KTb(MoO4)2 Green Phosphor with K+-Ion Conductivity: Derived from Different Synthesis Routes

Systematic Crystallization of NH4Ln(MoO4)2 as a Family of Layered Compounds (Ln = La-Lu and Y), Derivation of Ln2Mo4O15, Crystal Structure, and Photoluminescence

NH₄Ln(MoO₄)₂ (Ln = La-Lu lanthanide, Y) was crystallized via hydrothermal reaction as a new family of layered materials, from which phase-pure Ln₂Mo₄O₁₅ was successfully derived via subsequent annealing at 700 °C for the series of Ln elements excluding Ce and Lu. Detailed structure analysis revealed that the ionic size of Ln³⁺ decisively determined the crystal structure and Mo/Ln coordination for the two families of compounds. NH₄Ln(MoO₄)₂ was analyzed to be orthorhombic (Pbcn space group, no. 60) and monoclinic (P2/c, no. 13) for the larger and smaller Ln³⁺ of Ln = La-Gd and Ln = Tb-Lu (including Y), respectively, where both the crystal structures have a layered topology featured by the alternative stacking of a [Ln(MoO₄)₂]^- three-tier infinite anionic layer and interlayer NH₄⁺. Four types of crystal structures were found for the Ln₂Mo₄O₁₅ series, which are monoclinic (P2₁/a, no. 14) for Ln = La, triclinic (P1̅, no. 2) for Ln = Pr-Sm, triclinic (P1̅, no. 2) for Ln = Eu and Gd, and monoclinic (P2₁/c, no. 14) for Ln = Tb-Yb (including Y). The photoluminescence of NH₄Ln(MoO₄)₂ (Ln = Eu, Tb) and Ln₂Mo₄O₁₅:Eu³⁺ (Ln = La, Gd, Y) was thoroughly investigated in terms of spectral features, quantum efficiency, fluorescence decay, and CIE chromaticity. The thermal stability of luminescence was also studied for Ln₂Mo₄O₁₅:Eu³⁺, and the observed charge-transfer excitation components were successfully correlated with the features of the Mo-O polyhedron/unit.

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⁺.

KB3H8·NH3B3H7 Complex as a Potential Solid-State Electrolyte with Excellent Stability against K Metal

All-solid-state potassium batteries are promising candidates in the fields of large-scale energy storage owing to their intrinsic safety, stability, and cost-effectiveness. However, a suitable solid-state electrolyte with high ionic conductivity and favorable interfacial stability is a major challenge for the design and development of these batteries. Herein, we report the synthesis of new KB₃H₈·nNH₃B₃H₇ (n = 0.5 and 1) complexes to develop suitable solid-state K-ion conductors for batteries. Both the complexes undergo a reversible phase transition below the thermal decomposition temperature. The optimal KB₃H₈·NH₃B₃H₇ delivers a solid-state K-ion conductivity of 1.3 × 10^-4 S cm^-1 at 55 °C with an activation energy of 0.44 eV after a transition from a monoclinic to an orthorhombic phase, which is the highest value of K borohydrides reported to date and places KB₃H₈·NH₃B₃H₇ among the leading solid-state K-ion conductors. Moreover, KB₃H₈·NH₃B₃H₇ reveals a K-ion transference number of nearly 0.93, an electrochemical stability window of 1.2 to 3.5 V vs K⁺/K, a good capability of K dendrite suppression, and a remarkable stability against the K metal anode due to the formation of the stable interface. These performances make KB₃H₈·NH₃B₃H₇ a promising electrolyte for all-solid-state potassium batteries.