development of open flow cryostats (for temperature) and diamond anvil cells (for
pressure).
Since electrons are negatively charged, they interact with both the electrons and
nuclei of atoms in the diffraction process. The electron beam sources are relatively
common, and the high fluxes mean that relatively small crystals can be studied using
electron diffraction. It is particularly effective for studying thin samples. The wavelength can be readily tuned through the de Broglie expression, and the use of small
wavelengths leads to more information from the diffraction experiment. Radiation
damage of the crystals does occur and can be unacceptably high, and it is possible to
reduce these effects by doing the experiments at very low temperatures. It is difficult
to get useful information from electron diffraction studies of crystals which have
magnetic centres because the electrons are also deflected by the local magnetic
fields.
Neutrons have zero charge, and consequently furnaces and cryostats can be
placed around the crystal target and not attenuate the neutron beam greatly. Once
a neutron reactor was up and running, the costs of the neutrons and their costs and
those for the adaptation of facilities are not prohibitive. These days, the majority of
neutron studies are carried out at neutron spallation facilities (e.g. Oak Ridge and
ISIS) where the neutrons can effectively be switched on and off and there is no
longer the need to use a reactor in the old sense of the word. Neutrons have a
magnetic dipole moment, and therefore they are ideal for probing magnetic structures [67]. In recent decades, they have been very useful for studying antiferromagnetic superstructures in transition metal oxides [70]. Neutron techniques have probed
the vibrational, magnetic and lattice excitations (dynamics) of materials by measuring changes in the neutron momentum and energy simultaneously. The nuclear
interactions between the neutrons and the nuclei in molecules are not large, and
therefore relatively large crystals are required. It is particularly useful for studying
compounds containing light atoms and in particular hydrogen and deuterium. This
makes it suitable for studying organic polymers and biomolecules. The available
fluxes are relatively low compared with those for X-rays, but their wavelengths can
be changed over a wide range, and this means that they can be tuned to match the
atomic spacings of particular interest in the crystal. The neutrons represent a
relatively non-destructive probe, and the nuclear-neutron interactions are relatively
easy to calculate.
7.2 Powder X-Ray Diffraction
Recording the diffraction patterns for single crystals using the Bragg condition was
time-consuming because it required each diffraction spot to be measured individually. An obvious development was to study the effect of rotating the crystal or
studying crystalline samples consisting of a very large number of small crystals
randomly orientated, i.e. a finely ground powder. Powder X-ray diffraction was
independently discovered by Paul Scherrer and Peter Debye in Göttingen, Germany,
28
D. M. P. Mingos
pressure).
Since electrons are negatively charged, they interact with both the electrons and
nuclei of atoms in the diffraction process. The electron beam sources are relatively
common, and the high fluxes mean that relatively small crystals can be studied using
electron diffraction. It is particularly effective for studying thin samples. The wavelength can be readily tuned through the de Broglie expression, and the use of small
wavelengths leads to more information from the diffraction experiment. Radiation
damage of the crystals does occur and can be unacceptably high, and it is possible to
reduce these effects by doing the experiments at very low temperatures. It is difficult
to get useful information from electron diffraction studies of crystals which have
magnetic centres because the electrons are also deflected by the local magnetic
fields.
Neutrons have zero charge, and consequently furnaces and cryostats can be
placed around the crystal target and not attenuate the neutron beam greatly. Once
a neutron reactor was up and running, the costs of the neutrons and their costs and
those for the adaptation of facilities are not prohibitive. These days, the majority of
neutron studies are carried out at neutron spallation facilities (e.g. Oak Ridge and
ISIS) where the neutrons can effectively be switched on and off and there is no
longer the need to use a reactor in the old sense of the word. Neutrons have a
magnetic dipole moment, and therefore they are ideal for probing magnetic structures [67]. In recent decades, they have been very useful for studying antiferromagnetic superstructures in transition metal oxides [70]. Neutron techniques have probed
the vibrational, magnetic and lattice excitations (dynamics) of materials by measuring changes in the neutron momentum and energy simultaneously. The nuclear
interactions between the neutrons and the nuclei in molecules are not large, and
therefore relatively large crystals are required. It is particularly useful for studying
compounds containing light atoms and in particular hydrogen and deuterium. This
makes it suitable for studying organic polymers and biomolecules. The available
fluxes are relatively low compared with those for X-rays, but their wavelengths can
be changed over a wide range, and this means that they can be tuned to match the
atomic spacings of particular interest in the crystal. The neutrons represent a
relatively non-destructive probe, and the nuclear-neutron interactions are relatively
easy to calculate.
7.2 Powder X-Ray Diffraction
Recording the diffraction patterns for single crystals using the Bragg condition was
time-consuming because it required each diffraction spot to be measured individually. An obvious development was to study the effect of rotating the crystal or
studying crystalline samples consisting of a very large number of small crystals
randomly orientated, i.e. a finely ground powder. Powder X-ray diffraction was
independently discovered by Paul Scherrer and Peter Debye in Göttingen, Germany,
28
D. M. P. Mingos