CONTINUOUS ORDER-DISORDER TRANSITION IN LI3NI2NBO6 AND CR-DOPED LI3NI2NBO6 ROCK-SALT STRUCTURES

Li3Ni2NbO6 has a partially ordered rock salt superstructure in which N b occupies fully one set: of octahedral sites and Li/Ni are distribute d nonrandomly over three other sets of octahedral sites (Mather et at. , J. Mater. Chem. 5, 1177 (1995)). Over the temperature range 25-1100 degrees C the Li/Ni site occupancy distribution is largely unchanged. Above 1100 degrees C, Nb increasingly occupies the Li/Ni sites until, above 1300 degrees C, the cations are completely disordered. The trans ition appears to be fully continuous, with no evidence of a transition enthalpy by DTA, Cr3+ ions are able to substitute partially into both ordered and disordered structures by a range of mechanisms. The tempe rature range of the order-disorder transition decreases with Cr conten t, as shown in the phase diagram of the vacancy solid solutions, Li3-x Ni2-xCrxNbO6 (0 < x < 0.18). The materials are modest, semiconductors. (C) 1996 Academic Press, Inc.