The solid solution Sr_2−xLa_xSiO_4−xN_x (0 ≥ x ≥ 1), obtained by concomitant substitution of Sr²⁺ by La³⁺ and O²⁻ by N³⁻ in Sr₂SiO₄, has been prepared by solid state reaction at 1500 °C in N₂/H₂ of mixtures of SrO, La₂O₃, SiO₂ and Si₃N₄. Synchrotron radiation X-ray diffraction studies show a transition from the monoclinic P2₁/n symmetry of β-Sr₂SiO₄ to orthorhombic Pmnb α′,β-K₂SO₄type phase for x ≥ 0.2. The α′ phase is stabilized as a consequence of the increase in bond valence of the cations induced by nitriding. The activation with Eu²⁺ and Ce³⁺ of Sr_2−xLa_xSiO_4−xN_x produces luminescent materials with broad emission bands centered up to 650 nm under excitation at 405 nm. The emission wavelengths are tuned by La/N contents in a wide range with emission colours changing from yellow (x = 0.2) to orange-red (x = 1) for Eu²⁺phosphors and from blue-green (x = 0.2) to orange-yellow (x = 1) in Ce³⁺ samples.