Frenkel defect is noticed in_______. 

The unit cell with the following structure refers to______crystal system.

A substance ${\mathrm{A}}_{\mathrm{x}}{\mathrm{B}}_{\mathrm{y}}$ crystallises in a face centred cubic $\left(\mathrm{FCC}\right)$ lattice in which atoms $\text{'}\mathrm{A}\text{'}$ occupy each corner of the cube and atoms $\text{'}\mathrm{B}\text{'}$ occupy the centres of each face of the cube. Identify the CORRECT composition of the substance ${\mathrm{A}}_{\mathrm{x}}{\mathrm{B}}_{\mathrm{y}}$.