K5Eu1-xTbx(MoO4)4 Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb3+→Eu3+ Energy Transfer

This paper describes the influence of sintering conditions and Eu3+/Tb3+content on the structure and luminescent properties of K5Eu1-xTbx(MoO4)4(KETMO).KETMO samples were synthesized under two different heating and cooling conditions. AK5Tb(MoO4)4(KTMO) colorless transparent single crystal was grown by the Czochralskitechnique. A continuous range of solid solutions with a trigonal palmierite-type structure(alpha-phase, space groupR3??m) were presented only for the high-temperature (HT or alpha-)KETMO (0 <= x <= 1) prepared at 1123 K followed by quenching to liquid nitrogentemperature. The reversibility of the beta <->alpha phase transition for KTMO was revealed by adifferential scanning calorimetry (DSC)study. The low-temperature (LT)LT-K5Eu0.6Tb0.4(MoO4)4structure was refined in theC2/mspace group. Additional extrareflections besides the reflections of the basic palmierite-type R-subcell were present insynchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as anincommensurately modulated structure with (3 + 1)D superspace groupC2/m(0 beta 0)00and the modulation vectorq= 0.684b*. The luminescent properties of KETMO prepared at different conditions were studied andrelated to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to5D4 -> 7FJ(J=3-6) Tb3+transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highestluminescence intensity, which was similar to 20 times higher than that of LT-KTMO. The quantum yield lambda ex= 481 nm for the KTMO singlecrystal was measured as 50%. The intensity of the5D4 -> 7F5Tb3+transition increased with the increase ofxfrom 0.2 to 1 for LTand HT-KETMO. Emission spectra of KETMO samples withx= 0.2-0.9 at lambda ex= 377 nm exhibited an intense red emission at similar to 615nm due to the5D0 -> 7F2Eu3+transition, thus indicating an efficient energy transfer from Tb3+to Eu3+.