The microstructure, together with the composition, determines the properties of a material. The immediate environment around each atom (nearest neighbors) contains already a lot of information about the microstructure. Therefore, coordination numbers and local order parameters (LOPs) are ideally suited as descriptors to characterize materials. In this contribution, we exploit the recent introduction of LOPs [1] that were specifically developed to recognize key structural motifs. We highlight four cases in which the LOPs serve as versatile descriptors. First, we have scanned the database of The Materials Project, which currently contains &gt;60,000 inorganic crystalline structures obtained from relaxation using GGA-DFT. We determined the number of basic structural motifs (e.g., tetrahedra, octahedra) occurring in each structure. Correlation plots reveal islands of particularly frequently occurring pairs of (Ntet, Noct) across the database. A critical task, in this context, was finding sensible cutoff radii to determine nearest neighbors. Second, we address the technologically important question whether or not a hypothetical zeolite structure can be easily synthesized. Li et al. [2] have proposed criteria, which are based on average distances of nearest and next-nearest neighbors, that make reliable predictions of a zeolite's synthesizability. We show that a LOP can be used in place of the most important synthesizability criterion by Li et al. Third, we relaxed the tetrahedral (T) and octahedral (O) oxygen interstitial in hcp α-titanium, monitoring the formation energy and LOPs. While the values do not change for the O interstitial, relaxation of the T interstitial gives rise to a drop in both formation energy and LOP. Because interstitials are high-energy configurations, the faintest movement of an individual atom can lead to a dramatic change in the electron density and, thus, total energy. Therefore, our LOPs help to quickly identify the root cause (structural vs electronic) of a pronounced energy change. Finally, intercalant diffusion frequently proceeds from and to a site in which the migrating ion is octahedrally coordinated via a tetrahedrally coordinated intermediate state. Similar to the interstitial case, our LOPs complement the energetic information of the hop along the path with structural information in a compact way. [1] Zimmermann et al., J. Am. Chem. Soc., 10.1021/jacs.5b08098, 2015. [2] Li et al., Angew. Chem. Int. Ed. 52, 1673–1677, 2013.

Local order parameters: Descriptors for databases, synthesizability, interstitial relaxation, and diffusion paths

251st National Meeting (2016)

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