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

This work summarizes the results of structural stability, electronic properties and phonon dispersion studies of biocompatible ZrO2 compound in its cubic (c-ZrO2), tetragonal (t-ZrO2) and monoclinic (m-ZrO2) phases. The authors found that the monoclinic phase of zirconium dioxide is the most stable among the three phases in terms of total energy, lowest enthalpy, highest entropy, and other thermodynamic properties. The presence of rather weak frequencies for m-ZrO2 also confirms the monoclinic phase as a stable conformation of zirconia. Our analysis of the electronic properties showed that during the m-t phase transformation of ZrO2, the Fermi level first shifts by 0.125 eV toward higher energies and then decreases by 0.08 eV in the t-c cross-section. The band gaps for c-ZrO2, t-ZrO2, and m-ZrO2 are 5.140 eV, 5.898 eV, and 5.288 eV, respectively. Calculations to study the effect of 3.23, 6.67, 10.35 and 16.15 mol%Y2O3 on the structure and properties of m-ZrO2 showed that the enthalpy of m-YSZ decreases linearly and accompanies further stabilization of zirconium dioxide. Doping-induced phase transitions of ZrO2 were discovered under the influence of Y2O3 doping, due to which the position of the Fermi level changes and the band gap decreases. It has been established that, not only for pure systems but including those doped with Y2O3, the main contribution to the formation of the conduction band is made by the p-states of electrons.