FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 82

m n o

Three aspects of triphylite FeLi[PO 4 ] ( P nma)

p

The structure of FeSO 4 ( C mcm )

Fig. 10 Evolution of the structures of both Na 2 [SO 4 ] and FeLi[PO 4 ], all of them considered
as olivine-like compounds. The first row shows three aspects of the structure of I-Na 2 [SO 4 ](P6 3 /
mmc). In (a) the regular 6 3 layers are represented, formed by both S and Na atoms. The O atoms
show clearly the rotational disorder. In (b) the structure is projected on the (110) plane to show the
linear [NaS] blocks, with an S-Na-S angle of 180 .In(c) the Na-stuffed NiAs-type structure is
drawn. The second row contains the similar drawings for II-Na 2 [SO 4 ]( nma). Note the small

distortion of the graphite-like layers in (d), The lack of linearity in the [Na-S] blocks (angle
of 174 )(e) and the rather regular NiAs-type structure in (f). The third arrow corresponds to
III-Na 2 [SO 4 ](

P

mcm). Note the greater distortion of the hexagonal layers in (g) as well as the
lower value of the S-Na-S angle (170 )in(h). The NiAs-type arrangement is also rather
regular (i). The fourth arrow contains the same views of the high pressure

C

-FeLiPO 4 . Note the
irregularity of the [LiP] hexagons and the almost linear [FeP] blocks represented in (j) and (k),
respectively. These distortions make that this HP phase(

b

mcm) should be better conceived as a
Li-stuffed NiAs-type structure as drawn in (l). The fifth arrow collects the three views of the
triphylite FeLi[PO 4 ](

C

nma, olivine-like). The irregular [FeP] graphene-like layers are depicted
in (m). In (n), the [FeP] blocks of the PbO type are clearly outlined. The great distortion of the
Fe-stuffed NiAs-type structure represented in (o) clearly indicates that this phase should be
contemplated as a Li-stuffed FeP array [ C -FeS] of the PbO type, as shown in Fig. 8 .In(p),
the NiAs-type structure of the two phases of Mg[SO 4 ] to be compared with the corresponding
Na +

P

C -Mg[SO 4 ] structures drawn in (c), (f) and (i)

isostructural to Mg[SO 4 ], has been represented as a reference for the C -Mg[SO 4 ]

shown in Fig. 10c, f, i .
As already discussed, the hexagonal I-Na 2 [SO 4 ]( P 6 3 /mmc) is obtained at higher

temperatures, leading to a -[MgS] subarray which forms the graphite-like layers

depicted in Fig. 10a . These 6 3 nets might be regarded as a pseudo-[(II)-(VI)]
compound which are forming a network characteristic of a (IV-IV) structure, like

C