Instability of the Crystal and Electronic Structures due to Hydrogenation Influenced the Magnetic Properties of a La_2/3Ca_1/3MnO₃ Manganite

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We have investigated the hydrogen-annealing influence on the crystalline and electronic structures, and magnetic properties of La_2/3Ca_1/3MnO₃ (LCMO). The results have indicated that the annealing at 700 and 900°C (labeled as LCMO-700 and LCMO-900, respectively) readily reduced single-phase LCMO, resulting in a complex phase composition of several isostructural oxygen-deficient perovskite-type phases coexisting with the Ruddlesden-Popper (La/Ca)₂MnO₄-type phase. While a Mn³⁺/Mn⁴⁺ mixed valence is present in LCMO, the hydrogen-annealed samples mainly have Mn²⁺ and Mn³⁺ ions. Under such circumstance, large changes in magnetic parameters have been recorded, such as remarkable decreases in values of the magnetization, the Curie temperature (from 251 K for the as-prepared LCMO through 240 K for LCMO-700 to ∼221 K for LCMO-900), and the magnetic-entropy change. Particularly, the crystal and electronic-structure changes also enhance the magnetic inhomogeneity, resulting in a strong development of the Griffiths phase, and cause the first-to-second-order phase transformation. These results reflect the instability of the LCMO perovskite-type manganite versus hydrogenation.

Fig. 3 K-edge XAS spectra of the fabricated samples compared with those of manganese oxides as references. The arrow shows the absorption-edge (Abs. edge) shift of Mn.