ELECTRONIC-STRUCTURE OF TETRAGONAL BAK2-XBAXBI2O7 AND RELATED LAYER-TYPE BISMUTHATES

The electronic structure of body-centered-tetragonal BaK2BiO7, an ordered prototype of the recently synthesized perovskite-type layer compound BaK1.3Ba0.7Bi2O7, has been calculated with the use of the linear augmented-plane-wave method. In rigid-band terms, the results predict metallic properties for the entire BaK2-xBaxBi2O7 alloy series, with E(F)(x) rising with increasing x to gradually fill a pair of nearly two-dimensional (sp-sigma)* conduction bands. For compositions x > 0.8, the onset for filling the upper (sp-sigma)* subband produces a sharp increase in the density of states. The resulting value (per Bi) is about 50% greater than that for cubic Ba1-yKyBiO3 near the composition (y almost-equal-to 0.3-0.4) where T(c) (approximately 30 K) is a maximum. This suggests that a slightly increased Ba composition (x approximately 0.8) should provide more favorable conditions for possible superconductivity in these BaK2-xBaxBi2O7 alloys than those predicted for the nonsuperconducting x approximately 0.7 phase that has been prepared thus far.