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White-light-emitting Ca4–xF2Si2O7:xCe3+ phosphor and violet-light-emitting oxyfluoride Ca4F2Si2O7 host were synthesized by solid-state reaction. Ca4–xF2Si2O7:xCe3+ has strong absorption in the near-UV region (370 nm) and shows a broad emission in the range of 390–600 nm centered at 475 nm. Under 315 nm excitation, a narrow blue emission was observed. The Ca4F2Si2O7 host synthesized under the same reduction conditions exhibited violet emission due to the formation of anion-deficient nonstoichiometric Ca4F2−δSi2O7+δ/2 species. The broad emission of Ca4F2Si2O7:xCe3+ phosphor is attributed to Ce3+ occupying two crystallographic calcium sites available in the host. A white LED device was fabricated using Ca4F2Si2O7:Ce3+ without any additive phosphor, displaying excellent CIE chromaticity (0.29, 0.35) close to white emission with a color rendering index of 97. These exceptional optical properties of Ca4F2Si2O7:Ce3+ suggest the promising application of the single activator phosphor that could produce white light under near-UV-based LEDs.