Ab initio atomistic description of temperature-induced phase changes: The cases of zirconia and Ti-Y-co-doped zirconia

Zirconium dioxide, or zirconia, is a common and useful ceramic with a wide range of applications, from fuel cells to odontology. Its phase diagram is simple and well understood, having a structure which is monoclinic at temperatures up to 1500 K, tetragonal up to 2700 K and cubic up to 3000 K. Zirconia is rarely used in its pure form, being typically doped with ${\text{Y}}_{2}{\text{O}}_{3}$, MgO or ${\text{TiO}}_{2}$, and in this regime its phase diagram becomes much more complex. In this context, ab initio molecular dynamics (AIMD) can provide a detailed atomistic description of the phase diagram of this system, accurately describing its stable phases and transition regions. In this work, 3 mol-% ${\text{Y}}_{2}{\text{O}}_{3}$ (3YSZ) crystals doped with different Ti contents were studied at the density-functional level. For Ti contents varying from 0 to 30 at%, a global search algorithm was first used to explore the 0 K potential-energy surface and determine the most stable sites for the added Ti atoms. It was found that, at low Ti compositions ${X}_{\text{Ti}}$, small ${\text{TiO}}_{2}$ clusters form, followed by ${\text{TiO}}_{2}$ channels and infinite ${\text{TiO}}_{2}$ planes at larger ${X}_{\text{Ti}}$ values, and that the highest stability is achieved at 9% Ti. AIMD simulations within the isothermal-isobaric NPT ensemble were then performed to characterize the temperature-dependent phase changes as a function of the Ti content, where it was found that the Ti-doped structures presented considerably smaller volume changes near the phase-change critical temperatures. These findings suggest that YSZ materials doped with a small amount of Ti are both energetically and kinetically more stable than the undoped counterparts, in the ideal proportion of 3% ${\text{TiO}}_{2}$ for every 1% ${\text{Y}}_{2}{\text{O}}_{3}$ doping.