Question

The trigonal crystal system can also be referred to as:

• A. Hexagonal
• B. Rhombohedral
• C. Cubic
• D. Tetragonal

Hint:

Trigonal = Rhombohedral. Just remember: equal edges, but not all angles are right angles!

Answer 1

The Correct Option is B

The trigonal crystal system is one of the seven crystal systems in crystallography. It is unique because it can be described in more than one way.
Trigonal vs Rhombohedral:

• The trigonal system is sometimes called the rhombohedral system because the unit cell can be described as a rhombohedron — a solid with equal edge lengths but with angles that are not all 90°.
• Axial parameters: \[ a = b = c \quad \text{and} \quad \alpha = \beta = \gamma \neq 90^\circ \]
• This crystal system has a single three-fold rotational symmetry along one axis.

Why the Other Options Are Incorrect:

• (A) Hexagonal: Although related (both have a three-fold axis), the hexagonal unit cell is different — it has two equal axes at 120°, and a third perpendicular axis.
• (C) Cubic: All axes are equal and all angles are 90°, not suitable for describing the trigonal structure.
• (D) Tetragonal: Two equal axes, one different, all angles 90° — again, not matching the trigonal symmetry or unit cell geometry.

Hence, the correct equivalent term for the trigonal crystal system is Rhombohedral.