Turchenko V. Deficiency of structure, phase transitions and magnetoresistive properties of Ca2+ and Sr2+ doped manganite - lanthanum perovskites

The dissertation is devoted to definition of defect creation mechanism and influences of defects (cation and anion vacancies) on structural features, transport and magnetic properties of ceramic manganite - lanthanum perovskites. The lanthanum - calcium and lanthanum - strontium perovskites were prepared by ceramic method at a wide temperature range (900 - 1500 С). Ceramic samples were investigated by X-ray diffractin, pycnometric, optical and electron scanning microscope, magnetic, 55Mn NMR, resistive and magnetoresistive methods. It was determined that the real structure of manganite - lanthanum perovskites La1-xMx2+MnO3±delta (M2+= Ca2+ і Sr2+) contains anion and cation vacancies and electrical neutrality is attained by two mechanisms: transition Mn3+ -> Mn4+ and development of anion vacancies. The influence of anion vacancies concentration on value of thermal expansion factor was shown. Essential differences in temperatures of "metal-semiconductor" Tms and "ferro-paramagnetic" Tc phase transitions in Ca2 + and Sr2 + doped manganite - lanthanum perovskites were explained not only by differences of their ionic radii, but also by different oxygen nonstoichiometry and deficiency of structure. Concentration changes of activation energy correlate with changes in Mn3+/Mn4+ ratio and deficiencies of the crystal lattice.