A solid source material is described for forming a tungsten-containing film. The solid source material is tungsten hexacarbonyl, wherein content of molybdenum is less than 1000 ppm. Such solid source material may be formed by a process including provision of particulate tungsten hexacarbonyl raw material of particles of size less than 5 mm, wherein particles of size greater than 1.4 mm are less than 15% of the particles, and wherein content of molybdenum is less than 1000 ppm, and sintering the particulate tungsten hexacarbonyl raw material at temperature below 100° C. to produce the solid source material as a sintered solid.