Potassium, rubidium.

In an alloy of gold and cadmium, gold crystallizes in cubic structure occupying the corners only and cadmium fits into the face centre voids.

Number of gold atoms     =1/8 x 8 =1Au

Number of candium atoms =1/2 x 6 =3Cd

Hence, quantitative composition on = AuCd₃

Both naturally-occurring and artificial quartz crystal contains at least ninety-nine percent silicone dioxide, while cut glass crystal only possesses at most eighty percent silicone dioxide. 

Compounds having frenkel defect conduct electricity to a small extent. When electric field is applied, an ion moves from its lattice site to occupy a hole, it creates a new hole. In this way, a hole moves from one end to the other. Thus, it conducts electricity across the crystal. Due to the presence of holes, stability (or the lattice energy) of the crystal decreases.