- © The Mineralogical Society Of America
Infrared (IR) spectroscopy can provide information about details of the atomic structure, lattice dynamics and the chemistry of mineral phases that may not be readily obtained from other analytical techniques. IR spectra originate in transitions between vibrational energy levels of vibrating atomic groups and they are observed as absorption spectra. The classification of the vibrational quantum states and the description of the spectroscopic interaction are greatly simplified by exploiting the symmetry of the vibrating atomic groups. The mathematical framework of group theory is the basis of the quantitative description of the symmetry relations possessed by the vibrating groups. In the classical model of vibrational theory, point masses that are connected by elastic springs are allowed to undergo certain vibrational displacements about their equilibrium position. If the restoring force is directly proportional to the displacement of the point masses that represent the atoms of the vibrating group, then the vibrational motion is “harmonic”. The restoring force and the vibrational displacement of the atoms are related by a proportionality factor, named the force constant. This constant corresponds to the spring constant and is a measure of the bond strength.
Changes in the number or positions of IR absorption bands are mostly analyzed in terms of structural changes. In order to make a full correlation between vibrational spectra and structure, it would be necessary to know the atomic displacements associated with each vibrational mode. A reliable approach for discerning the origin of particular IR absorption bands is to combine theoretical considerations with empirical observations of phases having the same structure but differing in composition. By simply using the “harmonic oscillator model” for the energetics of IR-active vibrations, the substitution of one element by another will shift the wavenumber of a vibration according to the masses and the bonding behaviour of the respective atoms …