Ab-initio study on synthesis of new materials at high pressure : transition-metal nitrides and non-molecular CO2 phases

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 ( ∼ 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’s 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 “a-CO2” or “carbonia”, 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’s 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’t be easily accessible by other means.