This is a (1x3) structure

Rationale

The unit cell is illustrated below - note, this cell contains only one repeat unit and is the primitve unit cell for the adsorbate.

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 = a1 b2 = 3.a2

Hence, this structure is a (1x3) structure.

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