Effective oxygen content and properties of La0.74Ca0.26MnO3+d as a function of synthesis conditions

Using the thermogravimetric analysis measurements the effective oxygen cont ent was measured for La0.74Ca0.26MnO3+d samples synthesized at high oxygen pressure. All samples were single-phase with orthorhombic, Pbnm, structure for the entire range of effective oxygen contents achieved, 0 less than or equal to 3 + d less than or equal to 3.066. The unit-cell volume decreases with increasing d, confirming that the holes are doped uniformly. The magni toresistive transition temperatures increase intially with increasing effec tive oxygen contents but decrease for larger amounts of d > 0.008. Magnetic moment per Mn decreases with increasing d in agreement with the hole dopin g given by h = x + 2d, Structural and physical properties are consistent wi th the defect model of vacancies on the (La, Ca) and Mn sites in equal numb er, v similar to d/(3 + d). Results indicate that a small vacancy concentra tion on metal sites, v < 0.003, does not produce major structural or electr onic effects other than the effective hole doping that increases T-c. For l arger vacancy concentrations, scattering on defects may be a cause of obser ved decrease and broadening of T-c. (C) 1999 Academic Press.