that the
relationships remain at a topological level.
Our idea, already expressed in earlier works [ 4 , 14 , 15 ] , is that any topological
relationship should be contemplated as the result of chemical interactions and
should be explained in chemical terms. So, our aim in this work is to do a profound
“dissection” of the two structures of FeLi[PO 4 ] in the light of the extended
Zintl-Klemm concept (EZKC). We intend to find new structural correlations
which can help us to answer the two crucial questions quoted above, that is, why
the Li and Fe atoms separate in triphylite and to account for the exchange of
positions of the Li and Fe atoms in the high pressure

P

-phase. These kind of
questions are “rarely asked” in crystallographic publications and, as far as we
know, have ever been satisfactorily answered.

b

2 Description of the Triphylite FeLi[PO 4 ] Structure

As said above, the cation array of triphylite Fe[LiPO 4 ] (olivine-like) is an ortho-
rhombic distortion of the hexagonal Ni 2 In-type structure. The complete structure is
represented in Fig. 1 where it is shown how the octahedral cations (Fe and Li) are