In this work, we investigate the structure of argon adsorbed on pure-silica MEL at liquid nitrogen temperature. Our goal is to provide a microscopic interpretation for the appearance of a substep in the adsorption isotherm at intermediate loadings before saturation. For that purpose, we first perform time-of-flight neutron diffraction experiments of the loaded zeolite, before and after the substep. The measured spectra reveal that, after the substep, a considerable ordering of the adsorbate builds up, but there is also evidence of a zeolite structural change. These experimental data were then used in conjunction with Reverse Monte Carlo simulations to obtain theoretical structural models of the adsorbate/adsorbent system. Interestingly, the spatial distribution of the adsorbate predicted by Reverse Monte Carlo is considerably different from that obtained from grand canonical Monte Carlo simulations, even at half loading. We ascribe these discrepancies to deficiencies in the argon-zeolite interatomic potential. Besides, at high loading, we observe a different distribution of atoms along the channels parallel to the x-axis and those parallel to the y-axis. This might be attributed to a zeolite structural change distorting the tetragonal symmetry of MEL.

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