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Vacancy and antisite defect formation energies in D0₁₉-SnTi₃ are calculated by an ab initio approach. Based on a statistical-thermodynamic model, the defect concentrations are calculated as function of temperature and deviation from stoichiometry. The constitutional defects are antisite Sn atoms and antisite Ti atoms in Sn-rich and Ti-rich SnTi₃, respectively. The dominant thermal defects are composed of two antisites for stoichiometric alloys. For Sn-rich D0₁₉-SnTi₃ alloys, the thermal defect is an interbranch where one antisite Sn atom is replaced by four Ti vacancies. For Ti-rich D0₁₉-SnTi₃ alloys, the thermal defect is a five point defect comprising one antisite Ti and four Ti vacancies. The effective defect formation enthalpies are derived for different concentration regions of D0₁₉-SnTi₃. The Gibbs energy as well as the Sn and Ti chemical potentials in the D0₁₉-SnTi₃ are obtained as function of composition for various temperatures. The extension of the one phase domain of D0₁₉-SnTi₃ in the Sn–Ti phase diagram is discussed.