Improved Understanding of Atomic Ordering in Y₄Si_xAl_2–xO_9–xN_x Materials Using a Combined Solid-State NMR and Computational Approach

Abstract

Ceramics based around silicon aluminium oxynitrides are of both fundamental structural chemistry and technological interest. Certain oxynitride crystal structures allow very significant compositional variation through extensive Si/N exchange for Al/O which implies a degree of atomic ordering. In this study, solid-state 29 Si MAS NMR and variable field 1D and 2D 27 Al MAS NMR measurements are combined with Density Functional Theory calculations of both the structural and NMR interaction parameters for various points across the Y4Si2O7N2-Y4Al2O9 compositional range. This series provides numerous possibilities for significant variation of atomic ordering in the local ditetrahedral (Si,Al)2O7−xNx units. The two slightly structurally inequivalent aluminium sites in Y4Al2O9 are unambiguously assigned to the observed resonances. Computational findings on Y4Si2O7N2 system demonstrate that the single observed 29 Si NMR resonance covers a range of local inequivalent silicon environments. For the first time, the MAS NMR and neutron diffraction data from the Y4SiAlO8N structure have been directly reconciled, thus establishing aspects of atomic order and disorder that characterise this system. This comparison suggests that, although the diffraction data indicates long range structural order supporting a highly crystalline character, the short range information afforded by the solid-state NMR measurements indicates significant atomic disorder throughout the (Si,Al)2O7−xNx units.

Solid‐state nuclear magnetic resonance of spin‐9/2 nuclei 115In and 209Bi in functional inorganic complex oxides

“…T A B L E 3 Experimental and calculated 27 Al and 69,71 [84] b Estimated uncertainty in the fit given in parentheses.…”

Solid-State NMR of Spin-9/2 Nuclei 115In and 209Bi in Functional Inorganic Complex Oxides

“…33 Both nuclei are spin 9/2 with moderate gyromagnetic ratios ( ❑ ¿ disfavored. An analogy must be drawn to 93 Nb, the only other stable, odd-proton I = 9/2 nucleus (nota bene, there are several odd-neutron I = 9/2 nuclides- 73 Ge, 83 Kr, 87 Sr, and 179 Hf -but these have substantially lower gyromagnetic ratios). 93 Nb has found wider use than its (113)/115…”

“…where m = -1.027 and ref. = 572.35 ppm from Seymour et al87 c Calculated69,71 Ga shielding converted to shift according to the previous expression where m = -0.867 and ref. = 1502.63 ppm from Middlemiss et al86 d The69 Ga quadrupolar coupling constant is fixed relative to the71 Ga CQ according to the ratio of the69 Ga/71 Ga nuclear electric quadrupole moments, which is 17.1/10.7 = 1.60 35.…”