Natural abundance ¹⁷O MAS NMR and DFT simulations: New insights into the atomic structure of designed micas

Combining ¹⁷O Magic-Angle Spinning (MAS) NMR at natural abundance with DFT calculations is a promising methodology to shed light on the structure and disorder in tetrahedral sheets of designed micas with enhanced properties. Among brittle micas, synthetic mica is an important alternative to natural ones with a swelling sheet-like structure that results in many applications, by exploiting unique characteristics. Lowenstein's rule is one of the main chemical factor that determines the atomic structure of aluminosilicates and furthermore their properties. In the present article, ¹⁷O MAS NMR spectroscopy is used to validate (or not) the agreement of the Lowenstein's rule with the distribution of Si and Al sites in the tetrahedral sheets of synthetic micas. ¹⁷O MAS spectra of synthetic high-charged micas exhibit two regions of signals that revealed two distinguishable oxygen environments, namely Si-O-X (with X = Si, Al^tet, Mg) and Al^tet-O-Y (Y=Mg or Al^tet). DFT calculations were also conducted to obtain the ¹⁷O chemical shift and other NMR features like the quadrupolar coupling constant, C_Q, for all of the oxygen environments encountered in the two model structures, one respecting the Lowenstein's rule and the other involving Al^tet-O-Al^tet and Si-O-Si environments. Our DFT calculations support the ¹⁷O assignment, by confirming that Al^tet-O-3Mg and Al^tet-O-Al^tet oxygen environments show chemical shifts under 30 ppm and more important, with quadrupolar coupling constants of about 1 MHz, in line with the spectral observation. By quantifying the ¹⁷O MAS NMR spectra at natural abundance, we demonstrate that one of the synthetic mica compositions does not meet the Lowenstein's rule.