This is a (2x1) structure
The unit cell is illustrated below.
It is rectangular in shape and shows no rotational misalignment with the substrate unit cell. Using the substrate vectors given on the diagram we have the following relations :
|b1 = 2.a1||b2 = a2|
Hence, this structure is a (2x1) structure. In this instance it is particularly important to correctly define a1 , a2 , b1 & b2 since the (1x2) structure is completely different in nature.
What is the matrix description for this structure ?