FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 80

that the ambient pressure phase thenardite V-Na 2 [SO 4 ] undergoes, by increasing
temperature, the following transitions sequence [ 3 ]:

Thenardite V ð Na 2 SO 4 Þð F ddd Þ! III ð C mcm Þ! II ð P nma Þ

! I ð P 6 3 = mmc Þ

When these structures were described on the basis of their cation arrays [ 6 , 7 ] ), it
was thought that the three phases could be contemplated as slight variants of the most
symmetrical I-Na 2 [SO 4 ]( P 6 3 /mmc) whose cation subarray (Na 2 S) is of the Ni 2 In

type (also 6 3 /mmc). This can be easily understood by looking at Fig. 10b, e, h .

Recall that in the O'Keeffe and Hyde's approach [ 5 ], the cation array of olivine,
Mg 2 [SiO 4 ], isostructural to II-Na 2 SO 4 (

P

nma), was considered of the Ni 2 In type.
In connection with this relationship, it was shown later [ 4 ] that the two struc-
tures, III- and II-Na 2 [SO 4 ](

P

nma, respectively), should really regarded
as intermediate steps in a longer “structural transition” which ends in the Ni 2 In-type
structure (

C mcm and P

P 6 3 /mmc) formed by Na 2 S in the hexagonal I-Na 2 [SO 4 ]( P 6 3 /mmc).

Thus, FeLi[PO 4 ] and Na 2 [SO 4 ], with their two common phases ( C mcm and

P nma), follow, in part, a similar structural behaviour.

Recall that these features are in agreement with the topological analysis carried
out by Blatov et al. reported in this volume [ 9 ] .

4.1 FeLi[PO 4 ], C -Mg[SO 4 ] and C -Fe[SO 4 ] in the Light
of the EZKC

We must recall that the [Na 2 S] subarray in I-Na 2 [SO 4 ]( P 6 3 /mmc) was interpreted,

following the EZKC, as Na + [ -MgS] subarray was formed by

completely regular graphite-like layers like those represented in Fig. 10a [ 4 ]. The
reader is also referred to Fig. 3 to recall its relationship with the AlB 2 -type structure.
If we wish to deep in the application of the EZKC, Na 2 SO 4 can be formulated as
Na + [

C -MgS], where the [ C

C

-MgSO 4 ] if we admit that one Na atom transfers its valence electron to the
other one, converting it into pseudo-magnesium ( -Mg) and [NaSO 4 ] 1 into -Mg

[SO 4 ]. The consequence is that the structure of Mg[SO 4 ] itself should necessarily
appear in Na 2 [SO 4 ].
The impressive outcome is that Mg[SO 4 ], dimorphic, with a

C C

C mcm phase, stable

at ambient conditions, and a high temperature phase of nma symmetry [ 20 ] are just

the structures of both the III- and II-Na 2 [SO 4 ]phases(Fig. 10 ). The correlation is
much more far reaching, i.e. in the pseudo-structure

P

-Mg[SO 4 ]aswellasinrealMg
[SO 4 ], the [MgS] subarray is a slightly distorted NiAs-type structure, reproducing at
the same time, the NiAs structure of a metastable phase of MgS, obtained in thin films
[ 35 ] . Recall that bulky MgS crystals are of the NaCl-type, a structure that is related to
the NiAs-type. Thus, MgS, Mg[SO 4 ]and

C

-Mg[SO 4 ] all have a NiAs-type arrange-
ment as clearly shown in Fig. 10c, f, i, p .InFig. 10p , the structure of Fe[SO 4 ],

C