Hydrostatic Media Research Articles

An infrared and Raman spectroscopic study of PbSO4-anglesite at high pressures

Infrared and Raman spectra of the ABX4 compound anglesite (PbSO4) were collected to 43 GPa at 300 K in three separate pressure media. A major transition in the spectra initiates at 23 GPa that is characterized by shifting and/or splitting of most of the fundamental vibrations of the sulfate group. On decompression, PbSO4 reverts to the ambient anglesite (barite) structure. This transition is kinetically impeded within more hydrostatic pressure media and does not proceed to completion within a neon medium even at pressures as high as 43 GPa. The previously observed spectral shift at 13 GPa occurs in less hydrostatic media (such as 4:1 methanol:ethanol), and involves subtle splitting of a few modes, particularly the (SO4)2− tetrahedral stretching and bending-derived Raman and infrared modes. Another possible shift occurs near 33 GPa, and primarily involves the appearance of new sulfate antisymmetric bending modes in the infrared spectrum and the onset of new sulfate antisymmetric stretching modes in the Raman spectrum. It is likely that PbSO4 begins to convert sluggishly to the P212121 ABX4 structure starting at 23 GPa, with a possible additional crystallographic distortion or shift in compression mechanism occurring near 33 GPa. The spectral shift at 13 GPa in non-hydrostatic media may be a differential stress-induced structural distortion, or a subtle transition that is not accessible within more hydrostatic media. Our results emphasize that apparently equilibrium transitions such as that at 23 GPa may be notably suppressed within more hydrostatic media.

Zone-Collapse Amorphization Mimicking the Negative Compressibility of a Porous Compound

A strongly anisotropic architecture of stacked metal–organic grids, with pores arranged through their eyes, has been revealed for the new coordination polymer Ni(bipy)(hip)(H2O)2·DMF·CH3OH·H2O, where bipy is 4,4′-bipyridine and hip is 5-hydroxyisophthalic acid. This porous crystal, when submerged in various nonpenetrating oils used as the hydrostatic media, displays a negative volume compressibility. This counterintuitive effect results in the zone amorphization injecting the contents of collapsing pores into the retained crystalline portions of the sample. This additional contents of the pores supports the crystalline parts and prevents their amorphization.

Spectroscopic Study of the Effects of Pressure Media on High-Pressure Phase Transitions in Natrolite

Structural phase transitions in natrolite have been investigated as a function of pressure and different hydrostatic media using micro-Raman scattering and synchrotron infrared (IR) spectroscopy. Natrolite undergoes two reversible phase transitions at 0.86 and 1.53 GPa under pure water pressure medium. These phase transitions are characterized by the changes in the vibrational frequencies of four- and eight-membered rings related to the variations in the bridging T-O-T angles and the geometry of the elliptical eight-ring channels under pressure. Concomitant to the changes in the framework vibrational modes, the number of the O-H stretching vibrational modes of natrolite changes as a result of the rearrangements of the hydrogen bonds in the channels caused by a successive increase in the hydration level under hydrostatic pressure. Similar phase transitions were also observed at relatively higher pressures (1.13 and 1.59 GPa) under alcohol-water pressure medium. Furthermore, no phase transition was found up to 2.52 GPa if a lower volume ratio of the alcohol-water to natrolite was employed. This indicates that the water content in the pressure media plays a crucial role in triggering the pressure-induced phase transitions in natrolite. In addition, the average of the mode Grueneisen parameters is calculated to be about 0.6, whilemore » the thermodynamic Grueneisen parameter is found to be 1.33. This might be attributed to the contrast in the rigidity between the TO{sub 4} tetrahedral primary building units and other flexible secondary building units in the natrolite framework upon compression and subsequent water insertion.« less

High-pressure behavior in alpha -AlPO4: Amorphization and the memory-glass effect.

Previous studies have shown that \ensuremath{\alpha}-${\mathrm{AlPO}}_{4}$ (berlinite) undergoes a crystalline to amorphous transition at 15\ifmmode\pm\else\textpm\fi{}3 GPa. Moreover, it has been proposed that the high-pressure amorphous phase recrystallizes upon decompression, conserving both the structure and the crystallographic orientation of the original crystal. We show in this study that this so-called memory-glass effect is instead due to a polymorphic crystal-crystal phase transition occurring between 14 and 15 GPa on compression. This transition is fully reversible, with a hysteresis of 6--7 GPa, when hydrostatic media are used for compression. When quasihydrostatic pressure media are used, the decompression history is more complex. The samples develop a rim of back-transformed material, but the interior of the grains remains in the high-pressure phase. On recovery, this interior consists of amorphous material, coexisting with the back-transformed \ensuremath{\alpha}-${\mathrm{AlPO}}_{4}$ phase. Quasihydrostatic runs above 18 GPa show evidence for amorphization during pressurization. In some highly nonhydrostatic runs, the recovered sample contains both amorphous material, and a crystalline phase of ${\mathrm{AlPO}}_{4}$. This could correspond to a rutile-structured phase, or to a phase with the ${\mathrm{InPO}}_{4}$ structure, formed during decompression in the presence of shear stresses.

Sub-megabar Mössbauer studies using diamond anvil cells

The development and the use of the diamond anvil cell for Mossbauer Spectroscopy (MS) to pressures nearing 100 GPa are discussed. Three types of cells and their typical performance are given. Pressure calibration, hydrostatic media, gasketing, collimation, γ-ray absorption, and sample size for MS are reviewed. New MS results showing hysteresis in the room temperature high pressure transition in iron and showing the rapid rise ofTN in NiI2 with pressure are presented.

Zeolite Molecular Sieve 4A: Anomalous Compressibility and Volume Discontinuities at High Pressure

Unit cell parameters of synthetic zeolite 4A were measured at several pressures to 40 kilobars with both water and an alcohol mixture as hydrostatic pressure media. Compression in water was normal, with no observed phase transitions. Compression in alcohols was twice as great as in water, and three volume discontinuities were observed. These volume changes in alcohol were rapid with increasing pressure but sluggish in reverse. High-pressure "phases," all of which are dimensionally cubic, are progressively more compressible at high pressure. These unusual high-pressure phenomena, which indicate significant interactions between zeolite 4A and the hydrostatic media, are consistent with differences in zeolite adsorption of water alcohols.

Thermal Conductivity Cell as a Pressure Gauge

A thermal conductivity measurement cell is utilized as a high pressure gauge for hydrostatic media. It is based upon the pressure dependence of the thermal conductivity of the media. The gauge offers electrical readout and high sensitivity compared to a typical manganin resistance cell.