Compared to the experimental values, the DFT calculations deliver a shorter a axis and a longer c axis. Similar tendencies were observed and discussed in previous DFT studies of calcined SiO2-AST. The effect is fairly prominent for AlPO4-AST_QNU, where the relative deviation in a exceeds -2%, and moderately pronounced for SiO2-AST_TMA. In contrast, agreement with experiment is excellent for SiO2-AST_QNU. As the CP2K optimisations do not make use of symmetry information, slight deviations from ideal symmetry occur in the DFT-optimised structures (up to a few 1/100 Å). In order to restore the symmetry prior to the analysis, a symmetry search was carried out using Materials Studio. The results, tabulated in Table , show that all TMA-containing systems have 𝐼4 ̅ symmetry, the highest possible symmetry permitted by the chosen arrangement of the OSDA molecules in the ast cages. All QNU-containing systems have symmetry structures, and they are not further discussed here. In SiO2-and GeO2-AST_TMA, the fluoride anions are located at the cage centre, as evidenced by a very narrow distribution of the T-F distances. The distances agree well with those determined experimentally for AST-type SiO2 and GeO2 systems (d(Si-F)exp = 2.63 Å, d(Ge-F)exp = 2.75 Å). The Ge-O-Ge angles are significantly smaller than the Si-O-Si angles, in line with the smaller equilibrium angle. In AlPO4-and GaPO4-AST_TMA, the distributions of Al/Ga-F and P-F distances are also very narrow, with the Al/Ga-F distances being systematically shorter than the P-F distances. The same trend has been observed in crystallographic studies of d4r-containing alumino-and gallophosphates, and The higher degree of flexibility can be explained with the more ionic (= less directional) nature of the Al-O/Ga-O bonds in comparison to the dominantly covalent P-O bonds. A visualisation of the TMA cations and the surrounding ast cages is provided in the Supporting vious NMR study of SiO2-AST_TMA. On the other hand, they are up to ~10% smaller than the sum of van der Waals radii of hydrogen and oxygen of 2.70 Å, indicating a non-negligible electrostatic interaction. of the germanate ASU-7, where they were described as rectangular prisms. A closer look at these units reveals that the idealised symmetry is higher than that of a general rectangular prism (point group 𝑚𝑚𝑚), because three faces meeting at each corner are related by a threefold rotation axis (point group 𝑚3 ̅ ). This feature can be illustrated by drawing auxiliary O-O connections across the faces. As shown in Figure , the resulting shape resembles a pyritohedron, i.e., an irregular pentagonal dodecahedron consisting of identical, but irregular pentagons. In the following, these distorted d4r cages will be referred to as "pyritohedron-like" in order to distinguish them from undistorted "cube-like" cages. It is worth noting that published crystal structures of the GeO2 zeotypes ASU-7 and FOS-5 contain d4r units whose geometry deviates only slightly from point group symmetry 𝑚3 ̅ . Prior to a further analysis of the framework distortion in GeO2-AST_QNU, it is useful to take a look Taken together, there are two possible explanations for the framework distortion observed in GeO2-AST_QNU, and its absence in GeO2-AST_TMA: