Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 164

Fe 2 P-type transition involves a contraction from 63.1 ˚ 3
to 58.8 ˚ 3 per formula unit.
Our final comment will be devoted to remark the similarities between the Ba 3 Cl 3
subarray of BaCl 2 (Fig. 14e ), on the one hand, and the NiIn (CoSn-type) structure
(Fig. 14f ), on the other hand. The drawings evince that the BaCl subarray is related
to the NiIn structure in the same manner that cotunnite is related to Ni 2 In. Recall the
additional cotunnite

the high-pressure Co 2 Si !

! Ni 2 In transitions of Na 2 SatHP[ 7 ] (Fig. 1 ) and the d ! y

transition, reported for Ni 2 Si at HT [ 73 ] .

6.5 Oxides Derived from the Fe 2 P Structure: LiNaSO 4
and LiCaPO 4

In the previous sections, we have dealt with the structures of binary alloys of the
Fe 2 P type, but this skeleton also exists in the cation arrays of some oxides, such as
LiNaSO 4 [ 74 ] a nd LiCaPO 4 [ 75 ]. These oxides crystallize in the S.G.

P 3 1 c, a

subgroup of 6m2 (Fe 2 P) by doubling the c axis. The similarities between the

LiNaS subarray (LiCaP) and the structure of Fe 2 P are reflected in Fig. 19a, b .
LiNaSO 4 , above 750 K, transforms into a cubic phase, probably a filled blende-type
structure of which only the unit cell parameters are known [ 76 ]. A similar transition
has not been reported, however, for LiCaPO 4 .
If the Fe 2 P-type structure is close in energy to that of the PbCl 2 type, the former
might well be an alternative intermediate in the PbCl 2 ! CaF 2 transition, fitting
therefore into the general trend of the journey discussed in this chapter. Thus, in the
same manner that Ag 2 S undergoes a Co 2 Si

P

! CaF 2 transition [ 77 ] (see Scheme 1 ) ,

a similar Fe 2 P CaF 2 transition could also take place.

In this context, it should be expected that, by increasing temperature, the oxides
under discussion, i.e. LiNaSO 4 and LiCaPO 4 , could stabilize the lower pressure
phases, i.e. fluorite (or stuffed blende). It cannot b e discarded that the HT phase of
LiNaSO 4 could also adopt the cubic structure (

!

43m) of CsLiMoO 4 [ 78 ], also a
cubic phase in which the Na atoms are located at (1/2, 1/2, 1/2) forming with the

P

a b c

Fig. 19 (a) The structure of Fe 2 P projected on the ab plane. Green and brown spheres represent
Fe atoms. Purple spheres are P atoms. Both, Fe(1) and P atoms are connected by red lines .(b) The
similar structure formed by the cations array LiNaS in LiNaSO 4 . The Na atoms become C -Ne and
are represented by grey spheres .Li( green ) and S ( yellow ) atoms form the C -BeS subnet which is
connected with red lines .(c) Perspective view of part of the C -BeS subnet showing the three-,

four-, and six-coordination of S(1), C -Be(Li) and S(2), respectively