Effect of Fe–Fe interactions and <i>X</i>-site vacancy ordering on the OH-stretching spectrum of foitite

Abstract. The OH-stretching infrared absorption spectrum of a tourmaline sample close to the foitite end-member is interpreted in light density functional theory (DFT) modeling iron-bearing Y3Z6 clusters tourmaline. reflect Al-rich and Na-deficient character contain either two Fe2+ one Al3+ or ions at Y sites. are embedded host structure with dravite composition. For iron dimer models, structural vibrational properties corresponding ferromagnetic (FM) anti-ferromagnetic (AFM) arrangement spins effect vacancy ordering along [001] axis considered. A significant difference relaxed FM AFM observed, stemming from electron delocalization Fe–Fe bonding interactions cluster. These not allowed In this case, valence electrons opposite remain separately localized on Fe atoms. configuration more stable than theoretical provided that description on-site Coulomb repulsion Fe(3d) orbitals improved within DFT + U framework. Based results, bands 3630 3644 cm−1 spectra iron-rich tourmalines assigned WOH groups associated YFe22+YAl3+ environments an coupling surrounded by vacant X sites, respectively. major VOH experimental same basis, these interpretations extrapolated Mn-bearing tourmalines.