Seminarium Fizyki Materii Skondensowanej

Most abundant mineral compounds in the Earth's interior, upper mantlepart, are expected to be magnesium-orthosilicates solid-solutions withformula (Mg,Fe)_2SiO_4. Fe ions incorporated in the crystal structurecan significantly affect its phase stability, chemical, electronic,magnetic and optical properties. Complete phase diagram of Mg_2SiO_4end-member system at high pressure has been determined from thelattice dynamics calculated using the quantum DFT.The electronic structure in case of ferrous Mg_(2-x)Fe_(x)SiO_4 -spinel phase at 20 GPa pressure has been studied using the DFT+Umethod which incorporates the Hubbard model approach. The latticeconstant of the spinel (cubic lattice), ground state enthalpy andaverage magnetic moment localized on Fe ions have been examined withthe increasing Fe content x in the crystal structure. Dependence ofelectronic band structure and of its energy gap on the Feconcentration x has been determined from the calculated electronicdensity of states (EDOS) in full range of Fe's substitution fromMg_2SiO_4 to Fe_2SiO_4 end-member. Development from band insulator toMott's type of insulator has been observed with the increasing Fedoping in the crystal. Contributions of the Fe's (t_2g) and (e_g)orbitals to the EDOS have been determined and role of local Coulombcoupling in the band-gap formation has been discussed. The DFTcalculations have been performed and compared in case of ferromagneticand antiferromagnetic configuration. The basic characteristics ofoptical absorption in the spinel have been derived from the complexdielectric tensor and found to be in qualitative good accordance withexperimental data for lower Fe concentrations.