Question

Which primitive unit cell has all the edge lengths equal i.e. $a=b=c$ and all the axial angles equal to each other but not equal to ${90}^{\circ }$ i.e. $\alpha =\beta =\gamma \ne {90}^{\circ }$?

• A. Monoclinic
• B. Triclinic
• C. Trigonal
• D. Hexagonal

Answer 1

The correct option is C Trigonal

Trigonal crystal system is also called as rhombohedral crystal system. For trigonal crystal systems all the edge length or axial length are equal.
$a=b=c$
All three axial angles are equal but they are not equal to ${90}^o$
$\alpha =\beta =\gamma \ne {90}^o$


Calcite ($CaC{O}_3$) and $HgS$ (cinnabar) are example of trigonal crystals.