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 K₅Eu_1-xTb_x(MoO₄)₄ (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K₅Tb(MoO₄)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 R3̅m) 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-K₅Eu_0.6Tb_0.4(MoO₄)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 = 0.684b*. 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 ⁵D₄ → ⁷F_J (J = 3-6) Tb³⁺ 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 ⁵D₄ → ⁷F₅ Tb³⁺ 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 ⁵D₀ → ⁷F₂ Eu³⁺ transition, thus indicating an efficient energy transfer from Tb³⁺ to Eu³⁺.