Structural studies of (Ca,Sr)F2 single crystals with Raman and NMR spectroscopies

Single crystal CaF2 is the material of choice for optics in the vacuum UV, due in part to its high transmission at these wavelengths. Unfortunately, as the wavelengths decrease, intrinsic birefringence (also referred to as spatial dispersion) can become larger than the target birefringence for precision optics at 157 nm, even for a cubic material such as CaF2. One elegant solution to this problem is to capitalize on opposite signs for this intrinsic birefringence, such as mixing SrF2 or BaF2 with CaF2. If the proper fluorides are mixed with the correct concentration, the intrinsic birefringence of the mixed single crystals can be minimized. It is also important that the resulting mixed single crystal maintain the high symmetry of CaF2, which requires that the cations mix randomly in the CaF2 structure. We show through Raman and NMR spectroscopic studies that this random mixing of cations can be achieved for mixed (Ca,Sr)F-2 single crystals. A single Raman band is observed for all mixed crystals, which changes in both position and width upon addition of SrF2 to CaF2. F-19 NMR spectra also reflect a random distribution of cations around the different fluorine environments in the mixed crystals.