Blue-silica by Eu²⁺-activator occupied in interstitial sites

A blue-emitting SiO₂:Eu²⁺ compound has been successfully synthesized and characterized. The PL intensity of SiO₂:Eu_0.002²⁺ compound is about 24 times higher than that of the O-defective SiO₂ compound (without activators), which emits blue light. The valence state of the Eu ions responsible for the highly enhanced blue emission was determined to be Eu²⁺ using reference materials (EuCl₂ and EuCl₃) and XPS measurements. The Eu²⁺-activator ions occupied in the interstitial sites of the SiO₂ matrix were confirmed by FT-IR, XPS, and ²⁹Si MAS-NMR spectroscopy. Even though the void spaces formed structurally in both α-quartz and α-cristobalite can accommodate Eu²⁺ ions (ionic radius = 1.25 Å at CN = 8), SiO₂:Eu²⁺ compound fired at 1300°C under a hydrogen atmosphere is destined to be deficient in O or Si atoms, indicating the formation of the wider void spaces in the SiO₂ crystal lattice. A sputtered depth profile of SiO₂-related compounds obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) corroborates the O-defective SiO₂ induced by hydrogen. In particular, the interatomic potentials, depending on the interstitial positions of Eu atoms in α-cristobalite and α-quartz, are calculated based on Lennard-Jones and coulomb potentials. For α-cristobalite, the minimum potential value is -51.47 eV and for α-quartz, the value is 221.8 eV, which reveals that the Eu²⁺-activator ions more preferably enter the interstitial sites of α-cristobalite than those of α-quartz. Thanks to the stable Eu²⁺-activator ions enclosed by Si-O linkages, the SiO₂:Eu_0.002²⁺ compound emits blue light and its PL emission intensity is about 24 times higher than that of the O-defective SiO₂ compound. This phosphor material could be a platform for modeling a new phosphor and for application in the solid-state lighting field.