Crystallographic characterization and NO₂ gas sensing property of LnFeO₃ prepared by thermal decomposition of Ln-Fe hexacyanocomplexes, Ln[Fe(CN)₆]·nH₂O, Ln = La, Nd, Sm, Gd, and Dy

Finer perovskite-type LnFeO₃ (Ln = La, Nd, Sm, Gd, and Dy) powders were prepared by the thermal decomposition of heteronuclear complexes, Ln[Fe(CN)₆]·nH₂O. The prepared LnFeO₃ showed a p-type semiconductive behavior and the highest enhancement of conductance due to NO₂ exposure was observed for SmFeO₃ sensor. The atomic ratio of adsorbed oxygen O_ad increased with the surface coverage of Ln, expressed as Ln/(Ln + Fe). Experimentally, the atomic ratio of Ln/(Ln + Fe) was estimated to around 0.6 and the largest value, 0.65, was observed for SmFeO₃. For SmFeO₃, the distance between the central Sm ion and nearest four ions of oxygen is comparable with the sum of crystal radius of Sm³⁺ (C.N. = 6) and O^2-, and the distance between Sm ion and fifth and higher oxygen is longer than the expected length form Shannon's crystal radius. The observed longer length for C.N. = 5 or more is comparable to the length for Sm²⁺-O bond. The possible valence of Ln ion is directly related with the electron configuration of Ln species and only for Sm the existence of the divalent cation is expected. The divalency of the Sm³⁺ in the surface layer is suggested by the configurations of coordinated oxygen. The highest sensitivity for NO₂ observed for SmFeO₃ would be attributed to the formation of Fe defects due to the higher coverage of Ln and the divalency of Sm³⁺ in SmFeO₃.