. The asymmetric unit cell volume suggested the presence of one HTBZ and one HFU ion in the structure solution process. Hydrogen atoms, except those bound to aliphatic carbon atoms, were excluded from the initial model due to proton transfer considerations and the carboxylate group's conformational flexibility. To account for the slight excess of electron density from hydrogen atoms, the occupancy factors of the terminal oxygen atoms were allowed to vary within the range of 1-1.1. During the simulated annealing runs, bond distances and angles within the rigid bodies were kept fixed, while all possible dihedral angles were allowed to vary. The rigid bodies were freely translated and rotated in the asymmetric unit until convergence was achieved. Once the optimization reached a global minimum, the model underwent whole-pattern Rietveld refinement . Hydrogen atoms were added based on geometric considerations. During the refinement, all previously mentioned parameters were refined, including unit cell parameters, peak shape, and profile parameters, background coefficients (described as Chebyshev polynomial), bond lengths and angles within the rigid bodies, and thermal displacement parameters (defined as a single variable for each atomic type, except for hydrogen atoms which were calculated as the thermal displacement parameter of the bonded atom multiplied by a factor of 1.5). The Rietveld refinement process quickly converged with satisfactory figures of merit and a satisfactory difference curve. The Rietveld plot can be seen in Figure , and selected parameters and figures of merit are presented in Table . a The figures of merit are as defined in TOPAS-V6.