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 Oad 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 O2−, 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.