Multifunctional role of dysprosium in HfO2: stabilization of the high temperature cubic phase, and magnetic and photoluminescence properties

Abstract

Hafnium oxide (HfO₂) can exist in different crystalline structures such as monoclinic at room temperature, tetragonal at 1700 °C and cubic at 2600 °C. In the present study, nanocrystalline powders of HfO₂ synthesized by a Pechini type sol–gel technique show a monoclinic phase, P2₁/c, at room temperature. By incorporating Dy into the HfO₂ lattice, the intensity of all diffraction peaks corresponding to P2₁/c decreases and at a concentration of 11 at% of Dy, the monoclinic phase transforms completely to the cubic phase, Fmm, showing a mixed phase of monoclinic and cubic at intermediate concentrations (5–9 at%) of Dy. For the first time, we have stabilized the high temperature cubic phase of HfO₂ at room temperature by incorporating Dy. Selected area electron diffraction patterns confirm the monoclinic and the cubic phase as observed from the X-ray diffraction patterns. A mechanism for stabilization of the high temperature cubic phase in Hf_1−xDy_xO₂ has been analyzed based on the substitution of dysprosium for hafnium ions and the formation of oxygen vacancies. While ferromagnetic ordering at room temperature observed in HfO₂ nanoparticles is quenched after incorporating 1 at% of Dy, photoluminescence (PL) studies demonstrate excellent emissions in the blue and yellow region after exciting with UV light of wavelength 352 nm. Combining excitation and emission profiles, we have proposed a tentative energy band diagram illustrating the energetic processes taking place in Hf_1−xDy_xO₂.