Numerous images of this material appear in literature on transmission electron microscopy. The spacing gives a convenient test, but the specimens are beam sensitive and quickly lose their crystalline structure under the electron beam.
The resolution of crystal lattice planes is a useful test of the performance of an electron microscope, particularly mechanical and electrical stability. The crystal spacings are known accurately from X-ray data, and therefore the high magnification used can be calibrated with accuracy. The crystal specimens are mounted on perforated carbon films, and wherever possible, thin crystals which cross one of the holes should be selected, so that interfering structure from the support film is not encountered.
Good crystals can usually be located by checking the selected area diffraction pattern -unless a clear single crystal pattern is obtained the lattice planes will not be observed. Whena suitable crystal has been selected, a very high electron optical magnification should be used, so that the lattice planes can clearly be resolved on the fluorescent screen under the viewing telescope. The objective lens focus has to be adjusted carefully to optimise the contrast. The phase contrast of a given crystal lattice spacing is critically dependent on the amount of objective lens defocusing.
If the contrast is inadequate a significant improvement may be obtained by defocusing the condenser lens (reduction of illumination semi-angle). Further improvement may be brought about by tilting the illuminating beam so that the central beam and first order diffraction spot are symmetrical about the instrument axis (take care that the objective aperture is large enough to accept the diffracted beams).
This compound has a useful wide crystal spacing. However, it is rather susceptible to radiation damage, and it is therefore advisable to take great care not to study a crystal too long before attempting to photograph it.
The electron optical magnification should not be over 100,000x, or the beam intensity required will be too high. The lattice plane spacings lie parallel with the long axis of the crystal
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