Comprehensive study on the origin of orthorhombic phase stabilization in Gd-doped HfO₂ and DFT calculations

In recent times, ultra-thin films of hafnium oxide (HfO₂) have shown ferroelectricity (FE) attributed to the orthorhombic (o) phase of HfO₂ with space group Pca2₁. This polar o-phase could be stabilized in the doped thin film of the oxide. In the present work, both polar and non-polar o-phases of HfO₂ could be stabilized in Gd-doped bulk polycrystalline HfO₂. Rietveld analysis of XRD data shows that the relative population of o-phases in the presence of the monoclinic (m) phase of HfO₂ increases with increasing Gd-content. The local environment around the host atom has been investigated by time differential perturbed angular correlation (TDPAC) spectroscopy, synchrotron based X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements showed a reduction in grain size with increasing Gd-dopant indicating a solute drag effect. It could be established that the segregation of the Gd-dopant in the grain boundary is a thermodynamically favorable process and the solute drag effect plays an important role in nucleation of the o-phase in bulk HfO₂. Stabilization of Gd in both Pbca and Pca2₁ phases of HfO₂ was supported by defect formation energy calculations using density functional theory (DFT). The present study has important implications in future applications of HfO₂ in ferroelectric devices and in understanding the role of dopants in stabilizing the o-phase of HfO₂ in the bulk.