Chalcogenide alloys are usually p-type semiconductors. In GeTe, it was recently argued that p-type conductivity results from vacancies on Ge sites. In this work, we demonstrate that the creation of vacancies in GeTe, in which the co-existence of conventional covalent and dative bonds utilising the Te lone-pair (LP) electrons makes Ge(3):Te(3) bonding geometry possible, results in the formation of a triad of twofold coordinated Te atoms. Because of the different nature of bonding (conventional covalent vs. dative), only one of the three Te atoms naturally possesses LP electrons after rupture of the Ge-Te bonds. As a result of electron redistribution, three twofold coordinated Te atoms with LP electrons and concomitantly holes in the valence band are generated, which provides a natural explanation of p-type conductivity. We argue that a similar mechanism may be also operative in S- and Se-based alloys, providing a general explanation of p-type conductivity in chalcogenides.
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