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Raman spectra of Mn2(CO)10 and Re2(CO)10 have been studied to 27 and 98 kbar respectively, and the infrared (IR) spectrum of Re2(CO)10 to 30 kbar at ambient temperature. Both materials undergo a first-order structural phase transition at high pressure (Mn at approximately 8 kbar, Re at approximately 5 kbar) which is accompanied by dramatic changes in all regions of their vibrational spectra. These data are interpreted in terms of a change of molecular geometry from D4d to D4h. In the new high-pressure phases the M to carbonyl backbonding is increased significantly for the axial but not the equatorial carbonyls. The molecular torsion is implicated in the phase change but is not thought to drive it.

The so-called 'valence transition' in EuPd2Si2 has been observed using energy-dispersive synchrotron X-ray powder diffraction at elevated pressures and is centred at 8 kbar. The estimated increase in valence at the transition is 0.2, and is associated with crossover of 4f and 6d/s bands.

Using synchrotron energy-dispersive X-ray powder diffraction (EDXRPD) the authors have shown that K2SnBr6 transforms at pressures of 14 kbar and above from the parent monoclinic cell to a cubic structure of symmetry Fm3m, which is stable to 120 kbar. A different phase appears to exist above 120 kbar.

The structural behaviour of ammonium metavanadate, NH4VO3, under high pressures has been studied to 65 kbar using energy-dispersive synchrotron X-ray powder diffraction, and to 106 kbar by FTIR methods, in a diamond anvil cell. A reversible crystalline-to-disordered phase transition was found at 49 kbar. The transition is thought to be due to relative slipping of the vanadate chains parallel to their axes, the product being disordered, possibly amorphous, rather than crystalline because the process is hindered and the solid prevented from attaining periodicity by hydrogen bonding to ammonium ions.

A spectroscopic system used for the calibration of pressure using the ruby R-line method has been designed and interfaced to a BBC model B microcomputer. The methodology of the real-time software design is detailed including flow diagrams for the three main programs employed in the system. The software includes both ambient and high-pressure data acquisition and analysis programs. Suggestions are made for the application of the software system to other experimental tasks and also to other compatible microcomputers.

The aims were to understand and to learn how to control potentially destructive stresses which limit the performance of anvils in high pressure optical and X-ray cells. The calculations show that the largest compressive and shear stresses are near the working face of the anvil and, hence, that plastic flow will begin there, as has been observed. The largest tensile stresses are at the base, above the unsupported light port. Since diamond, a brittle material, is liable to failure in tension, these stresses are potentially destructive. The largest principal tensile stress is in a hoop direction: consequently diamond anvils are liable to basal failure by radial cracking, with ring cracking from tensile radial stresses important also. This type of failure has been observed in diamond anvil cells. In the light of these results, mechanisms of anvil failure are discussed and illustrated with photographs. Attention is drawn to the importance of fatigue as a cause of failure in diamond.

Potassium nitrate (KNO3) has seven polymorphs in the pressure range 0-4.0 GPa. The authors have studied it at room temperature as a function of pressure up to 9.3 GPa and have confirmed the structure of phase IV previously refined by neutron diffraction at 0.36 GPa. The compressibility measured in the range 0.3-9.3 GPa is found to be anisotropic with the axial compression ratios a:b:c=1.0:0.64:0.50. The relative merits of X-rays and neutrons for high-pressure studies are discussed.

The optical problems associated with infrared spectroscopy at very high pressures in a diamond anvil cell are considered and the main limitations identified. The principal difficulty is the conflict between optical requirements, which indicate the need for relatively large unsupported areas of diamond, and mechanical requirements which work conversely. A simple refracting beam condenser is described and the problems of coupling it to spectrophotometers are indicated.

Phase transitions have been observed in mercury (II) cyanide near 2.5, 89, 19 and 60 kbar. The I/II transition appears to be of second order with the structure of phase II closely related to that of the parent molecular type. Using Landau theory, three possible structures have been identified for the new phase II, of which S24 is marginally better in accord with the evidence. A major reconstruction of the lattice occurs at the II/III transition; the coordination number of mercury appears to increase to four, and the structure becomes covalently bonded, rather than molecular. Phase IV has a spectrum consistent with it having the cubic anti-cuprite structure adopted at STP by Zn(CN)2 and Cd(CN)2. Above 60 kbar, brown polymeric Hg(CN)2V is formed irreversibly. It has a single IR absorption

Raman spectra of CH3Hg (X=Cl,Br,I) have been studied as a function of pressure using diamond anvil cells. Phase transitions were discovered in the chloride at 1.5+or-0.3 and 5.9 kbar, with an almost immediate further transition near 6 kbar to a phase stable to at least 40 kbar. CH3HgBr shows a single transition at 12.5 kbar. Soft modes were found for all three materials. That for the bromide was followed by change of temperature as well as of pressure. The crystal structure of CH3HgBr was found to be like that of the chloride, but of a different space group (P421m, Z=4), allowing slight nonlinearity of the C-Hg-Br skeleton. On the basis of this structural evidence a phase diagram is proposed. A very narrow (about 0.1 kbar) phase in CH3HgCl may be incommensurate. Preliminary studies of CH3HgI show that its behaviour parallels that of its congeners, but that the crystallography is different in detail.

Infrared reflectance spectra are reported for oriented single crystals of MAMC and PAMC with the electric vector parallel to the anionic sheets for temperatures from 50 to 295K. Single-crystal Raman spectra were also obtained at similar temperatures. The main features of the spectra, particularly for the ambient temperature phases, are assigned with the aid of a factor group analysis. This phase for MAMC, although orthorhombic, behaves spectroscopically as pseudo-tetragonal, but PAMC is plainly orthorhombic. Modes above 260 cm-1 are attributed to cation internal modes, including torsions. The phase III (tetragonal) to phase IV (monoclinic) transition is distinguished in the Raman spectra by disappearance of an intense broad features at about 35 cm-1/.

The effect of pressure at low temperature (166.8K) upon the R1 and R2 lines of ruby has been investigated for the first time to establish a calibration for use in high-pressure/low-temperature work. A reinvestigation of the behaviour of R1 and R2 at ambient temperatures and at pressures up to ca 70 kbar supports previous work on the calibration under hydrostatic conditions but shows that in a 4:1 methanol:ethanol mixture conditions become nonhydrostatic at lower pressures (ca 50 kbar) than claimed (ca 100 kbar). The importance of monitoring both R1 and R2 is emphasised as their relative shifts are sensitive indicators of the presence, and approximate magnitude, of non-hydrostatic stress components. It is shown that although changes in half-bandwidths are also associated with development of shear stress, their use in estimating gradients is extremely hazardous and can indicate values greatly in excess of the known shear strength of the material.

Raman spectra have been obtained for HgCl2 to 145 kbar and HgBr2 to 100 kbar and phonon pressure dependencies established. The phase transition in HgCl2 said to be at 6.5 kbar, on NQR evidence, has been located at 2.2 kbar. A new first-order phase change has been found in HgCl2 at 79.0 kbar. Also in HgCl2, there are two bands that pressure scan differentially such that one overtakes the other, but they do not appear to be in resonance. Improved values for the phase transitions in HgBr2 are 4.0, 26.0 and 41.0 kbar. It is suggested that there are no structural equivalences between the two series of four polymorphs based upon the chloride and bromide respectively.

Diamonds intended for use as anvils in high-pressure devices for Raman and infrared spectroscopy must be selected carefully. For Raman use it is suggested that a useful criterion is whether or not the second-order spectrum of diamond can be seen above the accompanying fluorescence. Some type IIa stones are also suitable for Raman use and are superior to IIb stones in the infrared region above 2000 cm-1.