The Role of Zr on Monoclinic and Orthorhombic Hf x Zr y O 2 Systems: A First-Principles Study.

HfO 2 shows different polymorphs, including monoclinic and orthorhombic ones, that exhibit singular properties. Moreover, the character of HfO 2 is also influenced by the Zr atoms as a doping agent. Here, an extensive study of the monoclinic P2 1 /c and the orthorhombic Pca2 1 polymorphs of HfO 2 , Hf 0.75 Zr 0.25 O 2 , and Hf 0.5 Zr 0.5 O 2 is reported. For all six systems, density functional theory (DFT) methods based on generalized gradient approximations (GGAs) were first used; then the GGA + U method was settled and calibrated to describe the electrical and optical properties of polymorphs and the responses to the oxygen vacancies. Zr had different effects in relation to the polymorph; moreover, the amount of Zr led to important differences in the optical properties of the Pca2 1 polymorph. Finally, oxygen vacancies were investigated, showing an important modulation of the properties of Hf x Zr y O 2 nanostructures. The combined GGA and GGA + U methods adopted in this work generate a reasonable prediction of the physicochemical properties of o- and m-Hf x Zr y O 2 , identifying the effects of doping phenomena.