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Tellurium-based phase-change materials (PCMs) enable applications from optical and electronic data storage to thermoelectrics and plasmonics, which all demand precise control of electronic properties. These materials contain an unusually large number of vacancies: “stoichiometric” ones that stem from the chemical composition and “excess” vacancies that act like classical dopants. Here we show how both types of vacancies can be controlled independentlyin the solid solution Sn(Sb1–xBix)2Te4. We vary xin small steps over the entire compositional range and show that this has a profound effect on the material’s electronic nature: remarkably, we observe a change from p- to n-type conduction at x≈ 0.7, solely controlled by composition. Our findings lead to a new compositionally (that is, chemically) tunable materials platform that enables precise control of electrical properties.