Question

Name the crystal defect which lower the density in an ionic solid

Answer 1

give reason schottky defect lowers the density of a solid. Answer: If in an ionic crystal of type A+B-, an equal number of cations and anions are missing from their lattice sites so thatelectrical neutrality as well as stoichiometry is maintained this is called a Schottky Defect.

Answer 2

Answer:

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Explanation:

When we talk about crystalline solids, we say they have a definite structure. They have long term arrangement of their molecules. This means that they have repeated arrangement of a unit cell to form a space lattice structure. So the arrangement of molecules, in the long run, will remain the same.

However, these crystals are not perfect. They have defects or imperfections in their arrangement of constituent particles. These defects are a deviation from their uniform arrangement of particles. The defects are of two kinds

• Point Defects: When the deviation occurs around an atom/particle it is a point defect. It could be due to displacement, an extra particle or a missing particle.

• Line Defects: When there is an abnormality in the arrangement of an entire row, then it is a line defect.

Example: Titanium monoxide has a rock-salt structure. X-ray diffraction data show that the length of one edge of the cubic unit cell for TiO with a 1:1 ratio of Ti to O is 4.18Å, and the density as determined by volume and mass measurements is 4.92 g cm–3. Do the data indicate that defects are present? If so, are they vacancy or interstitial defects? [Ti = 47.88 u].  

Solution:  

The presence of vacancies (Schottky defects) at the Ti and O sites should be reflected in a lower measured density than that calculated from the size of the unit cell and the assumption that every Ti and O site is occupied. Interstitial (Frenkel) defects would give little of any difference between the measured and theoretical densities. There are four formula units per unit all so theoretical density is  

d = 4 (47.88+16) / 6.023 × 1023 (4.18 × 1023) = 4.81g cm–3  

This is significantly greater than the measured density. The crystal must, therefore, contain numerous vacancies. Because the overall composition of the solid is TiO, there must be equal number of vacancies on cation and anion sites.