Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 167

AB 2 O 4 oxides under
pressure, which can be illustrated with the olivine-like Mn 2 GeO 4 . Under pressure
the latter undergoes a first transition to the spinelloid

Liu and Bassett [ 16 ] have summarized the behaviour of some

mma), followed by
a second transition to a Sr 2 PbO 4 -type structure [ 19 ] . At more elevated pressures,
the compound decomposes into two components (perovskite + rocksalt) [ 16 ] .
On the contrary, the compound Ca 2 GeO 4 , also olivine type at ambient conditions
[ 20 ], shows a different response. At HT, Ca 2 GeO 4 transforms into a Ca-stuffed
tridymite-like structure, whereas, under pressure, the compound undergoes the
olivine

b -phase ( I

Sr 2 TiO 4 (Ba 2 SnO 4 ) transition, which also decomposes at more elevated
pressures [ 16 ]. These decomposition processes will be discussed in depth below.
More recent studies have shown that spinels of formula

!

M 2 MgO 4 , such as
Al 2 MgO 4 , may adopt under pressure any of the interrelated Fe 2 CaO 4 -, Mn 2 CaO 4 -
or Ti 2 CaO4-type structures [ 82 ]. The different M 2 XO 4 oxides do not follow a
uniform pattern. Thus, Al 2 ZnO 4 remains, up to 43 GPa, in the cubic spinel structure
[ 83 ], whereas Fe 2 ZnO 4 transforms into either a Fe 2 CaO 4 -oraTi 2 CaO 4 -type
structure beyond 24 GPa [ 84 ] .
In Mn-containing spinels Mn 2 X

O 4 , cubic-to-tetragonal transitions have been
observed at pressures as low as 12 GPa [ 85 ], and in a more recent paper [ 86 ]a
double phase transition has been reported for the cubic spinel Ga 2 ZnO 4 . A first
cubic tetragonal (Mn 2 ZnO 4 type) transition is followed by a second transition to

the orthorhombic marokite structure (Mn 2 CaO 4 ), related to that of Sr 2 PbO 4 type
(also Ca 2 SnO 4 ).
In Table 4 are collected several germanates, stannates and titanates and their
behaviour under pressure, as reported by Liu and Basset [ 16 ]. This information will
help the reader to follow all the structures we will describe next, as well as the phase
transitions and final decompositions experienced by these compounds under com-
pression.
The intermediate tetragonal spinel structure will be neglected because its cation
array is a slight distortion of the cubic polymorph. In the same way, some structure
types, such as Mn 2 CaO 4 (mineral marokite), Ti 2 CaO 4 and Fe 2 CaO 4 , strongly

!

Table 4 Crystal structures, at 1 bar (X), and decomposition products at high pressure (H) of the
compounds A 2 GeO 4 , A 2 SnO 4 and A 2 TiO 4
Compound Spinel

Ilmenite +
Rocksalt

Sr 2 PbO 4
Ca 2 SnO 4

Ba 2 SnO 4 Corundum
+ Rocksalt

Rocksalt
+ Rutile

Perovskite
+ Rocksalt

Rocksalt +
Fluorite a

Mg 2 GeO 4 X H H

Mn 2 GeO 4 X H(?) H H

Ca 2 GeO 4 H

Co 2 SnO 4 X H H

Zn 2 SnO 4 X H(?) H

Ca 2 SnO 4 X H H

b Mg 2 TiO 4 XH

H b
Cd 2 TiO 4 X(?) H H
a Recall the transformation SnO 2 (rutile) ! SnO 2 (fluorite). The transition TiO 2 (rutile) ! fluorite-
related also occurs some below 25 GPa
b

b

Fe 2 TiO 4 XH

These phase separations have been confirmed in the ilmenite-type titanates