Stable Compound of Helium and Sodium at High Pressure

Helium (He) is, on par with Neon, the most inert element in the Periodic Table. Indeed, no conclusive proofs about stable compounds containing chemical bonds with He at ambient conditions have been reported so far. However, pressure significantly affects chemical properties of elements. By using USPEX [1], a software which has been successfully used in the past to predict unexpected high pressure crystal structure [1], we found that above 160 GPa He and Sodium exothermically combine to form the compound Na2He, whose structure is reported in Fig. 1. Quasiharmonic free energy calculations based on computed phonon spectra indicate that the free energy of formation of Na2He is negative and that the latter is barely affected by the temperature (0-800 K range was considered). In order to understand the cause of stability of Na2He, we carried out a thorough study of its electronic structure at various pressures by means of several different approaches including the examination of the band structure and the analysis of real-space descriptors such as the electron density in the framework of the Quantum Theory of Atoms in Molecules [2], the Electron Localization Function [3] and the deformation density. By examining the band structure, we found that such compound is an insulator whose band-gap increases with pressure. Regarding real-space descriptors, two remarkable features of Na2He are the negative charge on He (obtained both using Mulliken and Bader partitioning) and the presence of interstitially localized electrons (i.e. Non-Nuclear Attractors), the latter being detectable in all the analyses above mentioned. In the range 160-350 GPa, the exothermicity associated to the formation of Na2He is mainly due to the volume reduction, while at higher pressures the electronic energy plays a prominent role in the stabilization of this compound. In this contribution we present the results of our study with particular emphasis on the role played by He in the stabilization of Na2He.

Structure of methane and ethane at high pressure

Methane is one of the most abundant hydrocarbon molecules in the universe and is expected to be a significant part of the icy giant planets (Uranus and Neptune) and their satellites. Ethane is one of the most predictable products of chemical reactivity of methane at extreme pressures and temperatures. In spite of numerous experimental and theoretical studies, the structure and relative stability of these materials even at room temperature remains controversial. We have performed a combined experimental and theoretical study of both methane and ethane up at high pressures up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopy and the ab-initio evolutionary algorithm, respectively. In the case of methane we have successfully solved the structure of phase B by determining the space group and the positional parameters of carbon atoms, and by completing these results for the hydrogen positions using the theoretical calculations. The general structural behavior under pressure and the relation between phase B and phases A and pre-B will be also discussed. For ethane we have determined the crystallization point, for room temperature, at 1.7 GPa and also the low pressure crystal structure (Phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been indentified, for the first time, at 18 GPa to a monoclinic phase III, the structure of which is solved based on a good agreement of the experimental results and theoretical predictions. We have determined the equations of state of methane and ethane, which provides a solid basis for the discussion of their relative stability at high pressures.

Computational Materials design by Evolutionary Structure Prediction

Crystal structure prediction (CSP) has been viewed as a major challenge in condensed matter science for a long time. Until recently, we developed a USPEX method based on evolutionary algorithms, and it proved to be a powerful tool enabling accurate and reliable prediction of structures from the beginning. How does it work - and why? In this lecture, I will summarize the principles, recent developments, and some applications of the USPEX code. 1) Optimizing chemical compositional space for compounds and co-crystals. A scheme is proposed to allow the automatic search for all the stable compounds with variation of chemical compositions. This function can be applied to study binary/ternary systems composed of both atomic/molecular blocks (Na-Cl, Mg-O, CaCl2-H2O, etc) [1]. 2) Predicting structures containing complex inorganic/organic molecular motifs. We designed a constrained evolutionary algorithm [2]. The key feature of this new approach is that each motif is treated as a building block which significantly reduces the search space. This method has been applied to a wide range of systems including inorganic complex, small molecular crystals, pharmaceuticals and even polymers crystals. 3) Predicting low dimensional system is different from predicting the bulk crystals. Surface brings another independent thermodynamic parameter, chemical potential. Since the stability of surface configuration depends on the chemical potential, the established phase diagram for multi-component system is quite different from that of bulk crystals [3].