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

Abstract The α-Zn2SiO4 (willemite) structure is known as an extremely versatile host matrix for phosphors. Here we show that Cr3+ has very low solubility in α-Zn2SiO4， and powder x-ray diffraction confirms that ZnCr2O4 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 Zn2+ and Si4+ vacancies occur with the addition of Cr3+ to α-Zn2SiO4 and charge neutrality requires that two Cr3+ ions are needed for each Zn2+ and Si4+ vacancy. The small addition of Cr3+ to α-Zn2SiO4 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 Cr3+ ion is closer to the Si4+ 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 Cr3+ addition than Zn 2p binding energy. The UV–vis absorption spectrum for α-Zn2SiO4:Cr3+ confirms that the Cr3+ d3 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 Cr3+ is distorted. The results confirm that the Cr3+ is not sited in either of the non-equivalent tetrahedral sites for Zn2+ in α-Zn2SiO4, as is typically reported for the other cations added to the host lattice.