Nitrogen-rich La–Si–Al–O–N oxynitride glass structures probed by solid state NMR

27Al NMR Studies of Aluminosilicate Glasses

“…(i) As anticipated from the correlation between the electron density and the chemical shift, the (Pauling) electronegativity of the ligand atom around Al affects δ iso [p] : for instance, each O!N replacement at an AlO 4 group typically increases δ iso [4] by around 10-12 ppm [41,[80][81][82]. The 27 Al deshielding effects from nearest-neighboring N atoms are evident from the NMR spectra of Fig.…”

“…The 27 Al deshielding effects from nearest-neighboring N atoms are evident from the NMR spectra of Fig. 15 in Section 9.2 that were collected from oxynitride AS glasses with increasing N contents [82]. While Al [4] reluctantly forms bonds to F, O!F substitutions at AlO 6 octahedra decreases δ iso [6] by a few ppm per F atom, altogether amounting to À 20 ppm lower 27 Al chemical shifts of 27 AlF 6 groups relative to 27 AlO 6 [83].…”

“…Whenever feasible, our group employs this procedure prior to any detailed rf power calibration, encompassing widely spanning types of NMR experimentation: (i) To locate suitable Hartmann-Hahn conditions for I ¼ 1=2 ! S > 1=2 cross-polarization (CP) [82,176,177], where ν nut CT for the S-spin is selected first, as its choice has the overall strongest bearings on the magnetization-transfer efficiency. (ii) To establish optimum conditions for homonuclear recoupling of spin-1/2 nuclei [178,179] or halfinteger spins [76,[171][172][173][180][181][182], as well as for (iii) spin-1/2 applications at fast MAS (>35 kHz), e.g., for identifying CP conditions and locating suitable proton decoupling powers [178].…”

“…15. 27 Al NMR spectra recorded at 9.4 T from La-Al-Si-O-(N) glasses of variable compositions [82]. Note the deshielding of 27 AlO 4 when the N-content of the glass is increased according to (A and B) 0%, (C and D) 17%, and (E and F) 36%, implying the formation of "mixed" Al(O, N) 4 tetrahedra [percentages represent 100 Â n N /(n N + n O )].…”

“…The smaller size of Mg 2+ relative to Ca 2+ is reflected by overall higher amounts of AlO 5 groups in MgO-Al 2 O 3 -SiO 2 glasses (or "mixed" Ca/Mg glasses) compared with their Ca counterparts of comparable compositions [54,66,[266][267][268]. Consequently, the significant field strength of trivalent RE 3+ species induces very substantial structural disorder effects and the features (1)-(3) manifest the strongest for RE 2 O 3 -Al 2 O 3 -SiO 2 glasses [61,62,76,82,89,104,116,248,269]. Large amounts of AlO 5 groups generally persist across all compositions obeying both conditions Eqs.…”