Achieving predicting power for the calculated properties of disordered materials: the role of exchange-correlation functionals within DFT

Disordered network-forming materials have proved a valuable playground for testing and improving the performances of current atomic-scale recipes aimed at a quantitative modeling of materials properties. For the specific family of chalcogenides (Ge_xSe_(1-x) in particular), a suitable bonding description has to able to account for the variety of changes characterizing the different concentrations. On the Se-rich concentration side, Se atoms are arranged in chains coexisting with GeSe₄ tetrahedra, the number of Se-Se homopolar bonds being very low. By increasing the content of Ge atoms, the overwhelming majority of Ge and Se atoms begin to be arranged in GeSe₄ tetrahedra, expected to be the predominant structural motif for x=0.33. Higher values of x correspond to the formation of Ge-Ge chains accommodating those Ge atoms that cannot find their place in GeSe₄ tetrahedra.

We have described the structural changes undergone by this family of disordered network-forming materials by resorting to first-principles molecular dynamics within density functional theory. Liquid and glassy systems have been considered by taking suitable statistical averages over extended time trajectories. Special attention has been payed to the role of the exchange-correlation functionals within DFT, due to the delicate interplay of ionic and covalent bonding inherent in the Ge-Se interaction. We show that functional featuring enhanced electron localization properties are the best suited to achieve a successful comparison with measured structural properties, both in real and in reciprocal space.

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