A detailed structural investigation of a series of fluoride phosphate laser glasses with nominal composition 25BaF 2 -25SrF 2 -(30-x)Al(PO 3 ) 3 -xAlF 3 -(20-z)YF 3 :zREF 3 with x = 25, 20, 15 and 10, RE = Yb and Eu and 0 ≤ z ≤ 1.0 has been conducted, using Raman, solidstate 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 27 Al/ 31 P 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 Yb 3+ spin probe (S = ½), revealing composition dependent echo-detected lineshapes, and strong hyperfine coupling with 19 F nuclei in hyperfine sublevel correlation (HYSCORE) spectra consistent with the formation of Yb 3+ -F bonds. In addition, photoluminescence spectra of Eu 3+ -doped samples reveal that the 7 F 2 -> 5 D 0 / 7 F 1 -> 5 D 0 transition intensity ratio, the normalized phonon sideband intensities in the excitation spectra as well as excited state lifetimes 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 (x = 25) the data suggest residual rare-earth-phosphate coordination.

In this contribution we present a detailed study of the effect of the addition of small to intermediate amounts of P 2 O 5 (up to 7.5 mol %) on the network organization of metaluminous sodium aluminosilicate glasses employing a range of advanced solid state NMR methodologies. The combined results from MAS, MQMAS (multiple quantum MAS), or MAT (magic angle turning) NMR spectroscopy and a variety of dipolar based NMR experiments 27 Al{ 31 P}-, 27 Al{ 29 Si}-, 29 Si{ 31 P}-, and 31 P{ 29 Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as 31 P{ 27 Al}-and 29 Si{ 27 Al}-REAPDOR (rotational echo adiabatic passage double resonance) NMRallow for a detailed analysis of the network organization adopted by these glasses. Phosphate is found as Q P 2 , Q P 3 , and Q P 4 (with the superscript denoting the number of bridging oxygens), the Q P 4 units can be safely identified with the help of 31 P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO 4 units that is needed for charge compensation of the Q P 2 units necessitates an alternative charge compensation scheme for these AlO 4 units via formation of Q P 4 units or oxygen triclusters. The dipolar NMR experiments suggest a strong preference of P for Al with an average value of ca. 2.4 P−O−Al connections per phosphate tetrahedron. P is thus mainly integrated into the network via P−O−Al bonding, the formation of Si−O−P bonding plays only a minor role.

The beneficial effect of magnesium oxide upon the performance of crack-resistant oxide glasses has been explored in a series of aluminoborosilicate glasses with the compositions 60SiO2–(20 – x)­Al2O3–xB2O3–20Na2O and 60SiO2–(20 – x)­Al2O3–xB2O3–10Na2O–10MgO. The simultaneous presence of both boron and aluminum oxides in these glasses produces a synergetic effect upon crack resistance (CR), whose structural origins are being explored by detailed 11B, 23Na, 27Al, and 29Si single and double resonance solid-state NMR studies. Aluminum is exclusively four-coordinated, whereas boron is found in both three- and four-coordination. Substitution of B2O3 with Al2O3 and Na2O with MgO leads to a dramatic reduction of N 4, the fraction of four-coordinate boron, accompanied by an increase in CR. 11B/27Al double resonance NMR studies show only weak interactions between the boron oxide and aluminum oxide components, giving no evidence of the formation of new structural units not already realized in the ternary aluminosilicate and borosilicate glass systems. Rather, the effect of magnesium can be related to a dramatic reduction of the fraction of four-coordinate boron species compared to the analogous sodium-based system. This reduction results from a preference of the sodium ions to charge-compensate anionic AlO4/2 – species, combined with an unfavorable interaction of four-coordinate boron with Mg2+. Overall, the results give important insights into the Mg-driven structural network changes in this four-component glass system, providing a structural rationale for the dramatic effect of magnesium upon the mechanical properties of these glasses.

Isolated acid sites were selectively generated by postsynthesis modification of a borosilicate zeolite beta. To this end, samples were prepared with pairs of adjacent boron sites balanced by Ca2+ ions, whereas isolated boron in the zeolite framework was balanced by NH4 + ions. To avoid undesired boron leaching, these ion exchange reactions were carried out in methanol solution rather than in water. Trigonal boron forms near the NH4 + ions by decomposing the latter into NH3 (and a proton), and selective extraction of the trigonal boron is achieved by water treatment, whereas the tetrahedral boron near Ca2+ ions remains in the zeolite framework. The vacancies were refilled with aluminum by treatment with an aqueous Al­(NO3)3 solution. Two Brønsted acid sites with 1H chemical shifts of 4.0 and 5.0 ppm exist in the dehydrated samples. 1H–27Al REAPDOR solid-state NMR measurements yield quantitative information on the local H–Al distances of isolated H–Al two-spin and H–Al–Al three-spin systems. The nearest H–Al distance is determined at 2.50 Å with an accuracy of 2% (or better) by fitting the oscillatory part of the REAPDOR curves, which was not observed before for zeolite acid sites. The second nearrest Al neighbors show a much larger distance of about 5 Å for the acid protons with a chemical shift of δ = 4 ppm. A second acid site at δ = 5 ppm has an approximately 50% occupation of a second Al neighbor at 3.73 Å, possibly within the same six-ring. This high resolution of dipolar interaction is not observed in a standard zeolite Al-beta prepared by direct synthesis. The method is suitable to identify well-defined local ordering in Al distributions of zeolites.

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 (C 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 C 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.