David M. Adams

Spectroscopy at very high pressures. X. Use of ruby R-lines in the estimation of pressure at ambient and at low temperatures

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.

X-ray diffraction study of hafnia under high pressure using synchrotron radiation

Abstract Hafnia, HfO 2 , has been studied to 20 GPa at ambient temperature using synchrotron energy-dispersive X-ray powder diffraction methods. Traces of the initial monoclinic (baddeleyite) phase could still be identified at 20 GPa, but a phase transition begins at 2.6 GPa to HfO 2 II, the lines of which can be indexed on an orthorhombic cell ( Pbcm , Z = 4). The relative intensities of the (111) and (−111) monoclinic reflections suggests that a further phase change may begin near 10 GPa but work at high temperatures is needed to establish this. The bulk compressibility of hafnia is 1.2 × 10 −4 kbar −1 . A Rietveld analysis of the baddeleyite phase gave results in close agreement with earlier work but showed the sample to be internally strained.

Reassignment of the vibrational spectra of π-cyclopentadienyl- and methyl-π-cyclopentadienyl-manganese tricarbonyl

Summary The long-standing assignment by Hyams, Bailey and Lippincott 1 of the vibrational spectrum of Mn(CO) 3 (π-C 5 H 5 ) is corrected: the π-C 5 H 5 part of the spectrum cannot be assigned on the basis of C 5ν “local” symmetry, and the a 1 C 3ν ) δ (MnCO) mode is at 668 cm −1 , not 543 cm −1 as previously claimed. New Raman data are given for Mn(CO) 3 (π-C 5 H 4 -CH 3 ) and assigned under C s rules for the ring. C 3ν symmetry is a good approximation for the Mn(CO) 3 moiety.

Vibrational spectroscopy at high pressure: Part 53. Alkali metavanadates and copper metagermanate

Abstract Essentially complete Raman and infrared spectra at ambient and 80 K have been obtained for the isostructural metavanadates AVO 3 , (A  K, Rb, Cs). An assignment is offered on the basis of line and factor group analyses. A similar study has been made on CuGeO 3 . Under pressure up to 100 kbar both KVO 3 and RbVO 3 show a single phase transition, at 56 and 53 kbar. The remarkable insensitivity of the transition pressure to change of cation implies that the major effect of pressure is chain torsion and repacking, driven by the need to reduce the cation coordination number.

A second‐order phase transition in Mn(CO)5Br

Raman spectra of Mn(CO)5Br are reported at hydrostatic pressures of up to 50 kbar. Existence of a phase change is shown by changes of slope in frequency/pressure plots. The phase transition is a second‐order transition. (AIP)

X-ray diffraction measurements on potassium nitrate under high pressure using synchrotron radiation

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.

Spectroscopy at very high pressures. XIII. Refracting beam condenser for infrared spectroscopy with a diamond anvil cell

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.

Spectroscopy at very high pressures. 14: Laser Raman scattering in ultrasmall samples in a diamond anvil cell

The problem of obtaining Raman spectra at high pressures with a diamond anvil cell is analyzed, and the successful use of a 90 degrees off-axis ellipsoidal mirror for collection in both 0 degrees and 180 degrees scattering modes is demonstrated with nu(OH) spectra of ice VI, VII, and vIII.

A high-pressure X-ray powder diffraction and infrared study of copper germanate, a high-Tc model

Copper germanate has been examined by synchrotron X-ray powder diffraction at high pressure in a diamond anvil cell at pressures up to 220 kbar, and by infrared spectroscopy to 162 kbar. These concur in showing a phase transition at 66-73 kbar: the X-ray powder diffraction data have been indexed successfully on the basis of a P21/a cell, deduced from a model of the phase change in which germanate chains slip relative to each other along their axes. Slow changes in diffraction pattern form 110-220 kbar suggests existence of a further, very sluggish, phase change. CuGeO3 turns black near 5 kbar due to a band shift. Contraction of the orthorhombic cell constants of the initial phase is markedly anisotropic, the principal changes being in the plane which contains the long Cu-O bonds. These are twice as easily compressed as the short Cu-O bonds.

Vibrational spectroscopy at high pressures—52. A study of the phase behaviour of K[M(CN)2], M = Ag, Au, at high pressure by Raman scattering

Abstract The high-pressure polymorphism of K[AuCN) 2 ] has been studied by Raman scattering at ambient temperature up to 25 kbar, revealing two phase transitions at 6.6 and 10.5 kbar. Little structural information can be gleaned for either of the new phases, since the I → II transformation is clearly first order in type. The reported increase in Au π-CN π* interaction is shown in a qualitative manner to manifest itself as a red shift in the absorption and fluorescence maxima. Preliminary work on the structurally similar K[Ag(CN) 2 ] indicates four transitions below 25 kbar at ca 1.5, 8.9, 15.0 and 21.0 kbar, confirming the mode behaviour reported earlier for phases 1Iand II.