title: Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework
author: Michael Fischer
description: In silicogermanate zeolites containing double four-ring (d4r) building units, the germanium atoms preferentially occupy the corners of these cube-like units, but the absence of long-range order precludes the determination of the preferred arrangements of Si and Ge atoms by means of crystallographic methods. If fluoride anions are present during the synthesis, they are incorporated into these cages. Because of the sensitivity of the 19F chemical shift to the local environment, NMR experiments can provide indirect insights into the predominant (Si,Ge) arrangements. However, conflicting interpretations have been reported, both with regard to the preference for, or avoidance of, Ge–O–Ge linkages, and concerning the equilibrium position of fluoride inside the cage, where fluoride might either occupy the cage center or participate in a partly covalent Ge–F bond. In order to shed light on the energetically preferred local arrangements, periodic dispersion-corrected density functional theory (DFT) calculations were performed for the AST framework, which is synthetically accessible across the range of (Si1–n,Gen)­O2 compositions (0 ≤ n ≤ 1). DFT structure optimizations for (Si,Ge)-AST systems containing fluoride anions and organic cations revealed that arrangements of Si and Ge which maximize the number of Ge–O–Ge linkages are energetically preferred and that fluoride tends to form relatively short (∼2.2–2.4 Å) bonds to Ge atoms surrounded by Ge–O–Ge linkages. The preference for Ge–O–Ge linkages disappears in the absence of fluoride. DFT-based molecular dynamics calculations were performed for selected AST models to analyze the dynamics of fluoride anions confined to d4r cages. These calculations showed that the freedom of movement of fluoride varies depending on the local environment and that it correlates with the average Ge–F distance. An analysis of the Ge–F radial distribution functions provided no evidence for a coexistence of separate local energy minima at the cage center and in the proximity of a germanium atom. The computational approach pursued in this work provides important new insights into the local structure of silicogermanate zeolites with d4r units, enhancing the atomic-level understanding of these materials. In particular, the findings presented here constitute valuable complementary information that can aid the interpretation of experimental data.