The competitive mechanisms of nano-SiO2 and reaction temperature on phase transformation and Eu2+ site occupation in Sr2SiO4:Eu2+ phosphor

Abstract

Sr2SiO4:Eu2+, which could be used as phosphor individually, employed as precursor to synthesize (oxy)nitride phosphors, or applied as basic structure model to tune new ones, is extremely useful. However, the phase transformation is inborn in its synthesis. Herein, the role of silicon source, reaction temperature and firing time on phase transformation, site occupation of Eu2+ activator, and energy transfer within different centers in Sr2SiO4:Eu2+ were investigated. The results show that both the increase of reaction temperature within 1150–1250 °C and the increase of nano-SiO2 concentration within 0–70% promote phase transition from high-temperature α′-Sr2SiO4 to low-temperature β-Sr2SiO4. Once temperature reaches up to 1300 °C, the relative content of α′-Sr2SiO4 intensifies sharply but brings with a by-product Sr3SiO5. Nevertheless, partial substitution of large-particles SiO2 with nano-SiO2 is helpful to suppress the Sr3SiO5 impurity. More interestingly, two competitive mechanisms of reaction temperature and nano-SiO2 concentration on the site occupation of Eu2+, accompanying with emission color shift, were observed; and the energy transfer from Eu(I) to Eu(II), which depends on site occupation, along with phase transformation was confirmed. The prolonged reaction time with aid of nano-SiO2 facilitates the site shift of Eu from low-energy Sr(II) to high energy Sr(I), which contributes to phase transformation from β- to α′-Sr2SiO4.