Select the incorrect statement:

Options

A. Schottky defect is shown by CsCl
B. Frenkel defect is shown by ZnS
C. Hexagonal close packing (hcp) & cubic close pack- ing (ccp) structure has same co-ordination no. of 12
D. At high pressure, the co-ordination number de- creases.

Answer

To solve the question of selecting the incorrect statement, we will analyze each option step by step.

### Step 1: Analyze Option A
**Statement:** Schottky defect is shown by CaCl.

**Analysis:** A Schottky defect occurs when an equal number of cations and anions are missing from the lattice, maintaining charge neutrality. Calcium chloride (CaCl₂) can exhibit Schottky defects due to the presence of both Ca²⁺ and Cl⁻ ions.

**Conclusion:** This statement is correct.

### Step 2: Analyze Option B
**Statement:** Frenkel defect is shown by ZnS.

**Analysis:** A Frenkel defect occurs when an ion is displaced from its lattice site to an interstitial site, creating a vacancy. Zinc sulfide (ZnS) is known to exhibit Frenkel defects, especially due to the smaller size of the Zn²⁺ ions compared to the S²⁻ ions.

**Conclusion:** This statement is correct.

### Step 3: Analyze Option C
**Statement:** Hexagonal close-packing (HCP) and cubic close-packing (CCP) structure has the same coordination number 12.

**Analysis:** Both hexagonal close packing (HCP) and cubic close packing (CCP) arrangements have a coordination number of 12, meaning each atom is in contact with 12 others.

**Conclusion:** This statement is correct.

### Step 4: Analyze Option D
**Statement:** At high pressure, the coordination number decreases.

**Analysis:** The coordination number refers to the number of nearest neighbors surrounding an atom in a crystal structure. Generally, when pressure increases, particles are forced closer together, which typically results in an increase in the coordination number, not a decrease.

**Conclusion:** This statement is incorrect.

### Final Conclusion
The incorrect statement is option D: "At high pressure, the coordination number decreases."

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