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

Nonstoichiometric phases of the compound La2NiO4 have been synthesized and chemically characterized. Their defect chemistry involves interst itial oxygens, oxygen vacancies, and lanthanum vacancies. The nonstoic hiometric phases follow the general formula La2-xNiO4+delta, with 0 le ss-than-or-equal-to x less-than-or-equal-to 0.15 and delta greater-tha n-or-equal-to 0 or delta < 0. The electrical properties of these compo unds 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 mea sured under isobaric conditions shows an upturn which for a long time has been interpreted as an indication of a semiconductor-to-metal tran sition. Careful measurements of the sample weight in this temperature range show, in fact, that the oxide does not maintain constant composi tion. 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 ra nge. (C) 1994 Academic Press, Inc.