CRYSTALLOGRAPHIC CHARACTERISTICS OF THE AL-CO DECAGONAL QUASI-CRYSTAL AND ITS MONOCLINIC APPROXIMANT TAU(2)-AL13CO4

The binary Al-Co decagonal quasicrystal (DQC) is stable between 800-degrees-C and 900-degrees-C and transforms to a primitive monoclinic phase (a = 3.984 nm, b = 0.8148 nm, c = 3.223 nm, and beta = 107.97 deg) above 950-degrees-C or below 750-degrees-C. The composition of this crystalline approximant is similar to the C-centered monoclinic Al13Co4 but with a and c parameters tau2 (tau = (1 + square-root 5)/2 = 1.61803. . . ) times larger. This tau2-Al13Co4 occurs frequently as (001) and (100) twins. The transformation of the Al-Co DQC to (100) twins of tau2-Al13Co4 has been followed by selected-area electron diffraction. With reference to the main zonal electron diffraction patterns and the X-ray diffractogram of the Al-Co DQC, the X-ray powder diffractogram of tau2-Al13Co4 with a giant unit cell can be successfully indexed.