MODEL FOR THE INSULATING BEHAVIOR OF PB-DOPED OR K-DOPED BABIO3

A simple model is presented to describe the nonmetallic behavior obser ved over wide doping ranges of perovskite-type BaBi1-xPbxO3 and Ba1-xK xBiO3 alloys. The model is based on the idea that the oxygen octahedra endure static distortion waves of a ''breathing'' type, with wave vec tors Q(x) nesting to the respective alloy Fermi surfaces as obtained f rom the energy bands of a virtual-crystal band model. For BaBi1-xPbxO3 , Bi-Pb ordering waves are added by placing the Bi atoms onto those oc tahedral sites that exhibit the largest breathing distortions. Using a simple tight-binding Hamiltonian based on realistic energy-band calcu lations, the model has been studied on large supercells, testing a mul titude of lattice configurations. Experimental consequences are discus sed.