7. Temperature factors

Atoms vibrate around an equilibrium position, even at absolute zero. The amplitude of vibration increases with temperature, in a variable way depending on the element. In general, and as a first approximation, the lower the boiling point of an element, the greater the amplitude of thermal vibration of the atom considered in a crystal at ordinary temperature, even if it is chemically bonded to a structure. It can also be said that, a priori, thermal agitation decreases as the number of covalent bonds contracted by an atom increases: this is particularly the case for a di-, tri- or tetracoordinated oxygen atom. Moreover, the higher the atomic mass, the lower the thermal vibration. Another type of temperature-dependent interaction is the X-ray-phonon interaction. It will not be discussed here, as it is not involved in structure determination.

At ordinary temperatures,...