About fifteen elements in the periodic table skip certain valences in all the compounds they form. This is understood to arise from a negative effective intra-atomic interaction. A model with such interactions as well as interatomic repulsions gives insulating behavior near half-filling and superconductivity farther from it, in agreement with the properties of BaPbₓ{Bi}_{1{{-}}x}O₃ and Baₓ{K}_{1{{-}}x}BiO₃.
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Chandra Mohan Varma (1988) studied this question.
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