91Ó°ÊÓ

The coordination number of a sphere occupying tetrahedral hole is 4 . They are smaller than the cubic or octahedral holes. Face- centred cubic arrangement has tetrahedral (8) and octahedral holes (4). The example used to see a tetrahedral structure is methane. Here, the carbon atom is attached to four equidistance hydrogen atoms placed at the corners of tetrahedrons.

Assume\(x\) be the number of phosphide ions present in the lattice. Hence, the number of tetrahedral holes is 2x.

One-fourth of the tetrahedral holes are occupied by cadmium ions.

The mathematical representation is as follows: \(\frac{1}{4} \times 2x = \frac{x}{2}\)

One-fourth of the tetrahedral holes are occupied by tin ions.

The mathematical representation is as follows: \(\frac{1}{4} \times 2x = \frac{x}{2}\)

Hence, the molecular formula of cadmium sulphide is \({{\rm{P}}_x}{\rm{C}}{{\rm{d}}_{\frac{x}{2}}}{\rm{S}}{{\rm{n}}_{\frac{x}{2}}}\).

To get the whole number, multiply with \(\frac{2}{x}\) then molecular formula is \({{\rm{P}}_2}{\rm{CdSn}}\).Thus, the molecular formula of cadmium, tin and phosphorus is \({{\rm{P}}_2}{\rm{CdSn}}\).