Our results suggest that isovalent Bi substitution on the Y site (Bi Y ) is preferred under Y-poor/Bi-rich conditions (Figure ), which is the expected behavior for Bi doping. In this case, no additional charge carriers (electrons or holes) are created, and the equilibrium Fermi energy (E F,eq ) is pinned in the gap such that Y 2 Te 3 is insulating. Under Y-rich/Bi-poor conditions on the other hand, Bi substitution on Te (Bi Te ) becomes energetically favorable. The tendency of Bi to substitute on chalcogen sites is not uncommon; for instance, Bi/Te and Bi/Se anti-site defects are found in Bi 2 Te 3 and Bi 2 Se 3 , respectively, and Bi-doping on the Se site has been able to achieve n-type SnSe. Interestingly, in Y 2 Te 3 , Bi Te is an amphoteric defect i.e., it acts as a donor when E F is close to the VB and an acceptor when E F is close to the CB. Under Y-rich/Bi-poor conditions, the equilibrium E F is closer to the CB where Bi Te is an acceptor defect (Figure ). Due to the high formation energy of Bi Te , the dopant does not generate compensating holes significantly, and we therefore predict negligible change in the free electron concentration.