METAL-INSULATOR-TRANSITION AND VARIATIONS OF CRYSTAL-STRUCTURES AND VALENCE STATES OF CR AND NI IONS IN LACR1-XNIXO3

A series of compounds LaCr1-xNixO3 (x = 0 to 1) with perovskite struct ure 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 o f chromium and nickel ions in LaCrO3, LaCr0.25Ni0.75O3 and LaNiO3 are calculated using a tight-binding approximation based on the EHMO metho d. The results indicate that both chromium in LaCrO3 and nickel in LaN iO3 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 Or-doped LaNiO3. Within a Zaanen-Sawatzky- Allen framework, the relationship between the metal-insulator transiti on and variations of crystal structure, valence states and the electro nic configurations of chromium and nickel ions in the Or-doped LaNiO3 are discussed. It may be assumed that the metal-insulator transition i n the compounds LaCr1-xNixO3 is related not only to a variation of cry stal structure, but also more directly to a variation of the valence s tate of the nickel ion and of the local coordination environment of th e nickel ion in these compounds.