Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 183

Fig. 29 The three polymorphs of Ca 2 GeO 4 . The stable phase at ambient conditions is the olivine-
like structure represented in (b). Both grey and green spheres represent Ca atoms and red spheres
are Ge atoms. When the olivine structure is heated above 1,726 K, the compound transforms to a
stuffed wurtzite-like structure, represented in (a). Atoms colour as in (b). (c) The structure of
Ca 2 GeO 4 at high pressure. It is of the Ba 2 SnO 4 type in which perovskite-like blocks CaGeO 3
co-exist with the CaO bilayers. The O atoms have been eliminated for clarity, except in the central
part of the figure where a CaGeO 3 cube has been outlined. The GeO 6 octahedron is depicted with
green lines . Compare Fig. 29a with Fig. 20c

If we analyse the structure of the oxides, we see that CaGeO 3 has two poly-
morphs at ambient pressure. One has the triclinic wollastonite-type structure. The
other one is an orthorhombic perovskite of the GdFeO 3 type whose cation array is
CsCl type. That is, the insertion of three O atoms provokes, in CaGe, the transition
of the CrB

CsCl, as it occurs, under pressure, in the alloy.
Unlike Ca 2 SiO 4 , which has a cation array of the anti-PbCl 2 type [ 5 ] at ambient
conditions, Ca 2 GeO 4 is olivine type (Fig. 29b ) and shows an interesting and
uncommon behaviour. When heated, the olivine-like structure remains stable up
to 1,726 K, transforming then into a Ca-filled wurtzite-like structure of formula Ca
(CaGeO 4 ), which is represented in Fig. 29a [ 21 ]. The wurtzite-like skeleton
is formed by the subarray (CaGeO 4 ) 2 , equivalent to the pseudo-formula Ca 2+
[

!

-TiGeO 4 ].
On the other hand, at 110 kbar and 900 K, the olivine-type structure transforms
into the Ba 2 SnO 4 -type structure (I4/mmm) [ 125 ]. This phase is represented in
Fig. 29c . Its comparison with that of Fig. 27a provides the unexpected result that
the olivine structure transforms, at high pressure, directly to the Ba 2 SnO 4 -type
structure, whereas it jumps directly into the Ca-filled tridymite at high temperature.
Thus, in both transitions no intermediate phase (spinel or cotunnite) has been
detected.
In the Ba 2 SnO 4 -like phase, we see that the CsCl-type fragments of composition
CaGeO 3 are intercalated with the CaO bilayers. That is, the irregular CaGe cubes
(CsCl type) observed in the perovskite-like phase of CaGeO 3 become completely
regular in the HP phase of Ca 2 GeO 4 which is of the Sr 2 PbO 4 type.

C