Electronic structure of the sodium and potassium electrosodalites (Na/K)8(AlSiO4)6

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Abstract
The alkali-doped electrosodalites form a unique class of magnetic materials where the magnetism comes from nonatom centered unpaired F-center electrons found inside body-centered cubic arrangements of alkali ion clusters supported by sodalite host lattices. In the present work the crystal structure of potassium-electrosodalite (PES) has been determined at 20 K using synchrotron powder diffraction. Using the linearized augmented plane wave method the electronic structure of sodium-electrosodalite (SES) and PES is calculated. While nonmagnetic and ferromagnetic phases of SES and PES are found to be metallic, DFT in both the LSDA and the GGA version correctly predicts the nonmetallic AFM phase to be the electronic ground state. Despite PES having a larger unit cell than SES the temperature of magnetic ordering is higher. This is found to be due to both a broadening of the bands belonging to the F-center states and to lower on-site correlations. Calculated optical properties are reported and the formation of F centers inside sodalite cages causes the materials to absorb light with energies around 1 eV and above, depending on structure. Non-nuclear maxima are found in the total electron density of both SES and PES and the spin density is found mainly in the center of the sodalite cage.