The lowest coordination number is 6 which is that exhibited by atoms at the kinks in the steps.

Rationale

The lowest coordination number is exhibited by atoms "on the outside" of the kinks in the steps. Such atoms have only three nearest neighbours in the surface layer of terrace atoms which terminates at the step, and another three in the layer immediately below ; a total of 6. This contrasts with the surface atoms on the terraces which have a coordination number of 9 and the normal step atoms which have a coordination number of 7.

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