Local Distortions in a Prototypical Zeolite Framework Containing Double Four-Ring Cages: The Role of Framework...

Cube-like double four-ring (d4r) cages are among the most frequent building units of zeolites and zeotypes. In materials synthesised in fluoride-containing media, the fluoride anions are preferentially incorporated in these cages. In order to study the impact of framework composition and organic structure-directing agents (OSDAs) on the possible occurrence of local distortions of fluoride-containing d4r cages, density functional theory (DFT) calculations and DFT-based molecular dynamics simulations were performed for AST-type zeotypes, considering four different compositions (SiO₂ , GeO₂ , AlPO₄ , GaPO₄ ) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO₂ -AST show significant deformations, with a pyritohedron-like distortion of the d4r cages occurring in GeO₂ - and GaPO₄ -AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO₄ - and GaPO₄ -AST. While the distortions occur at random in TMA-containing zeotypes, they exhibit a preferential orientation in systems that incorporate QNU cations. In addition to providing detailed understanding of the local structure of a complex host-guest system on the picosecond timescale, this work indicates the possibility to stabilise ordered distortions through a judicious choice of the OSDA, which might enable a tuning of the material's properties.