[en] The luminescence properties of Sm²⁺-activated stoichiometric and non-stoichiometric Ba₂B₅O₉Cl were investigated from 16 to 450 K. In stoichiometric Ba₂B₅O₉Cl, the emission spectra of Sm²⁺ are composed of 4f⁶→4f⁶ transitions over the 16-450 K temperature range studied. Luminescence was observed from Sm²⁺ in four inequivalent cationic sites at 16 K and two inequivalent sites at room temperature. In the non-stoichiometric compound, the emission is characterized by 4f⁶→4f⁶ transitions at low temperature and 4f⁵5d¹→4f⁶ transitions at high temperature. The Sm²⁺ doped into the Ca²⁺ and Sr²⁺ analogues in the M₂B₅O₉Cl series shows the broad band 4f⁵5d¹→4f⁶ luminescence in both the stoichiometric and non-stoichiometric compounds due to the increased ligand field caused by the smaller ionic radii of the metal ions