We report, to our knowledge for the first time, on the experimental observation that the maximum lattice deformation induced at the surface of iron doped lithium niobate crystal by thermal diffusion depends on both the Fe concentration and the reduction degree of the doped layer itself. By exploiting a simple linear model, we suggest a description of this experimental evidence and we point out a procedure that allows the characterization of the in-depth profile of the [Fe2+]/[Fe3+] ratio.