5 Mössbauer Spectroscopy with High Spatial Resolution …
239
Samples can be prepared by mounting them in epoxy and grinding and polishing
the disks to the appropriate thickness. The epoxy provides support and easy handling
for small grains and the same epoxy mount can be used for chemical determination
(for example using the electron microprobe) as well as Mössbauer spectroscopy. This
capability is particularly valuable for studies where Mössbauer spectroscopy is used
to calibrate standards for the flank method [33] and X-ray Absorption Near Edge
Structure (XANES) spectroscopy [34] (Fig. 5.11f). Preparation in epoxy also allows
optimal exposure of the region to be measured, such as in capsules recovered from
large volume high-pressure experiments [35] (Fig. 5.11g).
Single crystals provide information not available from powder measurements, but
require additional preparation to realise these benefits. Generally it is advantageous to
orient crystals prior to cutting, for example using cleavage directions or X-ray diffraction. Crystals can then be cut along desired directions with appropriate thickness.
For measurements in three orthogonal directions, a cube shape is ideal (Fig. 5.11h),
particularly for samples with low iron concentration that require substantial thickness. A slice is useful for measurements that involve rotating the crystal at an angle
inclined to the source radiation, e.g., [36] (Fig. 5.11i).
Inhomogeneous samples provide a wealth of information, but require more attention to preparation in order to facilitate documentation of the regions that are
measured. Thick sections offer the possibility to use optical images, and if sections
are polished, the same regions on the sections can be analysed using other methods
such as electron microprobe analysis, e.g., [37] (Fig. 5.11j, k). Important to note,
however, is that Mössbauer measurements in transmission geometry record iron
phases throughout the entire thickness of the sample section, not only on the surface.
One strategy to minimise inhomogeneity along the radiation path is to select regions
that show similar characteristics on both upper and lower surfaces of the section.
Inclusions in diamond can be measured in situ since carbon is a relatively light
element (Sect. 5.4.4). There are numerous advantages to keeping diamonds intact
(Fig. 5.11l), including preserving features such as diamond microstructure and inclusion associations that can be studied using non-destructive methods, for example
micro-computed tomography [38]. However chemical composition is difficult to
determine quantitatively when inclusions are enclosed in diamond, so one compromise is to polish the diamond until inclusions are partly exposed (Fig. 5.11m). Of
course inclusions can also be released entirely from their diamond hosts, usually
by breaking the diamond, in which case inclusions can be mounted on plastic foil
(Fig. 5.11d) or in epoxy (Fig. 5.11f).
Diamond anvil cells for Mössbauer measurements are generally prepared in
the same way as for other diamond anvil cell experiments. Special considerations for Mössbauer spectroscopy include ensuring an appropriate sample thickness
(Sect. 5.4.4) and using a gasket with high atomic weight (to collimate source radiation) that does not contain iron. Samples can be loaded as single crystals (Fig. 5.11n)
or as powder (Fig. 5.11o).
239
Samples can be prepared by mounting them in epoxy and grinding and polishing
the disks to the appropriate thickness. The epoxy provides support and easy handling
for small grains and the same epoxy mount can be used for chemical determination
(for example using the electron microprobe) as well as Mössbauer spectroscopy. This
capability is particularly valuable for studies where Mössbauer spectroscopy is used
to calibrate standards for the flank method [33] and X-ray Absorption Near Edge
Structure (XANES) spectroscopy [34] (Fig. 5.11f). Preparation in epoxy also allows
optimal exposure of the region to be measured, such as in capsules recovered from
large volume high-pressure experiments [35] (Fig. 5.11g).
Single crystals provide information not available from powder measurements, but
require additional preparation to realise these benefits. Generally it is advantageous to
orient crystals prior to cutting, for example using cleavage directions or X-ray diffraction. Crystals can then be cut along desired directions with appropriate thickness.
For measurements in three orthogonal directions, a cube shape is ideal (Fig. 5.11h),
particularly for samples with low iron concentration that require substantial thickness. A slice is useful for measurements that involve rotating the crystal at an angle
inclined to the source radiation, e.g., [36] (Fig. 5.11i).
Inhomogeneous samples provide a wealth of information, but require more attention to preparation in order to facilitate documentation of the regions that are
measured. Thick sections offer the possibility to use optical images, and if sections
are polished, the same regions on the sections can be analysed using other methods
such as electron microprobe analysis, e.g., [37] (Fig. 5.11j, k). Important to note,
however, is that Mössbauer measurements in transmission geometry record iron
phases throughout the entire thickness of the sample section, not only on the surface.
One strategy to minimise inhomogeneity along the radiation path is to select regions
that show similar characteristics on both upper and lower surfaces of the section.
Inclusions in diamond can be measured in situ since carbon is a relatively light
element (Sect. 5.4.4). There are numerous advantages to keeping diamonds intact
(Fig. 5.11l), including preserving features such as diamond microstructure and inclusion associations that can be studied using non-destructive methods, for example
micro-computed tomography [38]. However chemical composition is difficult to
determine quantitatively when inclusions are enclosed in diamond, so one compromise is to polish the diamond until inclusions are partly exposed (Fig. 5.11m). Of
course inclusions can also be released entirely from their diamond hosts, usually
by breaking the diamond, in which case inclusions can be mounted on plastic foil
(Fig. 5.11d) or in epoxy (Fig. 5.11f).
Diamond anvil cells for Mössbauer measurements are generally prepared in
the same way as for other diamond anvil cell experiments. Special considerations for Mössbauer spectroscopy include ensuring an appropriate sample thickness
(Sect. 5.4.4) and using a gasket with high atomic weight (to collimate source radiation) that does not contain iron. Samples can be loaded as single crystals (Fig. 5.11n)
or as powder (Fig. 5.11o).