Study on Structural Stability of ZrO2 and YSZ: Doping-Induced Phase Transitions and Fermi Level Shift

The article deals with the issues of structural stability, electronic properties and phonon dispersion of the cubic, tetragonal and monoclinic phase of ZrO2. The monoclinic phase of ZrO2 is found to be the most stable among all the phases of this compound, in terms of total energy comparison, the lowest enthalpy value, the highest entropy and the lowest negative modes found for m-ZrO2. An analysis of the electronic properties showed that during the m-t phase transformation of ZrO2, the Fermi level first shifts by 0.125 eV towards higher energies, and then, in the t-c section, goes down by 0.08 eV. In this case, the band gaps for c-ZrO2, t-ZrO2, and m-ZrO2 are 5.140 eV, 5.898 eV, and 5.288 eV, respectively. Calculations based on the analysis of the effect of doping with 3.23, 6.67, 10.35, and 16.15 mol. % Y2O3 on the m-ZrO2 structure showed that the m-YSZ enthalpy decreases linearly, which accompanies doped-induced phase transitions and further stabilization of monoclinic ZrO2. In this case, the position of the Fermi level changes stepwise, and the energy gap decreases. It has been found that not only for pure systems, including for Y2O3-doped systems, the p-states of electrons make the main contributions to the formation of the conduction band. The obtained results complement the database of research works carried out in the field of application of biocompatible zirconium dioxide crystals and ceramics for green energy generation and can be used in the design of humidity-to-electricity converters and the creation of solid oxide fuel cells based on ZrO2.