A composition formed from Group II fluorides in which the composition has little or no intrinsic birefringence at a selected wavelength. The composition is a mixed solid solution of CaF.sub.2 with a second crystal of SrF.sub.2 or BaF.sub.2. The resulting composition is in the form of Ca.sub.1-xSr.sub.xF.sub.2 or Ca.sub.1-xBa.sub.xF.sub.2, or a combination of SrF.sub.2 and BaF.sub.2, in the form of Ca.sub.1-x-ySr.sub.xBa.sub.yF.sub.2. The specific form of the composition that effectively nulls out the intrinsic birefringence at a selected wavelength within the UV range is determined in one preferred method from the magnitudes of the intrinsic birefringences of the components, CaF.sub.2, SrF.sub.2, and BaF.sub.2.