Solution: We know that in a monoclinic crystal system, there exists a unique
twofold axis, normally taken as the c-axis. This is called the monoclinic system of
first setting. Now, let us examine the following four possibilities of centering, viz.
the body-centering, the base-centering and face-centering in the primitive monoclinic one by one.
(a) Body-centering
Let us draw two monoclinic lattices side by side. Join the points as shown by
the dotted lines (Fig. 1.22a) This can equally well be described as a
base-centered (B-centering on ac-plane) lattice by appropriate choice of a and b
axes. Conversely, a B-centered lattice shown in Fig. 1.22b can equally well be
described as a body-centered monoclinic lattice. Thus, I B.
Fig. 1.21 Alternative way to
describe the centered
rectangular lattice
Fig. 1.22 a Body-centered (I) unit cells, b B-centered unit cells
22
1 Unit Cell Composition
twofold axis, normally taken as the c-axis. This is called the monoclinic system of
first setting. Now, let us examine the following four possibilities of centering, viz.
the body-centering, the base-centering and face-centering in the primitive monoclinic one by one.
(a) Body-centering
Let us draw two monoclinic lattices side by side. Join the points as shown by
the dotted lines (Fig. 1.22a) This can equally well be described as a
base-centered (B-centering on ac-plane) lattice by appropriate choice of a and b
axes. Conversely, a B-centered lattice shown in Fig. 1.22b can equally well be
described as a body-centered monoclinic lattice. Thus, I B.
Fig. 1.21 Alternative way to
describe the centered
rectangular lattice
Fig. 1.22 a Body-centered (I) unit cells, b B-centered unit cells
22
1 Unit Cell Composition