Low-temperature synthesis of SrAl2O4 by a modified sol-gel route: XRD and Raman characterization

Among other alkaline-earth aluminates, the monoclinic (M) polymorph of SrAl₂O₄ can be used as host material for Eu²⁺ luminescence based phosphors. With the aim of reducing the synthesis temperature of this polymorph, we have produced and characterized by XRD and Raman scattering solid solutions of the SrAl_2−xB_xO₄ system (x?0.3) obtained by two different methods, a ceramic route and a modified sol-gel synthesis. Though the addition of boron lowers the temperature of obtention of the M polymorph in both type of samples, lower B contents are needed to stabilize the M form as single phase for samples prepared by the sol-gel method than through the ceramic route. In the sol-gel method, the M polymorph can be obtained at temperatures as low as 1200 °C, with a Boron content of just 1%. Rietveld profile analysis allows us to conclude that coexistence of the monoclinic and hexagonal polymorphs of SrAl₂O₄ occurs for samples synthesized below an onset temperature of about 1000-1100 °C, that depends on the sample composition. Above those temperatures, only the monoclinic phase is formed.