Metal–Insulator Transition and Variations of Crystal Structures and Valence States of Cr and Ni Ions in LaCr1—xNixO3

A series of compounds LaCr 1-x Ni x O 3 (x = 0 to 1) with perovskite structure are synthesized by a citrate process and the crystal structures of these compounds are determined. A phase transition in this series of compounds occurs in the region of x = 0.6 to 0.7. The valence states of chromium and nickel ions in LaCrO 3 , LaCr 0.25 Ni 0.75 O 3 and LaNiO 3 are calculated using a tight-binding approximation based on the EHMO method. The results indicate that both chromium in LaCrO 3 and nickel in LaNiO 3 are positive trivalent. However, the valence state of the chromium ion will be larger than 3+, while the valence state of the nickel ion will be smaller than 3+ in Cr-doped LaNiO 3 . Within a Zaanen-Sawatzky-Allen framework, the relationship between the metal-insulator transition and variations of crystal structure, valence states and the electronic configurations of chromium and nickel ions in the Cr-doped LaNiO 3 are discussed. It may be assumed that the metal-insulator transition in the compounds LaCr 1-x Ni x O 3 is related not only to a variation of crystal structure, but also more directly to a variation of the valence state of the nickel ion and of the local coordination environment of the nickel ion in these compounds.