Influence of Sc substitution on the structure and properties of NASICON-type Na3Sc2−xRx(PO4)3 (R = Eu, Tb, Dy) phosphors

Na₃Sc_2-xR_x(PO₄)₃ (R = Eu, Tb, Dy; 0 ≤ x ≤ 0.2) phosphors were synthesized by a high-temperature solid-state reaction. Sc : R ratios for the NSP:xR samples were determined by ICP-MS, EDX-SEM and TEM-EDX measurements. An X-ray diffraction study revealed that solid solutions with a NASICON-type structure were formed at 0 ≤ x ≤ 0.1. The luminescence properties of Na₃Sc₂(PO₄)₃ and Na₃Sc_2-xR_x(PO₄)₃ (R = Eu, Tb, Dy) were studied in the range of 80-500 K. The highest R³⁺ luminescence intensity in Na₃Sc_2-xR_x(PO₄)₃ (R = Eu, Tb, Dy) depending on R was found for x = 0.05 in the case of Dy and x = 0.1 in the case of Eu and Tb. The temperature behaviour of the R³⁺ emission intensity of Na₃Sc_2-xR_x(PO₄)₃ (R = Eu, Tb, Dy) depends on R that replaces Sc. The decrease of the Eu³⁺ emission intensity depending on the transition energy by ∼26% and 18% at ∼420 K compared to T_R allowed us to consider NSP:0.1Eu³⁺ as a suitable phosphor for pc-LEDs. The temperature dependence of the Dy³⁺ emission for NSP:0.05Dy³⁺ demonstrates a strong thermal quenching. Different temperature dependences of the Tb³⁺ emission intensity of NSP:0.1Tb³⁺ were found for two excitation bands at λ_ex = 220 and 378 nm representing f-d and f-f intracentre transitions. No thermal quenching for f-f transitions takes place while the emission intensity for f-d transitions increases with a temperature rise from 80 to 500 K. The dielectric measurements for Na₃Sc₂(PO₄)₃ and Na₃Sc_1.9Eu_0.1(PO₄)₃ were provided on ceramic pellets sintered under vacuum using a spark plasma sintering technique. Different dependences of conductivity were found for two samples. The calculated conductivity for Na₃Sc_1.9Eu_0.1(PO₄)₃ with an R3̄c structure (σ_bulk = 6.4 × 10^-5 S cm^-1 at 300 K, 1.14 × 10^-3 S cm^-1 at 360 K and 5.0 × 10^-2 S cm^-1 at 500 K) is higher than that for pure α-Na₃Sc₂(PO₄)₃ but lower than that for β- and γ-Na₃Sc₂(PO₄)₃.