Four two-row crystals with different structures. (a) n = 16, , , and , it is nearly the same as Fig. 1(a) and the grains have inside square structures. (b) n = 20, , , and , it has 9 grains in the top row and 11 grains in the bottom row. (c) n = 20, , , and . The grains have hexagonal structures. (d) n = 20, , , and , it is something between panels (a) and (c).