Epitaxial Polycrystalline $CaF2$ Films

Epitaxial polycrystalline $CaF2$ films were formed by vacuum evaporation onto (100) cleavage faces of NaCl at 200 °C under high vacuum conditions. The epitaxial relationships are given as follows: complex $(110)CaF2∥(100)NaCl$ with average $〈111〉CaF2∥〈100〉NaCl$⁠. Transmission electron microscopy and diffraction were used to study the structure and microstructure of these films. Their polycrystalline nature was inferred from the spot electron diffraction patterns. These showed that, two or more sets of epitaxial $CaF2$ crystallites were oriented at certain definite angles relative to each other. Electron microscope dark field observations confirmed this interpretation. The complex electron diffraction patterns obtained from individual films were interpreted in terms of an extraordinary amount of double diffraction taking place at the edges between the various sets of crystallites.