Harnessing point-defect induced local symmetry breaking in a tetragonal-HfO₂ system through sterically mismatched ion doping

Defect-engineering is a frequent approach to modify the material's properties. The selection of a dopant is generally carried out by its similarity (size and valence state) to the host ions, which can only slightly tune the properties of the host materials. In this work, contrary to the traditional doping approach, sterically mismatched 'difficult-to-dope' ions featuring different ionic sizes and valence states were introduced as dopants, leading to local symmetry breaking. For this purpose, a non-ferroelectric tetragonal-HfO₂ with a 4/mmm point group was doped with sterically mismatched Y³⁺/La³⁺ ions. The larger ionic radius of Y and La (106 pm and 110 pm, respectively) has been expected to create a large point-defect-induced local symmetry breaking in the centrosymmetric tetragonal environment. Density-functional-theory-based ab initio calculations were performed to investigate the defect structure, where the local symmetry breaking and structural frustration caused by Y³⁺ and La³⁺ dopants resulted in a total atomic displacement of 2.769 and 3.507 Å, within the centrosymmetric environment. Importantly, this structural perturbation generates noticeable defect-induced dipole moments of 7.07 Debye and 9.62 Debye for Y³⁺ and La³⁺, respectively. The resultant dipole moment is attributed to the total ionic displacement of neighboring ions caused by local symmetry breaking through sterically mismatched ions with different ionic sizes (the larger the ionic size, the larger the ionic displacement and total dipole moment). In addition, La and Y co-doped HfO₂ was also investigated, where the co-doping led to a total dipole moment in the range of 7.12 - 8.68 Debye. The defect-dipolar behavior may provide another insight for understanding the polarization behavior in hafnium-based oxides as well as for manipulating material's properties.