Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 172

This interpretation could justify why some of the compounds listed in Table 5
produce ilmenites and rutiles as sub-products of decomposition. We will see later
that the appearance of perovskites can also be explained.

8.2 Extension to Mn 2 GeO 4 : The Olivine-,
Spinel- and Sr 2 PbO 4 -Type Structures

Mn 2 GeO 4 crystallizes in three polymorphs. At ambient conditions it is olivine-like
( nma) [ 17 ] , undergoing two high-pressure high-temperatures transitions [ 19 ] . The

first one is a tetragonal spinelloid

P

b -phase ( I mma), transforming at higher pressures

into the Sr 2 PbO 4 -type structure ( bam).

Although Mn 2 GeO 4 is isostructural with Ca 2 SnO 4 , a detailed discussion of its
structure will doubtless clarify many of the peculiarities discussed about Ca 2 SnO 4 .
The relationships that can be established between the cation subarray of Ca 2 SnO 4
and the corresponding Ca:Sn alloys will be of special importance.
In the Ca-Sn system, two phases have been reported. The first one is Ca 2 Sn

P

( nma, anti-PbCl 2 type) [ 94 ] . The second one is CaSn, CrB type, undergoing the

pressure-induced CrB

P

CsCl-type transition [ 2 ] . In addition to these alloys, it
should be recalled that the fragments of the SnO 2 (Fig. 22 ) can be justified if we
formulate the compound as (CaO) 2 SnO 2 .
We have seen that the existence of blocks of a bixbyite-type structure was
interpreted as if a II-IV compound (CaSn) was chemically equivalent to a III-III
subarray, such as elemental indium. Thus, (CaO)CaSnO 3 should be equivalent to
CaO(

!

-In 2 O 3 are fragments of real C -In 2 O 3 (also C -Sc 2 O 3 ).
Thus, the equivalence between the CaSn subarray and the In structure is evident.
Ca 2 SnO 4 can also be compared with the Ca 2 Sn alloy. The latter is anti-PbCl 2 ,
whereas the oxide stabilizes a Ca 2 Sn subarray which could not be related to any
known alloy. This was the reason why, in the above section, the Ca 2 Sn substructure
was fragmented to find relationships with other structures.
This feature contrasts with that shown by the related compounds Ca 2 Ge and
Ca 2 GeO 4 [ 94 ]. It is especially problematic when we try to explain the formation of
Ca 2 SnO 4 (Sr 2 PbO 4 type) in terms of the equivalence oxidation-pressure . Because
the Ca 2 Sn subarray is anti-PbCl 2 type, it should be expected that, when oxidized,
the Ca 2 Sn subarray should necessarily undergo the complete transition series Ni 2 Si
!

C -In 2 O 3 ), in which C

Ca 2 Sn . Of these, only the initial and the final states
have been identified, the former for Ca 2 Sn and the latter in Ca 2 SnO 4 . This implies
that Ca 2 Sn is forced to undergo the direct transition Ni 2 Si to Ca 2 Sn[O 4 ].
This extreme transition does not occur in Ca 2 GeO 4 (see Scheme 1 ) . At ambient
conditions the compound is olivine-like [ 20 ] . As discussed above, at high tempera-
ture, it transforms into a Ca-stuffed wurtzite-like network of composition CaGeO 4 ,
and under compression it becomes Ba 2 SnO 4 type [ 16 , 21 ]. Thus, Ca 2 Ge is observ-
able as Ca 2+ [

Ni 2 In ! TiSi 2 ! MgCu 2 !

-Ge 2 O 4 ], as Ni 2 In type and finally as Mo 2 Si type, which is the
structure type of the Ba 2 Sn subarray in Ba 2 SnO 4 .

C