Characterizing the First and Second ²⁷Al Neighbors of Brønsted and Lewis Acid Protons in Zeolites and the Distribution of ²⁷Al Quadrupolar Couplings by ¹H{²⁷Al} Offset REAPDOR

The application of ¹H­{²⁷Al} offset rotational-echo adiabatic-passage double-resonance (REAPDOR) NMR experiments enables the indirect observation of the first-order quadrupolar lineshapes for ²⁷Al nuclei in acidic zeolites H-beta and H,Na-A by observing the neighboring ¹H nuclei under the influence of dipolar coupling between them. The technique allows one to resolve the contribution of different Al environments, provided that these are characterized by different quadrupolar broadenings, reflecting different structural distortions. Therefore, Brønsted acid sites that typically show large quadrupolar coupling constants (<i>C</i>_Q) can be distinguished from extra-framework Lewis sites, which often have considerably smaller quadrupolar couplings. The ¹H­{²⁷Al} offset REAPDOR patterns can be simulated employing a Gaussian probability distribution using a library of reference offset REAPDOR curves (depending on quadrupolar coupling constant and asymmetry parameter). Monomodal and bimodal probability distributions of <i>C</i>_Q were used to fit the experimental data. This approach provides direct information on the neighborhood between Brønsted and Lewis acid sites. Finally, a theoretically expected frequency shift of the ¹H MAS NMR signals as a function of neighboring ²⁷Al nuclei in different spin states with large quadrupolar coupling is discovered experimentally for the first time for a zeolite Brønsted acid site by using offset REAPDOR

Network Structure and Rare-Earth Ion Local Environments in Fluoride Phosphate Photonic Glasses Studied by Solid-State NMR and Electron Paramagnetic Resonance Spectroscopies

A detailed structural investigation of a series of fluoride phosphate laser glasses with nominal composition 25BaF₂–25SrF₂–(30 – <i>x</i>)­Al­(PO₃)₃–<i>x</i>AlF₃–(20 – <i>z</i>)­YF₃:<i>z</i>REF₃ with <i>x</i> = 25, 20, 15 and 10, RE = Yb and Eu, and 0 ≤ <i>z</i> ≤ 1.0 has been conducted using Raman, solid-state nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopies. The network structure is dominated by the preferred formation of aluminum-to-phosphorus linkages, which have been quantified by means of ²⁷Al/³¹P NMR double-resonance techniques. The fluoride ions are found in mixed Al/Y/Ba/Sr environments accommodating the luminescent dopant species as well. The local environments of the rare-earth species have been studied by pulsed EPR spectroscopy of the Yb³⁺ spin probe (<i>S</i> = ¹/₂), revealing composition-dependent echo-detected lineshapes and strong hyperfine coupling with ¹⁹F nuclei in hyperfine sublevel correlation (HYSCORE) spectra consistent with the formation of Yb³⁺–F bonds. In addition, photoluminescence spectra of Eu³⁺-doped samples reveal that the ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ transitions intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and excited state ⁵D₀ lifetime values are systematically dependent on fluoride content. Altogether, these results indicate that the rare-earth ions are found in a mixed fluoride/phosphate environment, to which the fluoride ions make the dominant contribution. Nevertheless, even at the highest fluoride levels (<i>x</i> = 25), the data suggest residual rare-earth–phosphate coordination