Optical and transport properties of lanthanum-doped stannate BaSnO3

BaSnO 3 crystallizes in a cubic perovskite structure and exhibits insulating behaviour. It can be made conducting by reducing a small fraction of Sn 4+ into Sn 2+ under an O 2 -free atmosphere. This can be achieved through the solid solution Ba 1− x La x SnO 3−δ which is a mixed phase for x > 0.02, behaviour readily understood in terms of the lone pair cation Sn 2+ . 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 Sn 4+ and corroborating the delocalization of the stereo chemical pair 5s 2 . The band gap E g was found to be 3.12 eV; further indirectly allowed inter-band transition occurs at 2.85 eV. The transport properties of Ba 0.98 La 0.02 SnO 3−δ indicate n-type conductivity (σ 300 K = 4.03 Ω −1 cm −1 ), little temperature dependence, with an activation energy Δ E of ∼1 meV and an electron mobility μ 300 K ∼ 0.1 cm 2 V −1 s −1 , thermally activated. The conduction occurs by small polaron hopping between mixed valence Sn 4+/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 m o . 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.