Effective Oxygen Content and Properties of La0.74Ca0.26MnO3+das a Function of Synthesis Conditions

Using the thermogravimetric analysis measurements the effective oxygen content was measured for La0.74Ca0.26MnO3+dsamples synthesized at high oxygen pressure. All samples were single-phase with orthorhombic,Pbnm, structure for the entire range of effective oxygen contents achieved, 0<3+d<3.066. The unit-cell volume decreases with increasingd, confirming that the holes are doped uniformly. The magnitoresistive transition temperatures increase intially with increasing effective oxygen contents but decrease for larger amounts ofd>0.008. Magnetic moment per Mn decreases with increasingdin agreement with the hole doping given byh=x+2d. Structural and physical properties are consistent with the defect model of vacancies on the (La, Ca) and Mn sites in equal number,NU~d/(3+d). Results indicate that a small vacancy concentration on metal sites,NU<0.003, does not produce major structural or electronic effects other than the effective hole doping that increasesTc. For larger vacancy concentrations, scattering on defects may be a cause of observed decrease and broadening ofTc.