Yttrium lithium fluoride (LiYF<sub>4</sub>, sometimes abbreviated YLF) is a yellowish transparent birefringent crystal, typically doped with neodymium or and used as a gain medium in solid-state lasers. Yttrium is the substitutional element in LiYF<sub>4.</sub> The hardness of YLF is significantly lower than other commons crystalline laser media, i.e. yttrium aluminium garnet.