Innovative materials for phosphor-converted white light-emitting diodes (pc-LEDs) are much sought after due to the huge potential of the LED technology to reduce energy consumption worldwide. One of the main levers for furtheri mprovements are the conversion phosphors. The system Ca 1Àx Sr x AlSiN 3 :Eu 2 + currently provides one of the mosti mportantr ed-emitting phosphors for pc-LEDs. We report the discovery of the new polymorph b-Ca 1Àx Sr x AlSiN 3 :Eu 2 + which allows for significant improvements to LED efficacies. It crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 982.43(10) pm, b = 575.2(1) pm and c = 516.12(5)pm. Compared to a-Ca 1Àx Sr x AlSiN 3 :Eu 2 + ,i ts emission shows a significantly reduceds pectral full-width at half maximum (FWHM). With that, we demonstrated 3% efficacy increase for white light-emitting pc-LEDs. The new polymorph can easily be industrialised, because the synthesis works on the same equipment as a-Ca 1Àx Sr x AlSiN 3 :Eu 2 + .