Figure shows powder X-ray diffraction data and associated Rietveld refinement profiles for K-L and partially exchanged Li-L and Cs-L at ambient pressure. Crystal structures and cell parameters for all refined samples are given in the Appendix A Tables A1-A6. Rietveld refinement of high-resolution, powder X-ray diffraction data for our LTL zeolites reveal that extra-framework cations are located at sites B-D. Site B is at the centre of the can cages, site C is at the windows of the elliptical 8R channels and site D is at the periphery of the 12R channels, as shown in Figure . Site A, reported by Barrer at the centre of the d6rs is vacant in our as-synthesised K-L and cation exchanged Li and Cs-L zeolites. The occupancy at site A depends on the synthesis conditions, particularly the Si/Al ratio, where additional cations may be required to render an overall neutral structure. Analysis by energy dispersive X-ray spectroscopy (EDX) gave an average Si/Al ratio of 3.4(2) for the parent K-L sample. Refinement of cation occupancies for the parent K-L zeolite reveal that sites B and C are fully occupied by K cations, whereas site D is statistically occupied by 0.63 K + ions per unit-cell. This is consistent with a Si/Al of 3.1, which is in close agreement to the average value obtained from EDX data. Cation exchange at these sites occur in variable degrees to produce partially exchanged Cs and Li-L zeolites. Site B remains fully occupied by K + ions in both cases, as the small windows of the can cages restrict the exchange pathway. Site B can, therefore, be regarded as a locked site and is inert to cation exchange with both Li + and Cs + ions. Cations are preferentially exchanged at sites D and C, decreasing in the order D > C > B. Figure shows the refined, partially exchanged Cs-L structure. Site D undergoes a complete exchange of 0.63 K + ions per unit-cell and is repopulated by 0.72 Cs + ions per unit-cell. In comparison to the parent K-L structure (Figure ), Cs + ions located at site D are inset further into the 12R channels so that stability is achieved by accommodating their higher steric demands. Cs + ions can, therefore, be considered stable at site D and a 100% exchange rate proves that this is an open site. Refined occupancies reveal that site C is partially occupied by 0.45 and 0.38 K + or Cs + ions, respectively. Refinement models with K + solely populating site C result in occupancies > 1 and this excess electron density indicates the presence of Cs + . Site C is, in fact, not a closed site, as previously suggested by Newell and Rees, and can exchange K + ions with Cs + ions. Our refinement models show that site C is susceptible to exchange with Cs + ions and is similarly populated to its gallosilicate analogue, reported by Seoung et al., to have a 50:50 fractional occupancy at this same site. It is also important to note that the overall occupancy at site C by K + and Cs + ions combined is 0.83 per unit-cell compared to the full occupancy by K + ions in the parent K-L zeolite. To render an overall neutral structure, this is compensated for by the increased overall occupancy at site D which is 0.72 Cs + per unit-cell compared to the 0.63 K + in the parent K-L zeolite. This emphasises that Cs + ions preferentially occupy site D over site C as the free volume available is greater here to accommodate larger Cs + cations. Exchange of the parent K-L zeolite with Li + ions leads to a partially exchanged Li-L zeolite. Again, site B is inert to exchange and remains fully occupied by K + ions. Interestingly, unlike with Cs + ion exchange, site C is also locked and remains fully populated by K + ions. Only site D is shown to be susceptible to Li + exchange and our refinement models show it to be partially occupied by 0.11 and 0.57 Li + and K + ions, respectively. Refinement models with only K + ions at site D again show excess electron density, separate to that associated with the K + ions, and this identifies as Li + ions that sit further towards the periphery of the 12R channel. Figure illustrates the positions of Li + and K + ions at site D which vary in their y coordinates, sitting at y = 0.36 and y = 0.29, respectively. Partial exchange of K + ions for Li + ions at site D implies that Li + ions are less stable here than larger K + or Cs + ions. Because of their small ionic radii, Li + ions must sit much closer to the zeolite framework in order to achieve stability through electrostatic interactions with AlO4 tetrahedra. However, even when sitting at the very periphery of the channel walls, Li + ions still struggle to fully exploit the free volume available and cannot maximise their interactions with the framework at this site. K + and Cs + ions have larger ionic radii and are much better matched to the volume of this site. They benefit from stabilising interactions from the surrounding framework. Similarly, at site C, the windows of the elliptical 8R channels are large in comparison to the small Li + ions. Larger K + ions are more stable at this site and it would be unfavourable for K + ions to exchange with Li + ions here. The partial occupancy of Cs + and K + ions at site C indicates that Cs + ions are somewhat stable here, but K + ions are still preferable. Cs + ions are too large to achieve 100% occupancy at site C. The topology of the site also plays a role. For example, site D is more versatile, and cations can be displaced along the a/b-axis, either into the channel centre or towards the wall periphery. This is not possible at site C because it is a symmetric site. AlO4 tetrahedra may be randomly distributed surrounding the cation and electrostatic interactions can occur in multiple directions. Such detailed information of site occupancies is not attainable from semi-qualitative analysis, such as EDX. Refining site occupancies is, therefore, important in order to identify low exchange rates which could compromise the efficacy of the 1D chemical reactor for applications in catalysis or gas adsorption. Table shows how cation exchange at site D affects the 12R channel apertures (measured from O1-O1 and O2-O2). This can be used to control diffusion into and out of the channel. Steric and electronic effects at site D cause smaller Li + cations to retreat along the a/b direction towards the periphery of the channel and larger Cs + cations are displaced further into the centre of the channel. The free-access diameter is, therefore, smaller for the Cs-exchanged sample. This could be envisaged as a tool to control the entrance and exit of reactant and product molecules to the '1D reactor' during catalytic reactions. Water molecules are exclusively confined within the 1D channels of LTL zeolites.