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.

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High quality crednerite CuMnO 2 was prepared by solid state reaction at 950°C under argon flow. The oxide crystallizes in a monoclinically distorted delafossite structure associated to the static Jahn-Teller (J-T) effect of Mn 3? ion. Thermal analysis showed that it converts reversibly to spinel Cu x Mn 3-x O 4 at *420°C in air and further heating reform the crednerite above 940°C. CuMnO 2 is p-type, narrow semiconductor band gap with a direct optical gap of 1.31 eV. It exhibits a long-term chemical stability in basic medium (KOH 0.5 M), the semi logarithmic plot gave an exchange current density of 0.2 lA cm -2 and a corrosion potential of *-0.1 V SCE . The electrochemical oxygen insertion/desinsertion is evidenced from the intensity-potential characteristics. The flat band potential (V fb = -0.26 V SCE ) and the holes density (N A = 5.12 9 10 18 cm -3 ) were determined, respectively, by extrapolating the curve C -2 versus the potential to the intersection with C -2 = 0 and from the slope of the MottSchottky plot. From photoelectrochemical measurements, the valence band formed from Cu-3d wave function is positioned at 5.24 ± 0.02 eV below vacuum. The Nyquist representation shows straight line in the high frequency range with an angle of 65°ascribed to Warburg impedance originating from oxygen intercalation and compatible with a system under mass transfer control. The electrochemical junction is modeled by an equivalent electrical circuit thanks to the Randles model.

SrSnO 3-d , prepared in sealed ampoules, crystallizes in the perovskite structure. The band gap is directly allowed at 3.93 eV. The conductivity was found to change markedly and occurs by polaron hopping with activation energy of 0.22 eV. The thermal variation of the thermopower indicates an electron mobility µ e 300K = 3.15×10 -6 cm 2 ×V -1 ×s -1 ), thermally activated. The capacitance measurement shows a linear behavior from which a flat band potential of -0.20 V SCE and an electronic density of 5.56×10 18 cm -3 were determined. The conduction band edge lies below the H 2 O/H 2 level. Accordingly, SrSnO 3-d can be used for water photoreduction when combined with the delafossite CuFeO 2 as sensitizer.The development of optically active semiconductors (SC) is an emerging field of investigation for enabling photovoltaic and photoelectrochemical (PEC) conversions [1]. The stannates ASnO 3 , in which A is an alkaline earth, have gained an increasing interest owing to their technological applications as light-to-electrical and/or chemical energy conversion [2], chemical sensors [3], stable capacitors [4], and more recently water photoelectrolysis [5]. Some strategies to improve the efficiency of PEC devices have been attempted. One of them relies on the splitting between the bands at reduced cations like Sn 2+ or Sb 3+ [6]. These ions have outer ns 2 electrons at energy below the O 2 /H 2 O level in aqueous solutions, and oxygen evolution from water by solar light might be possible. In ASnO 3 , the top of the valence band (VB) is made up of the O 2-2p orbital, whereas the conduction band (CB) is formed mainly by the Sn5s level separated by a forbidden band (E g ) exceeding 3 eV [7]. SrSnO 3 is a white insulator that possesses interesting properties many of which are associated with vacancies, structural defects, or other imperfections. It occurs mostly as n-type material by doping with non-isovalent cations such as rare earth or elements of group V B respectively onto A or Sn sites [8] or through oxygen deficiency by heating under moderate reducing atmosphere [9]. Such processes enhance the spectral response to longer wavelengths and induce a concomitant reduction Sn 4+/2+ . The color turns to gray and one may expect the same origin and mechanism of conductivity. However, whereas little physical characterization of SrSnO 3 has been done [10], to our knowledge no previous investigations concerning the PEC properties have been reported. The presence of tin mixed valences is responsible for enhanced transport properties in deficient stannates, and this paper aims at investigating the electrical properties and PEC characterization of SrSnO 3-d . The main physical parameters, the band energy gap (E g ), the flat band potential (V fb ), and the ionized donor density (N D ), are determined on the basis of absorption spectra and PEC measurements. 1720040-5760/09/4503-0172