Infrared studies in the 1- to 15-micron region to 30,000 atmospheres

A pressure cell was constructed using a pair of type II diamonds for study of infrared spectra of solids in the 1- to 15-micron region. Using commercial infrared equipment, spectra can be studied routinely to calculated pressures as high as 30,000 atmospheres. Under pressure, bands generally shift to higher frequencies and decrease in intensity. The magnitude of both changes depends on the mode of vibration. Occasionally major changes in spectra occur. In calcite the carbon-oxygen symmetric stretching, mode v 1, becomes active at elevated pressures while the doubly degenerate v 3, stretching, and v 4, bending, frequencies split. From the shift in frequency of v 1 with pressure the "compressibility", (-1/Ro) (dR/dp), of the C-O bond length, R, is calculated to be 2.8×10-7/atmosphere. Major spectral changes are not observed in the same pressure ranges in other carbonates having the calcite or aragonite structures. The results for calcite may be explained by a shift of the CO 3 = ion from the trigonal axis under pressure.