A HIGH-RESOLUTION ELECTRON-MICROSCOPY STUDY OF THE DOMAIN-STRUCTURES OF MONOCLINIC TAU-2-AL13CO4

A high-resolution electron microscopy investigation was carried out to study the domain structure of monoclinic tau2-Al13Co4 whose beta almost-equal-to 108-degrees and parameters a and c are tau2 almost-equal-to 2.618 ... times, where tau = (1 + square-root 5)/2 is the golden number, greater than their counterparts in Al13Co4 (a = 1.5184 nm, b = 0.8122 nm, c = 1.2340 nm, beta = 107.90-degrees). Co atoms in the (010) layer form pentagons and 36-degrees rhombi with an edge length of 1.23 nm. Different tessellations of these pentagons form various domain structures, such as parallel domains with the domain boundary parallel to (100), (201BAR), (101BAR) and (001) and 36-degrees rotational domains (glide twins) with the domain boundary on (100) and (201BAR). Various complex domain structures resulting from a combination of parallel and rotational domains have also been found, and, in addition to pentagons and 36-degrees rhombi, other Penrose tiles such as 72-degrees rhombus, 'boat', and 'star' also occur.