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

Abstract

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, Altet, Mg) and Altet-O-Y (Y=Mg or Altet). DFT calculations were also conducted to obtain the 17O chemical shift and other NMR features like the quadrupolar coupling constant, CQ, for all of the oxygen environments encountered in the two model structures, one respecting the Lowenstein's rule and the other involving Altet-O-Altet and Si-O-Si environments. Our DFT calculations support the 17O assignment, by confirming that Altet-O-3Mg and Altet-O-Altet 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.