Structural Phase Transformation and Luminescent Properties of Ca_2–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ Orthosilicate Phosphors

Abstract

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–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ phosphor by Sr²⁺ substituting for Ca²⁺ within 0 ≤ <i>x</i> < 2. The crystal structures of Ca_2–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ are divided into two groups, namely, β phase (0 ≤ <i>x</i> < 0.15) and α′ phase (0.18 ≤ <i>x</i> < 2), and the phase transition (β → α′) mechanism originated from the controlled chemical compositions is revealed. Our findings verified that the phase transition <i>Pnma</i> (α′-phase) ↔ <i>P</i>2₁/<i>n</i> (β-phase) can be ascribed to the second-order type, and Sr²⁺ ions in Ca_2–<i>x</i>Sr_<i>x</i>SiO₄ preferentially occupy the seven-coordinated Ca²⁺ sites rather than the eight-coordinated sites with increasing Sr²⁺ 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 Ce³⁺ shift from 417 to 433 nm in the composition range of 0 ≤ <i>x</i> ≤ 0.8, and the difference in the decay curves can also verify the phase transformation process. Thermal quenching properties of selected Ca_2–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ 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