In situ Raman and Time-Resolved Luminescence Investigation of the Local Structure of ZrO2 in the Amorphous to Crystalline Phase Transition

Abstract

The local structure of europium doped and impregnated ZrO2 in the amorphous state and during crystallization is investigated by in situ X-ray diffraction and in situ Raman, high-resolution transmission electron microscopy (HRTEM) and time-resolved photoluminescence spectroscopy. From Raman spectra excited at three different wavelengths (λex= 488, 514, and 633 nm), both phonon modes of ZrO2 and photoluminescence (PL) corresponding to europium electronic transitions were investigated. In the as-synthetized state, samples were X-ray and Raman amorphous with few tetragonal (also monoclinic) crystallites being observed under HRTEM microscopy. In situ XRD patterns show that all samples crystallize in the tetragonal phase around 450 °C. The time-resolved PL spectra of europium doped and impregnated ZrO2 show spectral dynamics with time delay after lamp/laser pulse which is assigned to the coexistence of the different amorphous and crystalline components or unreacted europium precursor. From in situ Raman spectra, crystallization was detected at 300–350 °C, monitoring for the characteristic tetragonal-like 5D0–7F2 emission of europium at 606 nm. The ratio of tetragonal to amorphous emission increased abruptly from ca. 2–4% at 300–400 °C to almost 25% at 400–450 °C, whereas at 500 °C the emission is mostly tetragonal. A similar trend was found with the ex situ calcined samples, but relative strong tetragonal emission was observed at lower temperature in the range of 350 to 400 °C.