Oxygen content and structures of La1-xCaxMnO3+d as a function of synthesis conditions

Using thermogravimetric analysis measurements, oxygen 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 oxygen pressure. The oxygen content was found to decrease uniformly with increasing x for samples annealed at 12 and 140 atm O-2 at 800 degrees C, slowly tooled in 1, 20, and 100% O-2 as well as for samples in thermal equilibrium at high temperatures of similar to 1000-1400 degrees C, Annealing at a specific temperature and oxygen 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<root 2) and O* (a similar to b similar to cl root 2) structures, the rhombohedral R (3) over bar c structure is stabilized at high oxygen 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. (C) 1999 Academic Press.