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

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) sa mples prepared at high oxygen pressure. The oxygen content was found to dec rease 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 sam ples in thermal equilibrium at high temperatures of similar to 1000-1400 de grees C, Annealing at a specific temperature and oxygen pressure leads to a pproximately 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 report ed 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 rhom bohedral R (3) over bar c structure is stabilized at high oxygen contents f or x less than or equal to 0.14, A sequence of observed structures and a de crease of the unit cell volume with increasing d confirm that the additiona l holes are doped uniformly by cation-vacancy defects for all studied x. (C ) 1999 Academic Press.