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The atoms and ions of a crystalline lattice, which are bonded with each other with considerable interatomic forces, are not motionless. Due to the consistent vibration induced through thermal energy, they are permanently deviating from their equilibrium position. Elastic waves of different lengths, frequencies, and amplitudes run through crystalline solids at all times. The typical order of the atomic vibrations frequencies is 1013 Hz, and that of the amplitudes is 10−11 m.
The process of the atomic vibrations is important for materials of different classes: for metallic, covalent, ionic crystals, semiconductors, intermetallic compounds, interstitial phases. The amplitude-frequency characteristics of the vibrating spectrum of an alloy can be varied, for example by alloying, to produce a well-directed effect on the properties of the materials.
The phenomena of atomic vibrations reflecting the interaction of micro-particles with each other depend on the deep properties of the medium. The vibrational amplitude and the vibrational spectrum are determined by interatomic bonds.
The basic tool for the measurement of the mean-square amplitude of vibrations is X-ray diffraction. The heat vibrational motion of atoms, affecting the atom displacements, results in a weakening of diffracted lines (reflections). By measuring the intensity of the same reflections at two temperatures (for example, at a room and a high temperature) one can calculate mean-square amplitudes of the atom vibrations.
Correct data about the frequency spectrum is provided with the help of the technique of the neutron scattering by solids.