High temperature structural investigation of the delafossite type compound CuAlO2

Crystal structure parameters were determined for the delafossite type compound CuAlO₂ at 295, 450, 600, 750, 900, and 1200 K with single crystal high temperature X-ray diffraction technique. Anisotropic refinements result in conventional R values of 0.021, 0.027, 0.029, 0.030, 0.032, and 0.036 at respective temperatures. Crystals of CuAlO₂ have the rhombohedral space group $\text{R}\text{3}\text{m}$ with a = 2.8584(7), c = 16.958(3) Å and Z = 3 at 295 K. The mean thermal expansion coefficient for the dimension a is 11.0 × 10^?6 K^?1 about three times larger than 4.1 × 10^?6 K^?1 for c. In the structure, the AlO₆ octahedra are linked by their OO edges and form AlO₂ layers perpendicular to the c axis with the thickness corresponding to the height of an octahedron. With increasing temperature, the AlO₆ octahedra expand along the directions of the basal plane, while expansion scarcely occurs along the c axis. The Cu atom lying between the AlO₂ layers shows a large anisotropic behavior in the thermal vibration. The temperature factor for Cu atom in the basal plane becomes very large (0.044 Ų) at 1200 K, but the ratio of the temperature factor perpendicular to c to that parallel to c does not change appreciably with increasing temperature.