Constraint theory and carrier-type reversal in Bi-Ge chalcogenide alloy glasses

Replacement of B (B=S or Se) by Bi in Ge$B_4$ produces semiconductive glasses with n-type thermoelectric power, in contrast to almost all other chalcogenide glasses, which are p type. I describe theoretically a structural model which explains why carrier-type reversal occurs for Bi replacement, but not for Sb replacement, and with constraint theory I predict quantitatively the compositional threshold for the transition without adjustable parameters. The present model explains naturally why this electrical threshold coincides with the mechanical one for disordering the Bi local coordination environment. Predictions are made for the full ternary composition dependence.