Question

In which of the following systems, interfacial angles $\alpha =\gamma ={90}^o$ but $\beta \ne {90}^o$?

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

Answer 1

Answer: (A), (B)

The correct options are
A Monoclinic
B Rhombohedral

Name of system	Axes	Angles	Bravais Lattices
cubic	$a=b=c$	$\alpha =\beta =\gamma ={90}^o$	Primitive, Face-centred, Body centred $=$ $3$
Tetragonal	$a=b\ne c$	$\alpha =\beta =\gamma ={90}^o$	Primitive, Face-centred $=$ $2$
Rhombohedral or Trigonal	$a=b=c$	$\alpha =\beta =\gamma \ne {90}^o$	Primitive $=$ $1$
Orthorhombic or Rhombic	$a=b=c$	$\alpha =\beta =\gamma ={90}^o$	Primitive, Face-centred, Body centred, End centred $=$ $4$
Monoclinic	$a\ne b\ne c$	$\alpha =\gamma ={90}^o$; $\beta \ne {90}^0$	Primitive, End - centred $=$ $2$
Triclinic	$a\ne b\ne c$	$\alpha \ne \beta \ne \gamma ={90}^o$	Primitive $=$ $1$
Hexagonal	$a=b\ne c$	$\alpha =\beta ={90}^o$, $\gamma ={120}^o$	Primitive $=$ $1$

Hence, the lattices are monoclinic and rhombohedral.