Now, I will proceed with a very brief description of the two main parts that
form this thesis work, which are inspired by some new possibilities that high
pressure has opened for the synthesis of new phases and compounds. The
relevance of these new materials from a practical point of view, lies in the
possibility of having them recovered to ambient conditions and then used
in a wide variety of technical applications. The two examples covered here,
include a new class of transition-metal nitrides, and the synthesis of extended
forms of CO2.
In Chapter 3, it is shown that the new family of late transition-metal nitrides:
PtN2, OsN2 and IrN2, all synthesized at similar conditions ( 3c 50 GigaPascal
and 2000 K) [10, 11], shares common structural properties among its members
and opens the door to the synthesis of novel materials of this kind; with
possible technological applications since they can be recovered to ambient
conditions. The synthesis of these new nitrides is a clear example of how
pressure can be used to form compounds between species that do not mix at
ambient conditions.
Chapter 4 reports our studies on a different class of high pressure synthesis,
namely the chemical transformation of a molecular species (CO2) into an
extended compound with entirely different mechanical and electronic properties.
In particular it reports on the transition that molecular CO2 undergoes
at pressures above 40GPa and mild temperatures, into an extended glassy
phase. CO2\u2019s pressure-induced phase-transition from a molecular to an extended
phase was first observed in 1999 when V. Iota and collaborators at
Livermore, obtained a fully tetrahedral silica-like phase of CO2 whose precise
structure remains unresolved up to these days [12]. Recently, two new
extended phases that show strong similarities among themselves in many
aspects, have been reported [13, 14]. The first [13], is a non-molecular amorphous
phase named \u201ca-CO2\u201d or \u201ccarbonia\u201d, while the second [14], is a crystalline
phase indexed by its discoverers as stishovite-like, i.e. with six-fold
coordinated carbon atoms, that instead we believe is the crystalline counterpart
of carbonia. However, in contrast with what is observed in the case of
the transition-metal nitrides, for CO2 no recovery of any of the new extended
phases to ambient conditions has yet been possible. In fact, it is observed
that a-CO2 and phase VI go back to molecular phases at pressures around
20 GPa which coincides with the pressure at which the crossing between the
enthalpies corresponding to the molecular and tetrahedral phases takes place. Finally, also in Chapter 4, I consider some first-principles high-pressure chemistry
applied to the problem of the catalysis and recovery of new CO2 extended
phases. Here, I will show that by means of introducing a transition
metal (TM) as an impurity (Ti in our case) in a CO2 molecular sample (2%
concentration) an activation of the amorphization reaction is observed and
this leads to a transition that occurs much faster than in the case in which
no TM is used. It is also expected that attempts succeeding to lower the
transition pressure, will also lead to a lowering of the pressures at which the
CO2\u2019s non-molecular phases can be recovered, with the final goal of bringing
them to ambient conditions.
In summary, in this thesis it is shown how first-principles calculation techniques
can be effectively used in high-pressure physics and chemistry research
for clarifying very important issues regarding structural and electronic properties
that wouldn\u2019t be easily accessible by other means
