Abstract: Eighteen samples of the (dimim)[Fe(Cl1?xBrx)4] 0 ? x ? 1 solid solution were prepared in a one-pot synthesis. Single crystal and synchrotron X-ray powder diffraction studies show that all compounds are isomorphous with an orthorhombic structure (Ama2 space group) at room temperature. The crystal structure of these imidazolium molten salts can be described as layers of (dimim)+ cations (dimim= 1,3-dimethyl-imidazolium) extended in the bc-plane with an antiparallel stacking along the a-axis forming a herring-bond motive. The (FeX4)? (X = Cl and Br) anions form layers extended on the bc-plane that are intercalated between the organic layers and inverted with respect to the adjacent layer along the a direction. The evolution of the cell volume at this temperature follows the Vegard's law in the whole range of substitution with ?V ? 115 Å3 between non-substituted phases, showing a non-linear axial expansion in the b and c directions. In addition, the melting point of the solid solution does not follow the expected linear physical behavior. Finally, themagnetic study indicates the existence of overall antiferromagnetic interactions as predominant, which are stronger with the increase in the (FeBr4)? ion substitution. However, it was found that the 3D magnetic ordering of the non-substituted phases vanishes or it is below the minimum temperature reachable by the magnetometer devices (1.8 K) for the 25?50% bromide substitutions. This is connected with the appearance of different crystal phases as function of bromide composition at low temperature.

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