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The parameters of the unit cell of a substance are a = 2.5, b = 3.0, c = 4.0, α = 90°, β = 120°, γ = 90°. The crystal system of the substance is:

1. Hexagonal
2. Orthorhombic
3. Monoclinic
4. Triclinic

Correct answer: Monoclinic

Solution

With a != b != c and alpha = gamma = 90 but beta = 120 (not 90), exactly one angle is non-right. That is the defining condition of the monoclinic crystal system.

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