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

Using the thermogravimetric analysis measurements the effective oxygen content 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 magnitoresistive transition temperatures increase intially with increasing effective oxygen contents but decrease for larger amounts of d > 0.008. Magnetic moment per Mn decreases with increasing d in agreement with the hole doping given by h = x + 2d, Structural and physical properties are consistent with the defect model of vacancies on the (La, Ca) and Mn sites in equal number, v similar to d/(3 + d). Results indicate that a small vacancy concentration on metal sites, v < 0.003, does not produce major structural or electronic effects other than the effective hole doping that increases T-c. For larger vacancy concentrations, scattering on defects may be a cause of observed decrease and broadening of T-c. (C) 1999 Academic Press.