The ultrafine spinel NiFe1.9M0.1O4 powders (M=Al, Y, Cr, Fe) were synthesized by thermal decomposition of appropriate mixtures of complex compounds with acetylacetone - (2,4 pentadione) ligands ([M(AA)x]) at 500 °C. Samples were annealed in the temperature range 600-1000 °C and the crystallization process was followed by XRD. The largest crystallite size and the highest crystallization rate are observed for NiFe1.9Al0.1O4, while the opposite was found for NiFe1.9Y0.1O4. Strain value in NiFe1.9Y0.1O4 changes sign at temperatures between 700-800 °C, while for NiFe1.9M0.1O4 (M=Al, Cr) it has a positive sign and reaches the maximum value at 700 °C. The crystal structure of the samples annealed at 1000 °C was refined using the Rietveld profile method. Cation substitution in Ni-ferrite changes its structural and microstructural parameters, and can also significantly influence other physical properties.