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.