To solve the question regarding the correct statements about coordination numbers under varying pressure and temperature, we will evaluate each statement one by one.
### Step-by-Step Solution:
1. **Evaluate Statement 1**:
- Statement: "Under high pressure, the coordination number increases from 6 to 8."
- Explanation: When pressure is increased, the particles in a crystal are forced closer together, leading to a more compact structure. This often results in an increase in the coordination number. Therefore, this statement is **correct**.
2. **Evaluate Statement 2**:
- Statement: "Under high pressure, the coordination number decreases from 8 to 6."
- Explanation: This statement contradicts the first statement. Since we established that coordination number increases with pressure, this statement is **incorrect**.
3. **Evaluate Statement 3**:
- Statement: "At high temperature, the coordination number decreases."
- Explanation: As temperature increases, the kinetic energy of the particles also increases, which can lead to a decrease in the coordination number as the particles may not stay as closely packed. Thus, this statement is **correct**.
4. **Evaluate Statement 4**:
- Statement: "At 760 Kelvin, the cesium structure changes into NaCl structure."
- Explanation: The cesium chloride structure has a coordination number of 8:8, while the NaCl structure has a coordination number of 6:6. As temperature increases, the coordination number can decrease, which aligns with the transition from cesium structure to NaCl structure at high temperatures. Therefore, this statement is **correct**.
### Conclusion:
The correct statements are:
- Statement 1: Correct
- Statement 2: Incorrect
- Statement 3: Correct
- Statement 4: Correct
Thus, the correct options are **Statement 1, Statement 3, and Statement 4**.
### Final Answer:
The correct statements are 1, 3, and 4.
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