Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 155

When the three former structures are compared (Fig. 13 ), the suspicion is that the
transition Ni 2 Al Ni 2 In could occur through a continuous displacement of both

the Ni(1) and the Al [Fe, Si] atoms. In this way, the structure of Fe 2 Si [ 50 ] might be
regarded as an instantaneous “picture”, captured during the quenching process, that
is, as the Ni 2 Al-type structure is progressing towards the Ni 2 In type. This hypothe-
sis is supported by theoretical calculations carried out on Fe 2 Si, which predict the
Ni 2 In type (

!

6 3 /mmc) as the most stable phase (Rodr ´ guez-Hern ´ ndez et al. 2005,
unpublished results). In other words, at HT, the Ni 2 In-type structure begins the walk
towards the CaF 2 -type array, passing through the Ni 2 Al-type structure (question
mark in Scheme 1 ). Note that a small displacement of

P

0.15 ˚ of both Fe(2) and Si
atoms, in Fe 2 Si, leads to the Ni 2 In-type structure. Recall in this regard how CsLi
(CrO 4 ) reaches a cubic phase structure, related to fluorite, passing through the
intermediate cotunnite-type structure (see Scheme 1 ) .
The structures of Fe 2 Si and Ni 2 In (Fig. 13b, c ) also conform to the equivalence
between oxidation and pressure. Thus, in fayalite (Fe 2 SiO 4 ), the insertion of oxygen
stabilizes an undistorted Fe 2 Si subarray of the Ni 2 In type.

6 The Third Missing Link: The Fe 2 P Structure

In Sect. 4 , it was discussed the possibility that the Fe 2 P-type (anti-BaCl 2 ) structure
could be an intermediate arrangement in the Ni 2 In PbCl 2 transition [ 30 , 35 ] .

In support of this assumption are the phases observed in the double alkali sulphates
LiKSO 4 and LiNaSO 4 , colle c ted in Scheme 1 .
The structure of Fe 2 P(

!

62m) was reported as early as 1930 [ 51 ] . Since then,
more than 100 isostructural compounds have been collected in the structural
databases (ICSD, PCD). It has been pointed out [ 52 ] that no Fe 2 P polymorph is
known with the Co 2 Si (cotunnite-like) structure type. However, solid solutions
of the phosphides M 2 P( M

P

¼ Cr, Mn, Fe, Co, Ni), such as Mn 2 P, Fe 2 P and Ni 2 P,

are hexagonal ( 62m), as is barringerite, (Fe,Ni) 2 P, whereas Co 2 P remains in

the cotunnite-like Co 2 Si type. It is noteworthy that the orthorhombic phase of
(Fe, Ni) 2 P, known as the mineral allabogdanite and having the Co 2 Si-type structure,
has been found in the Onello meteorite [ 52 ]. On the contrary, FeMnP undergoes the
Fe 2 P

P

Co 2 Si phase transition above 1,473 K [ 53 ] .
Among the Fe 2 P-type compounds, one of them, BaCl 2 , is of special interest in the
context of this chapter. It is likewise included in Scheme 1 . At ambient conditions,
BaCl 2 is cotunnite type (Co 2 Si) (

!

nma) [ 54 ] , but on dehydra ti on of BaCl 2 ·2H 2 O
[ 31 ] , two additional polymorphs were obtained, i.e. Fe 2 Ptype(

P

P 62m) and Co 2 Si type

( P nma) (cotunnite-like). At even higher temperatures (1,200 K), a b -phase with the

anti-fluorite structure ( m3m) was observed [ 31 ] . Thus, that dehydration process has

revealed the important double transition Fe 2 P

F

CaF 2 .
More recent high-pressure studies [ 55 , 56 ] have permitted the observation of a
double transition cotunnite

! Co 2 Si !

Fe 2 P type at pressures of about 18
and 30 GPa, respectively. Thus, the hexagonal phase (

! post-cotunnite !

P 62m) of BaCl 2 can be