Emission colour tuning through coupled N/La introduction in Sr2SiO4:Eu2+
The solid solution Sr2−xLaxSiO4−xNx (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-K2SO4 type 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 Sr2−xLaxSiO4−xNx 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.