Oxygen content and structures of La[sub 1-x]Ca[sub x]MnO[sub 3+d] as a function of synthesis conditions

Using thermogravimetric analysis measurements, oxygen content was measured for La[sub 1[minus]x]Ca[sub x]MnO[sub 3+d] (0 [le] x [le] 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[sub 2] at 800 C, slowly cooled in 1, 20, and 100% O[sub 2], as well as for samples in thermal equilibrium at high temperatures of [approximately]1000--1400 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[prime] (cla < [radical]2) and O* (a [approximately] b [approximately] c/[radical]2) structures, the rhombohedral R[bar 3]c structure is stabilized at high oxygen contents for x [le] 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.