Effects of temperature and pressure on optical spectra

According to the crystal field model, the crystalline electrostatic field of ligands in a crystal structure influences the 3d orbital energy levels of a central transition metal ion in a coordination site. There is a two-fold influence of temperature on a crystal structure. First, increased thermal motion results in increased amplitudes of atoms vibrating about their crystallographic positions. Second, thermal expansion causes small increases in interatomic distances, so that according to eq. 

Increased pressure, on the other hand, results in compression of a crystal structure. The compressibilities of oxides and silicates show that the molar volumes of these phases decrease with rising pressure, indicating that interatomic distances within coordination sites become shorter. This is demonstrated by the data for Mantle minerals at elevated pressures summarized in table 9.1. Equation (2.20) 

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