The parameters of the unit cell of a substance are a = 2.5, b = 3.0, c = 4.0, $\alpha=90^\circ, \beta=120^\circ, \gamma=90^\circ$. The crystal system of the substance is :
Show Hint
Never assume typos in crystallography questions.
Always classify crystal systems strictly using
{both axial lengths and interaxial angles}.
Given unit cell parameters:
\[
a = 2.5,\quad b = 3.0,\quad c = 4.0
\]
\[
\alpha = 90^\circ,\quad \beta = 120^\circ,\quad \gamma = 90^\circ
\]
Clearly,
\[
a \neq b \neq c
\]
and
\[
\alpha = \gamma = 90^\circ,\quad \beta \neq 90^\circ
\]
Now compare with standard crystal systems:
Monoclinic:
\[
a \neq b \neq c,\quad \alpha = \gamma = 90^\circ,\quad \beta \neq 90^\circ
\]
Hexagonal:
\[
a = b \neq c,\quad \alpha = \beta = 90^\circ,\quad \gamma = 120^\circ
\]
The given parameters exactly match the monoclinic system and do not satisfy the condition $a=b$ required for hexagonal crystals.
\[
\boxed{\text{Crystal system = Monoclinic}}
\]
Hence, the correct option is (D) Monoclinic.
