Three-dimensional diffraction data collected from a small single crystal about 10 μm in size, using synchrotron radiation, were found to be scarcely affected by secondary extinction and provide important information on the structural chemistry of solid solutions as well as the electron density distribution in crystals. A detailed examination of the crystal structure of K 2 NiF 4 -type CaYAlO 4 coupled with molecular dynamics calculations revealed a wrinkling of the rock-salt-type monolayer caused by the difference in valency between the Ca and Y cations, and by the oxygen vacancies concentrated at the apical O2 sites.