Formation mechanism of scattering centers in BaMgF4 single crystals

Barium magnesium fluoride (BaMgF4—BMF) is a promising material for ultraviolet (UV) all solid-state lasers, but harmful scattering centers are always present in BMF crystals grown by the Czochralski method. In this work, scattering centers were observed only in the middle and upper parts of the as-grown crystals, whereas no light scattering was found in bottom parts. Furthermore, scattering centers can be removed by annealing at temperatures near the melting point, while they can also be created in the originally scatter-free crystals after being annealed at relatively low temperatures. Based on the results of post-growth heat treatments, the presence of a solid solution region near the melting point of BMF is proposed, and scattering centers were attributed to the precipitation of second-phase particles during crystal growth. Combining this hypothesis and the temperature distribution in the growth chamber, we are able to explain the scattering center formation mechanism and their inhomogeneous distribution in the as-grown crystals. This research will enable the growth of higher optical quality BMF crystals used in ultraviolet all solid-state lasers.