Structural Phase Transformation and Luminescent Properties of Ca(2-x)SrxSiO4:Ce3+ Orthosilicate Phosphors

The orthosilicate phosphors demonstrate great potential in the field of solid-state lighting, and the understanding of the structure-property relationships depending on their versatile polymorphs and chemical compositions is highly desirable. Here we report the structural phase transformation of Ca(2-x)SrxSiO4:Ce(3+) phosphor by Sr(2+) substituting for Ca(2+) within 0 ≤ x < 2. The crystal structures of Ca(2-x)SrxSiO4:Ce(3+) are divided into two groups, namely, β phase (0 ≤ x < 0.15) and α' phase (0.18 ≤ x < 2), and the phase transition (β → α') mechanism originated from the controlled chemical compositions is revealed. Our findings verified that the phase transition Pnma (α'-phase) ↔ P21/n (β-phase) can be ascribed to the second-order type, and Sr(2+) ions in Ca(2-x)SrxSiO4 preferentially occupy the seven-coordinated Ca(2+) sites rather than the eight-coordinated sites with increasing Sr(2+) content, which was reflected from the Rietveld refinements and further clarified through the difference of the Ca-O bond length in the two polymorphs of Ca2SiO4. The emission peaks of Ce(3+) shift from 417 to 433 nm in the composition range of 0 ≤ x ≤ 0.8, and the difference in the decay curves can also verify the phase transformation process. Thermal quenching properties of selected Ca(2-x)SrxSiO4:Ce(3+) samples were evaluated, and the results show that the integral emission intensities at 200 °C maintain >90% of that at room temperature suggesting superior properties for the application as white light-emitting diodes (w-LEDs) phosphors.