Effect of Changing Exposed Surfaces of SrF₂ Crystal on the Two-Dimensional Condensation of H₂O and Kr

We succeeded in preparing SrF2 samples having exposed surfaces different from the original surfaces by hydrothermal treatment. The original sample was synthesized by mixing aqueous solutions of Sr-(NC>3)2 and of NH4F and has a well-developed {100} surface. The hydrothermally treated samples have {110} or {111} surfaces as determined by X-ray and TEM methods. From the measurement of adsorption isotherms of H2O and Kr, it is clearly indicated that the {100} surface is homogeneous and the {111} surface heterogeneous for H2O adsorption and that both {100} and {111} surfaces are homogeneous for Kr adsorption. Moreover, the {111} surface is more homogeneous than the {100} for Kr adsorption. The strongly adsorbed H2O is present on the surface of every sample. It is concluded that strongly adsorbed H2O, which gives infrared absorption peaks at 3684, 2561, 1947, 1656, and 1000 cm1, is present on the {100} surface and that on this surface the two-dimensional condensation of H2O occurs.