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Properties of Undoped and Ca‐Doped LaMnO 3 − Non‐Stoichiometry and Defect Structure
Author(s) -
Yahia Mondher,
Batis Habib
Publication year - 2003
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200300025
Subject(s) - chemistry , orthorhombic crystal system , stoichiometry , octahedron , oxygen , crystal structure , analytical chemistry (journal) , lanthanum , formula unit , jahn–teller effect , solid solution , inorganic chemistry , vacancy defect , crystallography , manganese , aqueous solution , ion , organic chemistry , chromatography
Oxygen‐excessive LaMnO 3+ y samples with 0.02 ⩽ y ⩽ 0.26 have been prepared by thermal decomposition in air at 450 °C of a precursor obtained from an aqueous solution of lanthanum nitrate and manganese acetate (atomic ratio La/Mn = 1). The obtained solid is annealed at different temperatures from 700 to 1000 °C and under three oxygen pressures: 1.00, 0.21, and 10 −4 atm. Samples with 0.14 ⩽ y ⩽ 0.26 are simple‐phased rhombohedral perovskites, the distortion of which decreases as the oxygen content (and Mn 4+ content) increases. Samples with 0.02 ⩽ y ⩽ 0.09, obtained under low oxygen pressure (10 −4 atm), have orthorhombic symmetry. In this case, the order of lattice parameters c /√2 < a < b , characteristic of O′‐perovskites, is a consequence of the Jahn−Teller distortion of the oxide octahedron around the d 4 Mn 4+ cation. The volume ( V f ) per LaMnO 3+ y formula unit decreases regularly with increasing oxygen content (i.e., oxidation). Study has shown that the oxygen content is best described in terms of La and Mn vacancies, rather than of interstitial anions. Density measurements confirm the cation vacancy model involving equal amounts in La and Mn sub‐lattices. The infrared spectra of LaMnO 3+ y samples have been interpreted in connection with the structures of these solids. Correlations have been established between the spectral evolution and the crystal distortion of these compounds. La 1− x Ca x MnO 3± y oxides with 0 < x ⩽ 1 have been synthesised by the same route, at 700 °C and under an oxygen pressure of 0.21 atm. It is shown that a negative correlation exists between the calcium content x and the degree of non‐stoichiometry: at low Ca content, the oxygen content is higher. The ability to form over‐stoichiometric compounds in air decreases with increasing Ca content and mostly disappears at x ⩾ 0.7. Samples with 0.1 ⩽ x ⩽ 0.4 are simple‐phased rhombohedral perovskites with constant Mn 3+ /Mn 4+ ratios. Samples with 0.5 ⩽ x ⩽ 1 have orthorhombic symmetry. In this case, a linear variation of the Mn 4+ content as a function of the Ca content is observed. The defect structure of these compounds is described in terms of La and Mn vacancies when 0 < x ⩽ 0.7 (0 ⩽ y ⩽ 0.19) and anion vacancies when 0.8 ⩽ x ⩽ 1 (−0.04 ⩽ y < 0). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)