This is a c(2x2) structure


The centred unit cell is illustrated below - note, this cell contains two repeat units (in this case individual atoms), one in the centre and four "quarter atoms" at the corners of the unit cell.

It is square in shape and shows no rotational misalignment with the substrate unit cell but is larger by a factor of two in both linear dimensions. Since there is also an atom at the centre which is crystallographically equivalent to those at the corners, this is a centred (2x2), or c(2x2) structure.

By contrast, the diagram below shows the smaller primitive unit cell.

  How would the primitive unit cell for this structure be described using Wood's notation ?

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