ZSM‑5 Zeolite: Complete Al Bond Connectivity and Implications on Structure Formation from Solid-State NMR and Quantum Chemistry Calculations

Al site distribution in the structurally complex and industrially important ZSM-5 zeolite is determined by studying the spectroscopic response of Al­(OSi)₄ units and using a self-consistent combination of up-to-date solid-state NMR correlations (²⁹Si–²⁷Al and ¹H–²⁷Al <i>D</i>-HMQC) and quantum chemistry methods (DFT-D). To unravel the driving forces behind specific Al sitting positions, our approach focuses on ZSM-5 containing its more efficient OSDA, tetrapropylammonium

Transparency through Structural Disorder: A New Concept for Innovative Transparent Ceramics

Transparent polycrystalline ceramics present significant economical and functional advantages over single crystal materials for optical, communication, and laser technologies. To date, transparency in these ceramics is ensured either by an optical isotropy (i.e., cubic symmetry) or a nanometric crystallite size, and the main challenge remains to eliminate porosity through complex high pressure–high temperature synthesis. Here we introduce a new concept to achieve ultimate transparency reaching the theoretical limit. We use a controlled degree of chemical disorder in the structure to obtain optical isotropy at the micrometer length scale. This approach can be applied in the case of anisotropic structures and micrometer scale crystal size ceramics. We thus report Sr_1+<i>x</i>/2Al_2+<i>x</i>Si_2–<i>x</i>O₈ (0 < <i>x</i> ≤ 0.4) readily scalable polycrystalline ceramics elaborated by full and congruent crystallization from glass. These materials reach 90% transmittance. This innovative method should drive the development of new highly transparent materials with technologically relevant applications

Local Disorder and Tunable Luminescence in Sr_{1–<i>x</i>/2}Al_2–<i>x</i>Si_<i>x</i>O₄ (0.2 ≤ <i>x</i> ≤ 0.5) Transparent Ceramics

Eu-doped Sr_{1–<i>x</i>/2}Al_2–<i>x</i>Si_<i>x</i>O₄ (<i>x</i> = 0.2, 0.4, and 0.5) transparent ceramics have been synthesized by full and congruent crystallization from glasses prepared by aerodynamic levitation and laser-heating method. Structural refinements from synchrotron and neutron powder diffraction data show that the ceramics adopt a 1 × 1 × 2 superstructure compared to the SrAl₂O₄ hexagonal polymorph. While the observed superstructure reflections indicate a long-range ordering of the Sr vacancies in the structure, ²⁹Si and ²⁷Al solid-state NMR measurements associated with DFT computations reveal a significant degree of disorder in the fully polymerized tetrahedral network. This is evidenced through the presence of Si–O–Si bonds, as well as Si­(OAl)₄ units at remote distances of the Sr vacancies and Al­(OAl)₄ units in the close vicinity of Sr vacancies departing from local charge compensation in the network. The transparent ceramics can be doped by europium to induce light emission arising from the volume under UV excitation. Luminescence measurements then reveal the coexistence of Eu²⁺ and Eu³⁺ in the samples, thereby allowing tuning the emission color depending on the excitation wavelength and suggesting possible applications such as solid state lighting

Synthesis and Structure Resolution of RbLaF₄

The synthesis and structure resolution of RbLaF₄ are described. RbLaF₄ is synthesized by solid-state reaction between RbF and LaF₃ at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (<i>Pnma,</i> <i>a</i> = 6.46281(2) Å, <i>b</i> = 3.86498(1) Å, <i>c</i> = 16.17629(4) Å, <i>Z</i> = 4). One-dimensional ⁸⁷Rb, ¹³⁹La, and ¹⁹F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the ¹⁹F resonances is performed on the basis of both ¹⁹F–¹³⁹La <i>J</i>-coupling multiplet patterns observed in a heteronuclear DQ-filtered <i>J</i>-resolved spectrum and ¹⁹F–⁸⁷Rb HMQC MAS experiments. DFT calculations of both the ¹⁹F isotropic chemical shieldings and the ⁸⁷Rb, ¹³⁹La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF₄ and luminescence properties of Eu-doped LaRbF₄ are investigated

Synthesis and Structure Resolution of RbLaF₄

The synthesis and structure resolution of RbLaF₄ are described. RbLaF₄ is synthesized by solid-state reaction between RbF and LaF₃ at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (<i>Pnma,</i> <i>a</i> = 6.46281(2) Å, <i>b</i> = 3.86498(1) Å, <i>c</i> = 16.17629(4) Å, <i>Z</i> = 4). One-dimensional ⁸⁷Rb, ¹³⁹La, and ¹⁹F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the ¹⁹F resonances is performed on the basis of both ¹⁹F–¹³⁹La <i>J</i>-coupling multiplet patterns observed in a heteronuclear DQ-filtered <i>J</i>-resolved spectrum and ¹⁹F–⁸⁷Rb HMQC MAS experiments. DFT calculations of both the ¹⁹F isotropic chemical shieldings and the ⁸⁷Rb, ¹³⁹La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF₄ and luminescence properties of Eu-doped LaRbF₄ are investigated

New Transparent Glass-Ceramics Based on the Crystallization of “Anti-glass” Spherulites in the Bi₂O₃–Nb₂O₅–TeO₂ System

The crystallization of the Bi_0.5Nb_0.5Te₃O₈ (7.14 Bi₂O₃–7.14Nb₂O₅–85.72TeO₂) glass composition shows numerous original spherulite-like shape or “droplets” crystalline domains through the bulk. Both SEM-EDS and microprobe measurements demonstrate that both these droplets and the glassy matrix present the same composition. The combination of information obtained from various complementary techniques, electron probe microanalysis, Raman, and SEM-EDS, then leads to the identification of the spherulites as a new Bi_0.5Nb_0.5Te₃O₈ “anti-glass” phase. However, the crystallization mechanism is complex since microcracks are observed at the surface of the spherulites in some glass-ceramic materials, suggesting a confined crystal growth. Therefore, the crystallization process appears much different from a homogeneous congruent crystallization. The photoluminescence (PL) properties of the (1 wt %) Er₂O₃-doped Bi_0.5Nb_0.5Te₃O₈ glass-ceramics were investigated during isothermal crystallization of the parent glass at 380 °C. The evolution of the PL signal (⁴I_13/2 → ⁴I_15/2 transition) enables indirect detection of the first steps of crystallization. Moreover, the PL data indicate random nucleation with respect to the location of Er³⁺ ions, whereas the integrated PL intensity and lifetime values show very comparable evolution trends as a function of the crystallization rate