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