THE SEMICONDUCTOR-TO-METAL TRANSITION IN QUESTION IN LA2-XNIO4+DELTA (DELTA-GREATER-THAN-0 OR DELTA-LESS-THAN-0)

Nonstoichiometric phases of the compound La2NiO4 have been synthesized and chemically characterized. Their defect chemistry involves interstitial oxygens, oxygen vacancies, and lanthanum vacancies. The nonstoichiometric phases follow the general formula La2-xNiO4+delta, with 0 less-than-or-equal-to x less-than-or-equal-to 0.15 and delta greater-than-or-equal-to 0 or delta < 0. The electrical properties of these compounds have been measured between 300 and 1200 K. They are characterized below 600 K by a hopping conduction having a low and almost constant hopping energy (congruent-to 80 meV). Above 600 K, the resistivity measured under isobaric conditions shows an upturn which for a long time has been interpreted as an indication of a semiconductor-to-metal transition. Careful measurements of the sample weight in this temperature range show, in fact, that the oxide does not maintain constant composition. Taking this fact into account provides evidence suggesting that the ''transition'' to a metallic state does not take place. Instead a diffusive-type charge transport is involved in the high temperature range. (C) 1994 Academic Press, Inc.