Optical and transport properties of lanthanum-doped stannate BaSnO3

Abstract

BaSnO3 crystallizes in a cubic perovskite structure and exhibits insulating behaviour. It can be made conducting by reducing a small fraction of Sn4+ into Sn2+ under an O2-free atmosphere. This can be achieved through the solid solution Ba1−xLaxSnO3−δ which is a mixed phase for x > 0.02, behaviour readily understood in terms of the lone pair cation Sn2+. The magnetic susceptibility was measured down to 4.2 K and is less than 1.7 × 10−5 emu cgs mol−1 consistent with collective electron behaviour. The Mössbauer spectrum exhibits a wide unsplit peak with a quadrupole doublet of 3.18 mm s−1 and an isomer shift of 0.12 mm s−1 characteristic of Sn4+and corroborating the delocalization of the stereo chemical pair 5s2. The band gap Eg was found to be 3.12 eV; further indirectly allowed inter-band transition occurs at 2.85 eV. The transport properties of Ba0.98La0.02SnO3−δ indicate n-type conductivity (σ300 K = 4.03 Ω−1 cm−1), little temperature dependence, with an activation energy ΔE of ∼1 meV and an electron mobility μ300 K ∼ 0.1 cm2 V−1 s−1, thermally activated. The conduction occurs by small polaron hopping between mixed valence Sn4+/2+ ions. The observed conductivity is greater than that coming from La-substitution with one-electron doping implying the existence of oxygen vacancies. The electrons are believed to travel in the Sn-5s conduction band with an effective mass of 3.7 mo. The non-linear dependence of Ln σ versus T−1 at low temperatures could result from a predominant variable range hopping as suggested by the linear variation of log σ versus T−0.25. The electron localization may be attributed to the random distribution of lanthanum as well as oxygen vacancies.