Stabilization of Cubic Phase at Room Temperature and Photoluminescence Properties of Dy and Sm Co-Doped HfO2 Nanoparticles

HfO2 shows the monoclinic phase at room temperature (RT), whereas the technologically important high-k tetragonal and cubic phases are observed at ∼1700 °C and 2600 °C, respectively. Herein, we reveal that the high-temperature cubic phase of HfO2 is stabilized at RT after incorporating Dy and Sm codopant total concentration up to 13 at%. Below 13 at%, the monoclinic and cubic phases coexist, evidenced by Le-Bail profile refinement of the X-ray diffraction patterns. Transmission electron micrographs demonstrate average particle size as ∼31 and ∼10 nm for the monoclinic and cubic phase, respectively, which agrees with the crystallite size estimated from Debye–Scherrer equation. The monoclinic to cubic phase transformation is explained in terms of the oxygen vacancies formation and difference in ionic radii of Sm3+, Dy3+, and Hf4+ ions. Interestingly, electron spin resonance spectroscopy analysis indicates that while HfO2 exhibits oxygen vacancies, Dy and Sm co-doped HfO2 shows formation of magnetically inactive defect complexes. Moreover, low Dy and Sm co-dopant concentration in HfO2 produces strong emissions in green, yellow, and orange-red color regions under different excitation wavelength induced via exchange of excited electrons between nearby energy levels of Dy3+ and Sm3+. Such a weak energy transfer phenomenon is primarily governed through multipolar interaction mechanism.