Journal of Solid State Chemistry 144, 461 } 466 (1999) Article ID jssc.1999.8210, available online at http://www.idealibrary.com on Effective Oxygen Content and Properties of La 0.74 Ca 0.26 MnO 3d as a Function of Synthesis Conditions B. Dabrowski, P. W. Klamut, Z. Bukowski, R. Dybzinski, and J. E. Siewenie Department of Physics, Northern Illinois University, DeKalb, Illinois 60115 Received November 10, 1998; in revised form January 26, 1999; accepted January 27, 1999 Using the thermogravimetric analysis measurements the e4ec- tive oxygen content was measured for La 0.74 Ca 0.26 MnO 3 d sam- ples synthesized at high oxygen pressure. All samples were single-phase with orthorhombic, Pbnm, structure for the entire range of e4ective oxygen contents achieved, 043  d43.066. The unit-cell volume decreases with increasing d, con5rming that the holes are doped uniformly. The magnitoresistive transition temperatures increase intially with increasing e4ective 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, *&d/(3  d). Results indicate that a small vacancy concentration on metal sites, *(0.003, does not produce major structural or electronic e4ects other than the e4ective hole doping that increases T  . For larger vacancy concentrations, scattering on defects may be a cause of observed decrease and broadening of T  .  1999 Academic Press INTRODUCTION Substituted LaMnO  materials are of considerable cur- rent interest owing to a large decrease of resistivity in a magnetic "eld near the Curie temperature, ¹  (1). Classical theoretical models predict that the insulating state above ¹  results from the spin scattering of e  electrons (2, 3). Recent theoretical and experimental work has shown that addi- tional localization may arise from a strong Jahn}Teller (JT) electron}phonon coupling that produces large oxygen dis- placements above ¹  (4}10). These JT displacements are parallel to the Mn}O bonds and undergo considerable reduction below ¹  for La  Ca  MnO  samples near x"0.22 and 0.25 (11, 12). The strong dependence of resistiv- ity, ¹  , and structural properties on the average size, and the variance of sizes of the substituted alkaline and rare-earth ions in LaMnO  support a view of the powerful JT-elec- tron}phonon coupling (13, 14). Also at the Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wroclaw, Poland. The chemical stoichiometry that is a function of the synthesis conditions can also a!ect properties for lightly substituted samples, x40.15 (15}17). Research on pure and lightly substituted La  A  MnO  has shown that during synthesis under oxidizing conditions, large deviations from nominal stoichiometry are caused by the formation of va- cancies on the metal sites A(La, Ca, etc.) and B(Mn) while the oxygen network remains undefected (16, 17). Using den- sity measurements, structural studies by high-resolution electron microscopy and neutron powder di!raction, and thermodynamic considerations of defect models it was shown that vacancies on the A and B sites form in approx- imately equal amounts; i.e., v&v  &v  &d/(3#d) (15, 17, 18). These results were supported by the observations that no impurity phases appear after synthesis in air or oxygen (11, 12, 15}20). Most of the recent work on the moderately substituted materials, 0.15(x(0.30, does not seem to delibrate on the variable chemical stoichometry since variety of synthesis conditions were employed ranging from annealing at high oxygen pressure at 6503C to annealing in air or argon at 14003C followed by quenching or furnace cooling (19}21). No variation, or small variation of properties, was reported, but no quantitative measure of defects was given (20). Con- sequently, it is generally believed that samples near x"0.3 always form stoichiometric (12, 20). We have recently shown that contrary to earlier assertions considerable nonstoichio- metry and widely varying resistive and magnetic properties can be achieved under various synthesis conditions for La  Ca  MnO  samples with x"0.22 (11). Since the phase diagrams are complex and very sensitive to hole concentra- tion, it is, thus, of critical importance to report the proper material stoichiometry. Formation of vacant site defects in addition to a signi"cant increase of the average Mn oxida- tion level, h"x#2d&x#6v, may also introduce large local oxygen displacements. Vacant Mn sites and oxygen displacements, located on the electronically active Mn}O network, can yield charge localization and other e!ects that may be incorrectly interpreted as resulting from the elec- tron}phonon interactions. 461 0022-4596/99 $30.00 Copyright  1999 by Academic Press All rights of reproduction in any form reserved.