Abstract
We present a surface X-ray diffraction study of the {0 0 1} faces of potassium dichromate crystals in a humid environment. An etch-resistant layer develops in such an environment, which prevents the crystal from etching in a low-solubility solution. This layer is shown to be amorphous and is likely to be permeable for water molecules. The interface between the crystal and the amorphous layer is atomically flat. The crystal surface is not reconstructed, showing a potassium termination. The atomic structure of the surface is influenced by the relative humidity (RH), at 40% RH the top layer of molecules is expanded while at 100% RH these molecules are compressed. A first indication is given of a structural difference between the (0 0 1) and () faces, which is relevant for understanding the hypomorphism exhibited by this system.