Yttrium lithium fluoride

Yttrium lithium fluoride (LiYF4, 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 LiYF4. The hardness of YLF is significantly lower than other commons crystalline laser media, i.e. yttrium aluminium garnet.