Characterization of defects and the local structure in natural and synthetic alunite (K, Na, H₃O)Al₃(SO₄)₂(OH)₆ by multi-nuclear solid-state NMR spectroscopy

Abstract

The local structural environments in a series of natural and synthetic alunite samples [ideally AAl₃(SO₄)₂(OH)₆, A = H₃O⁺, D₃O⁺, Na⁺, and K⁺] have been probed by solid-state ¹H, ²H, ²³Na, ²⁷Al, and ³⁹K NMR spectroscopy. The natural alunite [KAl₃(SO₄)₂(OH)₆] and synthetic hydronium alunite samples contain few structural defects, whereas the synthetic natroalunite and alunite samples have ca. 10% Al vacancies based on ²⁷Al NMR. A new ²⁷Al local environment (Al_D) was observed and assigned to Al with one Al vacancy in the first cation sphere. Three different proton environments, Al₂-OH, Al - OH₂, and H₃O⁺ are detected by ¹H and ²H MAS NMR. The hydronium ion (H₃O⁺) is only observed in hydronium alunite, and is associated with the stoichiometric regions of the sample. It was not detected in ¹H and ²H NMR spectra of alunite and natroalunite despite K (Na) occupancies of significantly less than 100%, as determined from elemental analysis. Thus, our NMR results suggest that the common assumption, namely that an A vacancy and an Al³⁺ vacancy are compensated by adding an H₃O⁺and 3H⁺ (creating 3 Al-OH₂ groups), respectively, is too simplistic. Instead, a significant fraction of the Al³⁺ vacancies are compensated for by 4 H⁺ ions, resulting in 4 Al-OH₂ groups per vacancy. This substitution is accompanied by the simultaneous deprotonation of a H₃O⁺ ion present on the A site. The resultant H₂O molecule is unnecessary for charge balance, accounting for the A-site deficiency often observed. The presence of Al³⁺ and A⁺ vacancies appears closely correlated based on NMR.