Chemistry Reference
In-Depth Information
Concurrent Pathways in the Phase Transitions
of Alloys and Oxides: Towards an Unified
Vision of Inorganic Solids
´
ngel Vegas
Abstract The study of phase transitions is usually restricted to two to three
transformations. Examples of such transitions include the CaF
2
!
PbCl
2
!
Ni
2
In in alloys, the NaCl
!
CrB
!
CsCl or the well documented transformation
olivine
O
4
. These transitions, traditionally regarded as
partial processes, have prevented the construction of wider structure maps. One of
the scarce examples of these maps was reported by L´ger and Haines (Eur J Solid
State Inorg Chem 34:785-796, 1997) concerning the phase transitions of
!
spinel of the oxides
A
2
X
AX
2
compounds (dihalides and dioxides), where increasing the coordination number
of the
atom is linked to the pressure increase. The structural information,
collected in these maps, is always of interest because it limits the number of
possible transition paths which may relate a structure-type into another. However,
a careful analysis of the partial phase transitions undergone by different com-
pounds, at high temperature and high pressure, reveals that the partial transitions
are not isolated processes but they overlap, forming a long, rational pathway that
connects all the structures in a coherent manner. Alloys and their related oxides
show a similar trend along their concurrent pathways which complement each
other. In this work,
A
the analysis is restricted to the
AX
2
alloys and their
corresponding oxides
AX
2
O
4
, and the results demonstrate that there exists a unify-
ing principle that can be inferred through the simultaneous analysis of all the phase
transitions involved in the concurrent
structural journeys
carried out by both types
of compounds. The
AX
2
alloys begin the
walk
in the fluorite-type structure, ending
in the MoSi
2
-type structure. In the case of the oxides
AX
2
O
4
, their cation arrays
follow a concurrent pathway that, starting at the filled fluorite-type structure, ends
in the final Sr
2
PbO
4
-type structure. These structural “
journeys
” also allows for the
discovery of several “
missing links
”(
structure types
) which fit into the general
sequence and help one understand the whole transitions pathway as a rational
process, which takes place simultaneously in the alloys as well as in the cation
arrays of the oxides. Very recent works show that alkali metals (Na and K) also join
