Phosphor-converted white LEDs rely on combining a blue-emitting InGaN chip with yellow and red-emitting luminescent materials. The discovery of cyan-emitting (470-500 nm) phosphors is a challenge to compensate for the spectral gap and produce full-spectrum white light. Na<sub>0.5</sub>K<sub>0.5</sub>Li<sub>3</sub>SiO<sub>4</sub>:Eu<sup>2+</sup> (NKLSO:Eu<sup>2+</sup>) phosphor was developed with impressive properties, providing cyan emission at 486 nm with a narrow full width at half maximum (FWHM) of only 20.7 nm, and good thermal stability with an integrated emission loss of only 7% at 150 °C. The ultra-narrow-band cyan emission results from the high-symmetry cation sites, leading to almost ideal cubic coordination for UCr<sub>4</sub>C<sub>4</sub>-type compounds. NKLSO:Eu<sup>2+</sup> phosphor allows the valley between the blue and yellow emission peaks in the white LED device to be filled, and the color-rendering index can be enhanced from 86 to 95.2, suggesting great applications in full-spectrum white LEDs.