Chemistry Reference
In-Depth Information
Fig. 4 The atomic form factor of a C atom (in 1s
2
2s
2
2p
2
electronic configuration). Core electron
scattering is in
blue
, Valence electron scattering is in
red
and total scattering in
black
Waller factor implies a larger penalty to high scattering angles. Therefore, reducing
the damping produced by
T
(S) could bring the intensity of a high resolution
reflection from hardly observable (at ambient temperature) to significant (at low
temperature). For reflections at lower resolution the gain is smaller, but also less
important because these reflections are inherently more intense (given the atomic
form factors; see Fig.
4
). It should be noted that for ionic crystals and minerals, the
atomic displacements are already quite small at room temperature, and therefore the
low temperature gain is not so large.
An additional advantage of the very low temperature is the smaller dependence
u
2
of
on the temperature. Thus, fluctuations of the temperature during an experi-
ment are less troublesome. It should be taken into account that experiments without
temperature control might easily experience
h
i
5
C fluctuations (night/day varia-
tion, heating caused by other machines, etc.). This produces a fluctuation of 1-2%
of the intensity for a reflection at 1.0
˚
resolution and up to 10% for a reflection at
0.5
˚
resolution (assuming it is measurable).
2.2.2 Thermal Diffuse Scattering
As seen in (6), the integrated intensities contain a contribution from Bragg diffrac-
tion as well as from diffuse scattering and background. TDS is caused by energy
