EFFECTIVE OXYGEN CONTENT AND PROPERTIES OF La0.74Ca0.26MnO3+d AS A FUNCTION OF SYNTHESIS CONDITIONS

摘要

Using the thermogravimetric analysis measurements the effective O content was measured for La0.74Ca0.26MnO3+d samples synthesized at high O pressure. All samples were single-phase with orthorhombic, Pbnm, structure for the entire range of effective O 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 initially with increasing effective O 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, nu approx 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 increases Tc. For larger vacancy concentrations, scattering on defects may be a cause of observed decrease and broadening of Tc. 23 refs.