no.of triangular voids in fcc structure

Dear student,

Each sphere is in contact with six of its neighbours and the two dimensional coordination number is 6. The centres of these six spheres are at the corners of a regular hexagon Fig (b), hence this packing is called two dimensional hexagonal close-packing. It can be seen that in this layer there are some voids. These are triangular in shape. The triangular voids are of two different types. In one row , the apex of the triangles are pointing upwards and in the next layer downwards.