The unit cell parameter a was calculated from (110) reflection at ~60.6°. Its value depends on the nature of the doping metal, as it reflects cation-cation distance within the hydrotalcite layer, Table . In the HTs doped with M 2+ metals, the ionic radii of the divalent ions isomorphically substituting Mg 2+ , are within close range to that of Mg 2+ (0.72 Å), see Table . As a result, the cell parameter a remains unchanged (3.06 Å). On the contrary, in the HTs doped with M 3+ metals, such as Fe 3+ , there is a more substantial difference in the M 3+ ionic radius relative to that of Al 3+ (0.54 Å vs 0.65 Å for Al 3+ vs Fe 3+ ). This difference manifests itself in structural changes, reflected in the value of a: in the sample doped with 15% Fe 3+ the parameter a increases to 3.09 Å compared to the neat HT sample.