Effects of Cr3+ addition on the structure and optical properties of α-Zn2SiO4 synthesized by sol-gel method

Abstract The α-Zn 2 SiO 4 (willemite) structure is known as an extremely versatile host matrix for phosphors. Here we show that Cr 3+ has very low solubility in α-Zn 2 SiO 4 ， and powder x-ray diffraction confirms that ZnCr 2 O 4 spinel is segregated even at 0.05 mol% Cr addition， while Raman spectroscopy shows that a silicate phase is segregated at the same 0.05 mol% Cr addition and these results confirm that both Zn 2+ and Si 4+ vacancies occur with the addition of Cr 3+ to α-Zn 2 SiO 4 and charge neutrality requires that two Cr 3+ ions are needed for each Zn 2+ and Si 4+ vacancy. The small addition of Cr 3+ to α-Zn 2 SiO 4 results in a small lattice expansion and because the Cr–O and Zn–O bond lengths are similar, and the Si–O bond length is shorter, it is expected that the Cr 3+ ion is closer to the Si 4+ vacancies resulting in a lattice expansion. This is supported by X-ray photoelectron spectrometry which shows that the binding energy of Si 2p, and especially O 1s, are more affected by Cr 3+ addition than Zn 2p binding energy. The UV–vis absorption spectrum for α-Zn 2 SiO 4 :Cr 3+ confirms that the Cr 3+ d 3 ion is in its preferred octahedral coordination but the photoluminescence (PL) measurements show emission in the yellow red region (CIE coordinates x = 0.52 and y = 0.45), rather than red, suggesting that the octahedral coordination for Cr 3+ is distorted. The results confirm that the Cr 3+ is not sited in either of the non-equivalent tetrahedral sites for Zn 2+ in α-Zn 2 SiO 4 , as is typically reported for the other cations added to the host lattice.