Influence of Sc substitution on the structure and properties of NASICON-type Na₃Sc_2-xR_x(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 similar to 26% and 18% at similar to 420 K compared to T-R allowed us to consider NSP:0.1Eu(3+) as a suitable phosphor for pc-LEDs. The temperature dependence of the Dy3+ emission for NSP:0.05Dy(3+) demonstrates a strong thermal quenching. Different temperature dependences of the Tb3+ emission intensity of NSP:0.1Tb(3+) were found for two excitation bands at lambda(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 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 R (3) over barc structure (sigma(bulk) = 6.4 x 10(-5) S cm(-1) at 300 K, 1.14 x 10(-3) S cm(-1) at 360 K and 5.0 x 10(-2) S cm(-1) at 500 K) is higher than that for pure alpha-Na3Sc2(PO4)(3) but lower than that for beta- and gamma-Na3Sc2(PO4)(3).