YVO4 (Yttrium Vanadate) is an optical crystal that has a high birefringence. It is a common material used in various applications in the optical field such as waveplates, polarizers, and optical isolators. When undoped, YVO4 has a cubic crystal structure, and its optical properties are primarily determined by its large birefringence and high refractive index. This makes YVO4 a popular choice for optical components that require high transmission, high damage threshold, and high stability.