Metastabilitatea structurii de tip K₂NiF₄ în soluţiile solide Eu_2-xSr_xNiO₄

The results of the crystal chemical investigation of the solid solution Eu_2-xSr_xNiO₄ are reported. Modeling the chemical bond by the valence bond model, we conclude that the substitution Er³⁺/Sr²⁺ stabilizes the K₂NiF₄-like structure by the partial oxidation of the Ni²⁺ ions. Additionally, the results are used to underscore the role of the nine-fold coordination of the Eu³⁺ / Sr²⁺ cations as a "mixing" the twelve-fold coordination in EuNiO₃ perovskite and the six-fold one of Sr²⁺ in rock structure of SrO. By increasing the Sr²⁺ amount, the structure will be stabilized because the bond distances of the two parts of the nine-fold coordination tend to become appropriate.