The appearance of colour in solid alkali metal halides is generally due to

Options

A. Schottky defect
B. Frenkel defect
C. Both Frenkel and Schottky defects
D. Cation excess defect

Answer

To solve the question regarding the appearance of color in solid alkali metal halides, we can follow these steps:

### Step 1: Understand the Types of Defects in Ionic Crystals
Ionic crystals can have different types of defects, which can affect their properties, including color. The two main types of defects are:
- **Schottky Defect**: Occurs when there are vacancies of cations and anions in the crystal lattice, typically in ionic compounds where the size difference between cations and anions is small.
- **Frenkel Defect**: Occurs when a cation leaves its normal position and occupies an interstitial site, usually when the anion is significantly larger than the cation.

### Step 2: Identify the Relevant Defect for Color Appearance
In the case of alkali metal halides, the appearance of color is primarily due to the presence of excess cations in the lattice. This is known as:
- **Cation Excess Defect (Metal Excess Defect)**: This defect occurs when there are more cations than anions, leading to the presence of unpaired electrons in the crystal lattice.

### Step 3: Explain the Mechanism of Color Appearance
The unpaired electrons in the cation excess defect can absorb energy from visible light. When these electrons are excited to higher energy levels, they can emit light in the visible spectrum, resulting in the appearance of color. For example:
- In sodium chloride (NaCl), when heated in the presence of sodium vapor, excess sodium ions can create anionic vacancies. The unpaired electrons in these vacancies are referred to as **F-centers**, which can absorb light and give the crystal a yellow color.

### Step 4: Conclude with the Correct Answer
Based on the above understanding, the appearance of color in solid alkali metal halides is generally due to the **Cation Excess Defect**.

### Final Answer:
The appearance of color in solid alkali metal halides is generally due to **Cation Excess Defect**.

---