NASICON-type Na 3.6 Lu 1.8- x (PO 4 ) 3 : x Eu 3+ phosphors: structure and luminescence.

Na 3.6 Lu 1.8- x (PO 4 ) 3 : x Eu 3+ phosphors were synthesized by a high-temperature solid-state reaction. A powder X-ray diffraction study revealed that homogeneous solid solutions with a NASICON-type structure were formed at 0 ≤ x ≤ 0.7. The Na 3.6 Lu 1.8 (PO 4 ) 3 structure was refined from the powder X-ray diffraction data and the cation distribution in the lattice sites of the NASICON-type structure was revealed. The refinement indicates structural disorder caused by the displacement of a part of Lu cations along the c axis inside the (Lu/Na)O 6 octahedra that is confirmed by the broadened emission lines of Eu 3+ , which substitutes Lu cations. The highest Eu 3+ luminescence intensity is found in Na 3.6 Lu 1.8- x (PO 4 ) 3 : x Eu 3+ for x = 0.5, whereas a further increase of the Eu 3+ content leads to concentration quenching that is shown to occur due to the dipole-dipole interaction. An enhanced temperature stability of the Eu 3+ emission was observed at the excitation energy of 3.23 eV. At this excitation energy, thermal quenching of the emission caused by the 7 F 0 → 5 L 7 transitions is compensated by the intensity increase of the emission related to the 7 F 1 → 5 G J transitions, which occurs due to the increase of the 7 F 1 level population, induced by a temperature rise.