First-principles calculations of the structural, electronic, elastic and thermodynamic properties of MgAl2O4:Ti3+ and ZnAl2O4:Ti3+

Abstract

Detailed first-principles calculations of structural, electronic, elastic, thermodynamic and vibrational properties of two spinel crystals, MgAl2O4 (MAO) and ZnAl2O4 (ZAO): neat and doped with Ti3+ ions, at ambient and elevated hydrostatic pressure, are reported. Special attention is given to the location of Ti3+ 3d level in the hosts’ band gap. Various exchange-correlation functionals are employed for that purpose; the best agreement with the experimental data is obtained for the M06 functional, which places the Ti3+ state at 4.39 eV above the valence band top in MgAl2O4 and at 4.08 eV in ZnAl2O4. Crystal field splitting of the Ti3+ 3d states is calculated for different pressures; dependence of the crystal field strength 10Dq on pressure and Ti3+–O2− distance is analyzed. Our calculations of the Debye temperature (based on the knowledge of elastic constants) result in close agreement with the corresponding experimental data. Doping with Ti3+ ions leads to a slight decrease of the elastic parameters and lowering the Debye temperature by 20–40 K, because the Ti3+–O2− chemical bonds become longer and softer when compared with the Al3+–O2− ones in undoped materials. As a result, slight red shift of the most prominent features in the vibrational spectra is expected; this is confirmed by the performed calculations. Obtained results give a deeper insight into the properties of doped optical materials, highlight the effect of added impurity ions on their physical parameters and may serve as useful guides for smart materials engineering with wide opportunities of fine tuneability of their most important characteristics for potential applications.