Vibrational spectroscopy at high pressures.

A review with 133 References of the application of vibrational spectroscopy at high pressures to the study of solids is given, Two new designs of diamond anvil cell (DAC) are described and compared with designs of others. The use of the DAC in combination with various spectroscopic instruments is described and the calibration of the cell, taking into account the pressure gradient present across the anvil faces, is reported. The spectroscopic and physical properties of two anvil materials, diamond and sapphire, are described with particular attention being paid to the fluorescence properties of various types of diamond under laser excitation. Another design of high pressure cell, which applies hydrostatic pressure to samples, is reported for use in Raman spectroscopy. The main area of compounds investigated at high pressures using the DAC were complexes of the type A2IMX6 (A = monatomic cation, X = halogen). A wide range of these materials were studied in the far-i.r. and members of the series A2IPtCl6 were also examined in the Raman. For the latter compounds symmetry species were related to observed pressure sensitivities of vibrations. Unusual behaviour of the v4 bending mode at elevated pressures was found for complexes where M possessed a lone pair of electrons. Possible mechanisms to account for the behaviour of the mode are discussed. Some hexammine salts of cobalt and nickel halides were also examined both at liquid nitrogen temperature and at high pressure respectively. The appearance of an ammonia torsion mode at liquid nitrogen temperature is shown. An attempt to obtain quantitative results from the observed pressure dependencies was not possible owing to the nature of the complexes examined.