Abstract Combining the experimental characterization with the large-scale density functional theory calculations based on finite-element discretization (DFT-FE), we address the stabilization of polar orthorhombic phases (o-HfO₂) in Al:HfO₂ nanofilms by means of the atomic registry distortions and lattice deformation caused by Al substitutional defects (Al_Hf) and Schottky defects (2Al_Hf + V_O) in tetragonal phases (t-HfO₂) or monoclinic phases (m-HfO₂). The phase transformation directly from the t-HfO₂ into polar o-HfO₂ are also elucidated within a heterogeneous distribution of Al dopants in both t-HfO₂ bulk crystal structure and Al:HfO₂ nanofilm. It is revealed using large-scale DFT calculations that the Al substitutional defects (Al_Hf) or the Schottky defect (2Al_Hf + V_O) could induce the highly extended atomic registry distortions or lattice deformation in the t- and m-HfO₂ phases, but such effects are greatly diminished in ferroelectric orthorhombic phase. By purposely engineering the multiple Al_Hf defects to form dopant-rich layers in paraelectric t-HfO₂ nanofilm or bulk crystal, the induced extended lattice distortions surrounding the defect sites exhibit the shearing-like atomic displacement vector field. The large-scale DFT calculations further predicted that the shearing-like microscopic lattice distortions could directly induce the phase transformation from the t-HfO₂ into polar orthorhombic phase in both Al:HfO₂ bulk crystal and nanofilms, leading to the large remanent polarization observed in Al:HfO₂ nanofilms with the presence of Al-rich layers. The current study demonstrates that the ferroelectricity of HfO₂ bulk crystal or thin film can be optimized and tuned by delicately engineering both the distribution and concentration of Al dopants in atomic layer deposition without applying the top capping electrode, providing the extra flexibility for designing the HfO₂ based electronic devices in the future.