Identify the reagent used in Etard reaction

An element with a molar mass 27g mol$^{-1}$ forms a cubic unit cell. Calculate the number of atoms present in a unit cell if the density of metal is 2.7g cm$^{-3}$.
$[a^3 \times N_A = 40$ cm$^3$ mol$^{-1}]$

A compound formed by elements A and B crystallizes in a cubic structure where A atoms are at the corner of the cube and B atoms are at the center of the cube. What is the formula of the compound?

Which of the following dopant is NOT used in Ge to obtain n-type semiconductor?