Si-29 and Al-27 MAS-NMR spectroscopy of beta-eucryptite (LiAlSiO4): The enthalpy of Si,Al ordering

By combining data from oxide melt solution calorimetry and Si-29 MAS-NMR sp ectroscopy, the enthalpy for short-range SI,Al ordering in beta-eucryiptite (LiAlSiO4,) was determined for a series of samples prepared by glass annea ling. Si,Al ordered beta-eucryptite gives a single Si-29 NMR peak for two u nresolved crystallographic sites, but two distinct Al-27 resonances that di ffer in both chemical shift and quadrupolar coupling. Samples of beta-eucry ptite crystallized from glass contain additional Si-29 NMR peaks, indicatin g significant levels of short-range Si,AI disorder. This disorder decreases exponentially with annealing time at 1173 K, from 0.55(+/-0.04) Al-O-Al li nkages per formula unit after one hour to 0.05(+/-0.01) after 70.5 hours. T he decrease in the concentration of Al-O-Al linkages with annealing time co rrelates linearly with enthalpies of drop-solution in molten lead berate, g iving an enthalpy of Delta H-ord = -26 +/- 3 kJ/mol for the reaction: Al-O- Al + Si-O-Si --> 2(Si-O-Al). Additional NMR results are presented for samples synthesized along the SiO2 -LiAlSiO4 join (quartz to beta-eucryptite). Solid solution samples with com positions 20 and 69 mol% quartz appear to have very few Al-O-Al linkages (< 0.04 per formula unit).