Oxygen isotope effect inLa0.8Ca0.2MnO3+δand its dependence on oxygen stoichiometry

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The oxygen isotope effect of the ferromagnetic transition temperature was investigated in various samples of the manganite ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3+\ensuremath{\delta}},$ as a function of sintering conditions. Sintering at 1200 \ifmmode^\circ\else\textdegree\fi{}C and above leads to oxygen-stoichiometric comparison samples with an isotope exponent of ${\ensuremath{\alpha}}_{\mathrm{O}}=0.41\ifmmode\pm\else\textpm\fi{}0.02,$ and transition temperatures ${T}_{C}(16)=189\ifmmode\pm\else\textpm\fi{}2 \mathrm{K}$ and ${T}_{C}(18)=180\ifmmode\pm\else\textpm\fi{}2 \mathrm{K}.$ Samples sintered at 1000 \ifmmode^\circ\else\textdegree\fi{}C, and below, always have oxygen excess. The oxygen excess leads to vacancies at the $A(\mathrm{L}\mathrm{a},\mathrm{C}\mathrm{a})$ and $B(\mathrm{Mn})$ sites, leading to a decrease in unit cell volume. The unit cell volume is identical for both isotopes, and this implies equal oxygen content for the isotopic comparison samples. The isotope exponent ${\ensuremath{\alpha}}_{\mathrm{O}}$ for oxygen excess samples is much larger than 0.4. Its value, as well as the transition temperatures ${T}_{C},$ depends on the ${\mathrm{Mn}}^{4+}$ concentration, as well as the defect structure induced by A and B sites vacancies.