Effect of cation vacancies in the local structure and transport properties ofLaMnO3+δ:A MnK-edge x-ray-absorption study

Abstract

The effect of cation vacancies on La and Mn sites in the electronic structure of manganese perovskites has been investigated by means of Mn K-edge x-ray-absorption spectroscopy. ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}},$ with $\ensuremath{\delta}=0,$ 0.07, and 0.15 samples were studied as a function of temperature and were compared with ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ($x=0.15$ and 0.33) and ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{Mn}}_{0.95}{\mathrm{Al}}_{0.05}{\mathrm{O}}_{3}$ compounds. The x-ray-absorption near-edge structure spectra of ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}}$ samples show an electronic state for the Mn atom very similar to that found in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ compounds. Extended x-ray-absorption fine structure spectra show a static Jahn-Teller distortion of the ${\mathrm{MnO}}_{6}$ octahedra for $\ensuremath{\delta}=0,$ which is drastically reduced as \ensuremath{\delta} is increased. As it occurs in ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ magnetoresistive compound, for $\ensuremath{\delta}=0.07$ and $\ensuremath{\delta}=0.15$ samples, the first oxygen coordination shell in the paramagnetic phase can be interpreted either in terms of a large Debye-Waller factor or in terms of a local distortion (0.16 and 0.1 \AA{}, respectively). Otherwise, this distortion remains in the low-temperature phase for both compounds and can be correlated to the absence of metallic behavior.