However, for n = 23 onwards will give us sin h > 1, which is not possible.
Therefore, the possible orders of (111) reflection are up to twenty-second.
(iii) Rotation/ Oscillation Method
For the a-axis rotation/oscillation photograph of the crystal material, the separation
of n
th layer line (with respect to zero-layer) and the lattice parameter of the sample
are, respectively, given by (Fig. 9.4)
S n ¼
nk=a
ð
Þ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
1 À nk=a
ð
Þ
2
q
R
and a ¼
nk
S n
R
2
þ S
2
n
À
Á 1=2
where R is the radius of the camera used.
Alternatively, for the a-axis (b-axis or c-axis) rotation/oscillation photograph of
the crystal material, the lattice parameter can also be obtained by using the
expression
a ¼
nk
sin tan À1 S n =R
ð
Þ
½
Š
Fig. 9.4 Formation of layer
line from cones of the
diffracted rays
9.1 Steps in Crystal Structure Determinations
361