The coverage for this structure is θ = 1/3 (θ= 0.33)

Rationale

In this instance , if we let the rhombohedral substrate unit cell be of unit area then the (3x3)R30 cell of the adsorbate has an area of 3 units and contains one adsorbate atom : hence

Coverage = ( 1 / 3 ) x 1 = 0.33

Note - in this instance it is still an easy matter to determine the relative area of the adsorbate unit cell since it is has the same shape as the substrate cell. More generally, the area of the adsorbate unit cell relative to that of the substrate can be obtained from the magnitude of the determinant of the matrix required to describe the adsorbate unit cell.

i.e. Relative Area = | (m11 x m22 ) - (m12 x m21 ) |

e.g. in this instance, Relative Area = |(-2)-(1)| = |-3| = 3

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