Two new barium magnesium fluorides, Ba.sub.2 Mg.sub.3 F.sub.10 and BaMg.sub.3 F.sub.8, can be prepared by solid state reactions of barium fluoride and magnesium fluoride by heating in controlled atmospheric conditions at a temperature from about 700.degree. C to about 850.degree. C. In the BaF.sub.2 -MgF.sub.2 binary system, compositions containing from about 40 to 85 mole percent of MgF.sub.2 when activated by europium alone or in combination with manganese have been found to luminesce when excited by ultraviolet light, cathode rays, or with x-ray bombardment. Lamps incorporating these materials afford benefits of high temperature stability and x-ray intensifying screens incorporating these materials yield a speed equal to or better than present screens employing calcium tungstate.