K 5 Eu 1- x Tb x (MoO 4 ) 4 Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb 3+ → Eu 3+ Energy Transfer.

This paper describes the influence of sintering conditions and Eu 3+ /Tb 3+ content on the structure and luminescent properties of K 5 Eu 1- x Tb x (MoO 4 ) 4 (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K 5 Tb(MoO 4 ) 4 (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 R 3̅ m ) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ x ≤ 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-K 5 Eu 0.6 Tb 0.4 (MoO 4 ) 4 structure was refined in the C2/m 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 C2/m (0β0)00 and the modulation vector q = 0.684 b *. 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 5 D 4 → 7 F J ( J = 3-6) Tb 3+ 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 ex = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the 5 D 4 → 7 F 5 Tb 3+ transition increased with the increase of x from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with x = 0.2-0.9 at λ ex = 377 nm exhibited an intense red emission at ∼615 nm due to the 5 D 0 → 7 F 2 Eu 3+ transition, thus indicating an efficient energy transfer from Tb 3+ to Eu 3+ .