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

Na3Sc2−xRx(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 Na3Sc2−xRx(PO4)3 (R = Eu, Tb, Dy) were studied in the range of 80–500 K. The highest R3+ luminescence intensity in Na3Sc2−xRx(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 Na3Sc2−xRx(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 TR 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 λex = 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 Na3Sc1.9Eu0.1(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 Na3Sc1.9Eu0.1(PO4)3 with an R3c structure (σbulk = 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.