Solid State - Result Question 9
####9. Which primitive unit cell has unequal edge lengths $(a \neq b \neq c)$ and all axial angles different from $90^{\circ}$ ?
(a) Hexagonal
(b) Monoclinic
(c) Tetragonal
(d) Triclinic
(2019 Main, 10 Jan I)
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Answer:
Correct Answer: 9. (d)
Solution:
- Triclinic primitive unit cell has dimensions as, $a \neq b \neq c$ and $\alpha \neq \beta \neq \gamma \neq 90^{\circ}$.
Among the seven basic or primitive crystalline systems, the triclinic system is most unsymmetrical. In other cases, edge length and axial angles are given as follows :
Hexagonal : $a=b \neq c$ and $\alpha=\beta=90^{\circ}, \gamma=120^{\circ}$
Monoclinic : $a \neq b \neq c$ and $\alpha=\gamma=90^{\circ}, \beta \neq 90^{\circ}$
Tetragonal : $a \neq b \neq c$ and $\alpha=\beta=\gamma=90^{\circ}$
