Sodium doped sodium sodalite: magnetic coupling between F centers and hyperfine interactions with framework atoms

Abstract

Density functional calculations (B3LYP hybrid functional) are used to study the electronic structure of sodium doped sodium sodalite. Two types of finite models of the aluminosilicate framework are designed. Ring-type models are employed to examine the magnetic coupling between nearest neighbour (nn) and next nearest neighbour (nnn) pairs of F centers. An antiferromagnetic coupling is predicted with coupling constants for nn and nnn pair interactions of about 12 and 3 meV, respectively. Models consisting of two four-membered aluminosilicate rings having a central T site in common are used to calculate the spin density at the Si and Al framework atoms surrounded by an increasing number of F centers. Our results support the paramagnetic shift model introduced by Engelhardt etal. (J. Chem. Soc., Chem. Commun., 1996, 6, 729) to explain the multiple lines of the ²⁹Si and ²⁷ Al MAS NMR spectra. The calculations also support the experimentally observed spin density at the nuclei of the (Na⁺)₃ centers which are neighboured to paramagnetic Na ₄³⁺ centers.