Role of Oxygen Vacancies in Stabilizing the Orthorhombic Phases of Hf0.5Zr0.5O2 Nanoparticles

In this work we study the stabilization of the orthorhombic phases in small Hf0.5Zr0.5O2 nanoparticles (average size ~ 7 nm) annealed under different oxygen partial pressures. Concentration of the oxygen vacancies, which is determined by annealing conditions, was estimated from the electron paramagnetic resonance spectra and X-ray photoelectron spectroscopy. The fraction of the orthorhombic phases, that is determined by the X-ray diffraction and nuclear magnetic resonance, depends on the concentration of oxygen vacancies. Phenomenological calculations based on Landau-Ginzburg-Devonshire theory considering trilinear coupling between nonpolar, antipolar and polar phonon modes, indicate that chemical strain induced by oxygen vacancies can stabilize the orthorhombic phase o-III with the ferroelectric long-range ordering in small Hf0.5Zr0.5O2 nanoparticles. The theory confirms the stability of ferroelectric polarization in the vacancy-enriched Hf0.5Zr0.5O2 nanoparticles. The increase in the intensity of the dielectric permittivity maximum, observed near 350 - 380 K in the PVDF matrix with the Hf0.5Zr0.5O2 nanoparticles annealed in the CO+CO2 atmosphere, is clearly associated with the increase in oxygen vacancies concentration. The vacancies lead to the defect-induced elastic dipole formation and to the increase in ionic conductivity, which decreases the depolarization field and may induce the ferroelectric-like phase transition in the vacancy-enriched Hf0.5Zr0.5O2 nanoparticles. Due to the interfacial effects the negative capacitance states may be realized in weakly screened and spatially isolated Hf0.5Zr0.5O2 nanoparticles embedded in the PVDF matrix.The present approach based on oxygen-vacancy-induced elastic and screening effects may provide a route for engineering ferroelectric-like states in other nanoscale ferroic oxides.