containing 70 elements and used the structural data to define the characteristic ionic
and covalent radii of the atoms in the compounds. For the synthetic solid-state
chemists and physicists, the technique proved to be invaluable for determining
whether the powder was pure. The impurity could also be identified if its diffraction
data had been recorded previously, and its relative concentration could also be
established from the intensities of the lines [75–77].
It also enabled chemists to rapidly measure how the cell dimensions and perhaps
the phase of the crystal changed as a function of temperature. Such studies were of
great importance in the development of metallurgy during the twentieth century.
Powder diffraction was quickly adopted because it did not require specimens which
formed large crystals, and many samples could be studied rapidly. It was widely
applied in mineralogy, petrology, metallurgy and materials science. One of the
difficulties of the method lies in the indexation of the diffraction lines for samples
which crystallised in lower-symmetry space groups. The determination of more
complex structures had to wait for the development of new refinement techniques
[78, 79]. This led to a complete renewal of the powder diffraction method, with
applications to the study of different classes of new materials in chemistry, materials
science and biology. The X-ray powder diffraction file now contains nearly a million
entries. In recent years increased computer speeds and more sophisticated
programmes have made it possible to use the technique for more complicated
organic and organometallic compounds.
The diffraction lines in a powder diagram are also very sensitive to the degree of
perfection of the crystal. A general analysis, taking distortions within the grains into
account, was given by A. R. Stokes and A. J. C. Wilson [80, 81]. The profile of the
lines depends both on the size of grains, as mentioned above, and on the distribution
of defects. B. E. Warren and his school studied in the 1950s the influence of online
shape of microtwins and stacking faults such as those introduced during cold
working of metals and alloys. During annealing of these materials, the grains
recrystallise along preferred orientations, their size increases, and the diffraction
lines become discontinuous [82, 83].
7.3 Neutron Diffraction
The de Broglie equation established that neutrons generated in a nuclear reactor
(or more recently a spallation source) had wave lengths in the same range as that
established for X-rays and consequently could also be diffracted by crystals. The
high initial cost of neutron sources and the diffracted intensities are rather weaker
than those obtained with X-rays because the diffraction centres are not the electrons
but the nuclei. Neutron diffraction has proved to particularly useful for locating
hydrogen atoms in structures because the hydrogen atom has only a single electron
and it is not a strong diffraction centre [84–88]. Since the nuclei and the neutron are
both small, significant scattering only occurs when the neutron is close to the
nucleus. The advantage is that the scattering does not fall off with Bragg angle as
30
D. M. P. Mingos
and covalent radii of the atoms in the compounds. For the synthetic solid-state
chemists and physicists, the technique proved to be invaluable for determining
whether the powder was pure. The impurity could also be identified if its diffraction
data had been recorded previously, and its relative concentration could also be
established from the intensities of the lines [75–77].
It also enabled chemists to rapidly measure how the cell dimensions and perhaps
the phase of the crystal changed as a function of temperature. Such studies were of
great importance in the development of metallurgy during the twentieth century.
Powder diffraction was quickly adopted because it did not require specimens which
formed large crystals, and many samples could be studied rapidly. It was widely
applied in mineralogy, petrology, metallurgy and materials science. One of the
difficulties of the method lies in the indexation of the diffraction lines for samples
which crystallised in lower-symmetry space groups. The determination of more
complex structures had to wait for the development of new refinement techniques
[78, 79]. This led to a complete renewal of the powder diffraction method, with
applications to the study of different classes of new materials in chemistry, materials
science and biology. The X-ray powder diffraction file now contains nearly a million
entries. In recent years increased computer speeds and more sophisticated
programmes have made it possible to use the technique for more complicated
organic and organometallic compounds.
The diffraction lines in a powder diagram are also very sensitive to the degree of
perfection of the crystal. A general analysis, taking distortions within the grains into
account, was given by A. R. Stokes and A. J. C. Wilson [80, 81]. The profile of the
lines depends both on the size of grains, as mentioned above, and on the distribution
of defects. B. E. Warren and his school studied in the 1950s the influence of online
shape of microtwins and stacking faults such as those introduced during cold
working of metals and alloys. During annealing of these materials, the grains
recrystallise along preferred orientations, their size increases, and the diffraction
lines become discontinuous [82, 83].
7.3 Neutron Diffraction
The de Broglie equation established that neutrons generated in a nuclear reactor
(or more recently a spallation source) had wave lengths in the same range as that
established for X-rays and consequently could also be diffracted by crystals. The
high initial cost of neutron sources and the diffracted intensities are rather weaker
than those obtained with X-rays because the diffraction centres are not the electrons
but the nuclei. Neutron diffraction has proved to particularly useful for locating
hydrogen atoms in structures because the hydrogen atom has only a single electron
and it is not a strong diffraction centre [84–88]. Since the nuclei and the neutron are
both small, significant scattering only occurs when the neutron is close to the
nucleus. The advantage is that the scattering does not fall off with Bragg angle as
30
D. M. P. Mingos