This is the first report to investigate on the electrical conductivity of unusual high valence state pentavalent chromium oxides in terms of defect thermodynamics. Calcium chromium(V) apatite was prepared as a single phase, and the electrical conductivity was examined as a function of oxygen partial pressure p O2 . The conductivity was increased at higher p O2 in wet atmosphere but it was not dependent on p O2 in dry atmosphere. The water concentration cell gives stable electromotive force. The features disclosed that the chromium(V) apatite reveals proton electron mixed conductivity under wet oxidative condition.The apatite is a group of rare earth or alkaline earth salts of tetrahedral MO 4 oxoanions (M = P 5+ , Si 4+ , Ge 4+ , As 5+ etc.). Early studies have revealed that there are many interesting characteristics in these materials. The most important one, however, is their oxide ionic conductivity. Apatite oxide ion conductors (rare earth silicates) were first found by Nakayama et al. in 1995. 1,2 They show fairly high oxide ionic conductivity in an intermediate temperature range of 500-800 • C. 3 In addition, they have ionic transport numbers which are very close to 1.0 with a wide oxygen partial pressure range, 4 thus being considered as candidates for intermediate temperature solid oxide fuel cell (IT-SOFC) electrolytes.Typical apatite-type oxides tend to be electronically insulator because of the nature of the constituent cations with closedshell configuration. Meanwhile, apatite-type calcium chromate(V), Ca 5 (CrO 4 ) 3 (OH), comprising from Cr V O 4 3− oxoanions are reported to reveal semiconductor characteristics at elevated temperature because of the mobile electrons derived from d 1 configuration Cr V ions. 5 Hence, it is reasonably expected that this compound retains superimposed electronic conduction and ionic conductivity at intermediate temperatures. Such a material is promising for the cathode/anode materials of fuel cells, gas separation membranes and so on. 6,7 In this study, we investigate on the electrical conduction behavior of apatitetype calcium chromate(V) under various oxygen partial pressure in order to explore the feasibility of mixed ion electron conductivity in apatite phase.
ExperimentalApatite-type calcium chromate was prepared by thermal decomposition of Ca II -Cr VI mixed precursors. 8,9 Ca(CH 3 COO) 2 (99.9% purity, Kanto) and CrO 3 (99.9% purity, Kanto) powders were dissolved into Milli-Q water at a stoichiometric Ca/Cr molar ratio. The solution were heated at 140 • C in order to evaporate water and thus, the Ca(II)-Cr(VI) mixed precursor powders were obtained. The precursors were once heated at 500 • C for 1 h in air and then annealed at 860 • C for 6 h in air and quenched at room temperature. Finally, the products were annealed at 930 • C in a wet Ar atmosphere (p H2O = 3 kPa) prepared by bubbling in water at 25 • C, giving desired apatite compound. The dark green powder thus obtained was examined by X-ray diffractometry (XRD) to check the phase purity. Scanning electron m...