OXYGEN CONTENT AND STRUCTURES OF La1-xCaxMnO3+d AS A FUNCTION OF SYNTHESIS CONDITIONS

摘要

Using TGA measurements, O content was measured for La1-xCaxMnO3+d (0 less than or equal to x less than or equal to 0.38) samples prepared at high O pressure. The O content was found to decrease uniformly with increasing x for samples annealed at 12 and 140 atm O2 at 800 C, slowly cooled in 1, 20, and 100% O2, as well as for samples in thermal equilibrium at high temperatures of approx 1000-1400 C. Annealing at a specific temperature and O pressure leads to approximately constant hole doping over an extended range of Ca substitution x. The most favorable synthesis conditions for obtaining stoichiometric, d=0, samples were determined. The data suggest that several studies reported in the literature were done on significantly nonstoichiometric materials. Structural results show that, in addition to the orthorhombic Pbnm O' (c/a< square root of 2) and O* (a approx b approx c/square root of 2) structures, the rhombohedral R-3c structure is stabilized at high O contents for x less than or equal to 0.14. A sequence of observed structures and a decrease of the unit cell volume with increasing d confirm that the additional holes are doped uniformly by cation-vacancy defects for all studied x. 42 refs.