Complex effect of partial substitution of La3+ by Ca2+ on the stability of fast oxide-ion conductor La2Mo2O9

摘要

Demixing/recombination and metastability phenomena are observed in the La2-2xCa2x Mo2O9-x solid solutions of LAMOX-type fast oxide-ion conductors (0.005 <= x <= 0.05). The demixing process is more visible at high Ca content. The effects of composition, sample shape (powders with two degrees of fineness, ceramic pellets, single crystals) and thermal history are studied by using high-temperature X-ray diffraction, differential scanning calorimetry (DSC) and polarized microscopy. All these parameters are shown to have an influence on the stability of, and transformations between, the ordered a and the disordered P forms of the LAMOX compounds. Powder samples with low Ca content (x = 0.01, 0.02) have been stabilized in the a form, and powder samples with higher Ca content (x = 0.03, 0.04, 0.05) are in the P form. Upon heating, the low-Ca-content compounds (x = 0.01 and x = 0.02) exhibit the alpha/beta transition, whereas those with higher Ca content (x = 0.03 and x = 0.04) show a succession of beta/alpha/beta phase transformations (metastability of the beta form). The compound with the highest Ca content (x = 0.05) remains in the P form over the whole thermal range. Upon cooling, all samples exhibit the beta/alpha phase transition. When x = 0.04, powder annealing suppresses all the phase transformations upon heating, but not the beta/alpha transition upon cooling, whereas sample shaping/sintering as a pellet stabilizes the beta form over the whole thermal range upon heating and cooling. Single crystals with compositions x = 0.005 and x = 0.025 can be obtained either in the a or in the form by slow cooling or by quenching, respectively. In these oxide-ion conductors with a high concentration of point defects, internal or external stress appears to strongly affect the vacancy equilibrium but not the demixing/recombination phenomenon.