KEuTb(MoO) Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb → Eu Energy Transfer.

This paper describes the influence of sintering conditions and Eu/Tb content on the structure and luminescent properties of KEuTb(MoO) (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A KTb(MoO) (KTMO) colorless transparent single crystal was grown by the Czochralski technique. A continuous range of solid solutions with a trigonal palmierite-type structure (α-phase, space group 3̅) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ ≤ 1) prepared at 1123 K followed by quenching to liquid nitrogen temperature. The reversibility of the β ↔ α phase transition for KTMO was revealed by a differential scanning calorimetry (DSC) study. The low-temperature (LT)LT-KEuTb(MoO) structure was refined in the space group. Additional extra reflections besides the reflections of the basic palmierite-type R-subcell were present in synchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as an incommensurately modulated structure with (3 + 1)D superspace group (0β0)00 and the modulation vector = 0.684*. The luminescent properties of KETMO prepared at different conditions were studied and related to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to D → F ( = 3-6) Tb transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highest luminescence intensity, which was ∼20 times higher than that of LT-KTMO. The quantum yield λ = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the D → F Tb transition increased with the increase of from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with = 0.2-0.9 at λ = 377 nm exhibited an intense red emission at ∼615 nm due to the D → F Eu transition, thus indicating an efficient energy transfer from Tb to Eu.