Nanostructural Units in Disordered Network-Forming Materials and the Origin of Intermediate Range Order

Abstract

Disordered network-forming materials are characterized by structural order extending well beyond the first shell of neighbors. For these reasons, reliable atomic-scale modeling is ideally suited to complement experiments in the search of the microscopic origins of this behavior. A key to understand why these systems have specific structural properties is to focus on the nanostructural units by which they are composed. By analyzing the role played by these units, one is able to put forth a valuable rationale accounting for the occurrence of intermediate range order. In this review, we present recent results obtained via first-principles molecular dynamics on a set of disordered network-forming materials, with special emphasis on the prototypical system GeSe₂. In a short introduction we begin with explicit examples of differences, at the structure factor and pair correlation level, between networks exhibiting intermediate range order and those purely disordered at any length scale. Concerning our theoretical approach, we rely on density functional theory and first-principles molecular dynamics to follow the time trajectories at finite temperature of these networks and obtain statistical averages to be compared with the experimental quantities. Specific methodological issues pertaining to the simulation of disordered materials are analyzed in detail (size of the computational cell, role of exchange–correlation functional, and production of an amorphous phase). Then, three specific points are addressed by considering both experimental and simulation results: first, the atomic-scale signature of intermediate range order as it manifests itself via the appearance of the first sharp diffraction peak in the total neutron structure factor; second, the correlation existing between fluctuations of concentration on the intermediate distances scale and the shape taken by the partial structure factors; and third, the establishment of the nanostructural units responsible for the occurrence of the first sharp diffraction peak in the concentration–concentration structure factor. All these examples are substantiated by extensive reference made to existing and ongoing first-principles molecular dynamics simulations.