K₅Eu_1–<i>x</i>Tb<i>_x</i>(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. 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-K5Eu0.6Tb0.4(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 → 7FJ (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 λex = 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 λex = 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+

Bi₃(PO₄)O₃, the Simplest Bismuth(III) Oxophosphate: Synthesis, IR Spectroscopy, Crystal Structure, and Structural Complexity

The bismuth­(III) oxophosphate Bi₃(PO₄)­O₃ was obtained by hydrothermal synthesis. The unit cell has <i>a</i> = 5.6840(6) Å, <i>b</i> = 7.0334(7) Å, <i>c</i> = 9.1578(9) Å, α = 78.958(2)°, β = 77.858(2)°, γ = 68.992(2)°, <i>V</i> = 331.41(6) ų, space group <i>P</i>1̅, and <i>Z</i> = 2. The crystal chemical formula that reflects the presence of oxo-centered tetrahedra and triangles is ^2D[O^IIIO^IV₂Bi₃]­(PO₄). The crystal structure contains [O₃Bi₃]³⁺_∞∞-heteropolyhedral corrugated layers parallel to (001), which alternate along [001] with isolated (PO₄) tetrahedra. The structural complexity parameters are <i>v</i> = 22 atoms, <i>I</i>_G = 3.459 bits/atoms, and <i>I</i>_G,total = 76.107 bits/unit cell, and thus Bi₃(PO₄)­O₃ is the simplest pure bismuth­(III) oxophosphate

Bi3(PO4)O3, the Simplest Bismuth(III) Oxophosphate: Synthesis, IR Spectroscopy, Crystal Structure, and Structural Complexity

The bismuth(III) oxophosphate Bi3(PO4)O3 was obtained by hydrothermal synthesis. The unit cell has a = 5.6840(6) Å, b = 7.0334(7) Å, c = 9.1578(9) Å, α = 78.958(2)°, β = 77.858(2)°, γ = 68.992(2)°, V = 331.41(6) Å3, space group P1̅, and Z = 2. The crystal chemical formula that reflects the presence of oxo-centered tetrahedra and triangles is 2D[OIIIOIV2Bi3](PO4). The crystal structure contains [O3Bi3]3+∞∞-heteropolyhedral corrugated layers parallel to (001), which alternate along [001] with isolated (PO4) tetrahedra. The structural complexity parameters are v = 22 atoms, IG = 3.459 bits/atoms, and IG,total = 76.107 bits/unit cell, and thus Bi3(PO4)O3 is the simplest pure bismuth(III) oxophosphate