Evolutionary search for novel superhard materials: Methodology and applications to forms of carbon and TiO2

We have developed a method for prediction of the hardest crystal structures in a given chemical system. It is based on the evolutionary algorithm USPEX (Universal Structure Prediction: Evolutionary Xtallography) and electronegativity-based hardness model that we have augmented with bond-valence model and graph theory. These extensions enable correct description of the hardness of layered, molecular, and low-symmetry crystal structures. Applying this method to C and TiO2, we have (i) obtained a number of low-energy carbon structures with hardness slightly lower than diamond and (ii) proved that TiO2 in any of its possible polymorphs cannot be the hardest oxide, its hardness being below 17 GPa.Comment: Submitted in November 2010; revised in March 2011; resubmitted 24 June 2011; published 12 September 2011. 8 pages, 2 tables, 3 figure

Fe-C and Fe-H systems at pressures of the Earth's inner core

The solid inner core of the Earth is predominantly composed of iron alloyed with several percent Ni and some lighter elements, Si, S, O, H, and C being the prime candidates. There have been a growing number of papers investigating C and H as possible light elements in the core, but the results are contradictory. Here, using ab initio simulations, we study the Fe-C and Fe-H systems at inner core pressures (330-364 GPa). Using the evolutionary structure prediction algorithm USPEX, we have determined the lowest-enthalpy structures of possible carbides (FeC, Fe2C, Fe3C, Fe4C, FeC2, FeC3, FeC4 and Fe7C3) and hydrides (Fe4H, Fe3H, Fe2H, FeH, FeH2, FeH3, FeH4) and have found that Fe2C (Pnma) is the most stable iron carbide at pressures of the inner core, while FeH, FeH3 and FeH4 are stable iron hydrides at these conditions. For Fe3C, the cementite structure (Pnma) and the Cmcm structure recently found by random sampling are less stable than the I-4 and C2/m structures found here. We found that FeH3 and FeH4 adopt chemically interesting thermodynamically stable structures, in both compounds containing trivalent iron. The density of the inner core can be matched with a reasonable concentration of carbon, 11-15 mol.percent (2.6-3.7 wt.percent) at relevant pressures and temperatures. This concentration matches that in CI carbonaceous chondrites and corresponds to the average atomic mass in the range 49.3-51.0, in close agreement with inferences from the Birch's law for the inner core. Similarly made estimates for the maximum hydrogen content are unrealistically high, 17-22 mol.percent (0.4-0.5 wt.percent), which corresponds to the average atomic mass in the range 43.8-46.5. We conclude that carbon is a better candidate light alloying element than hydrogen.Comment: Published in Physics-Uspekhi: full text will soon appear at http://ufn.ru/en/articles/2012/5/c/ (currently, only abstract is available