The crystal site engineering and turning of cross-relaxation in green-emitting β-Ca3(PO4)2-related phosphors

Series of Tb3+-doped phosphates Ca9–xZnxTb(PO4)7 and Ca9–xZnxLa0.99(PO4)7:0.01 Tb3+ with β-Ca3(PO4)2-type structure were synthesized using a standard high temperature route. The luminescence properties of these compounds were determined by the photoluminescence emission (PL), the excitation (PLE) spectra and lifetime measurements. A green-emitting LED device was fabricated using the most effective one. No concentration quenching was detected in the whole region of solid solutions. Upon Ca2+ → Zn2+ substitution the crystal structure transformation from the polar to centrosymmetric. The ways of control the PL spectra and cross-relaxation process were determined. In both solid solutions a “green” magneto-dipole 5D4 → 7F5 transition is predominant. The integral intensity increases in 3 times in the most effective Ca8ZnTb(PO4)7 phosphor. The measured quantum yields (QY) reached 31%. Thus, a target substitution of Ca2+ by Zn2+ is a new path for crystal site engineering and allows to obtain an efficient green luminescence of Tb3+ ions in the β-Ca3(PO4)2-type hosts.