There is provided a substrate material for an optical component for X-rays of wavelength _R. The substrate includes (a) a glass phase made of amorphous material having a positive coefficient of thermal expansion, and (b) a crystal phase including microcrystallites having a negative coefficient of thermal expansion and a mean size of less than about 4 _R. The substrate material has a stoichiometric ratio of the crystal phase to the glass phase such that a coefficient of thermal expansion of the substrate material is less than about 510^-6 K^-1 in a temperature range of about 20 C. to 100C. The substrate material, following a surface treatment, has a high spatial frequency roughness (HSFR) of less than about _R/30 rms.