Question

Which of the following statement(s) for crystals having Schottky defect is/are correct?

• A. Schottky defect arises due to the absence of cation and anion from the position which it is expected to occupy
• B. Schottky defect are more common in ionic compounds with high coordination numbers
• C. The density of the crystals having Schottky defect is larger than that of the perfect crystal
• D. The crystal having Schottky defect is electrically neutral as a whole

Answer 1

Answer: (A), (B), (D)

The correct options are
A Schottky defect arises due to the absence of cation and anion from the position which it is expected to occupy
B Schottky defect are more common in ionic compounds with high coordination numbers
D The crystal having Schottky defect is electrically neutral as a whole

Schottky Defect:

This defect is caused when some of the lattice points are unoccupied called vacancies or holes as shown in figure (a). The number of missing positive and negative ions is the same so that the crystal remains neutral in all.

Schottky defects are more common in ionic compounds with high coordination number and where the sizes of positive and negative ions are almost equal. For example, NaCI, KCI, CsCI and KBr.

The number of defects increases with increase in temperature. The number of defects increases to one in $106$ sites at $775$ K and one in $104$ sites at $1075$ K. The presence of large number of Schottky defects in crystal results in significant decrease in its density.