S. Block

Calibration of the pressure dependence of the R1 ruby fluorescence line to 195 kbar

The pressure dependence of the R1 ruby fluorescence line has been calibrated at 25 °C against the compression of NaCl. Pressures are determined using the Decker equation of state for NaCl. The dependence is linear to 195 kbar following the equation PNaCl=2.746(Δλ), where P is in kbar and Δλ in A. The uncertainty in the value of the slope, dp/dλ, expressed in terms of a 95% confidence interval is 2.746±0.014 kbar A−1. The coefficient of the quadratic term (Δλ)2 is not significantly different from zero; and the quadratic term makes indeed a negligible contribution to the fit. Taking into account the reported uncertainty associated with the Decker equation of state for NaCl, the value of the slope is 2.740±0.016 kbar A−1 within a 95% confidence interval.

An Optical Fluorescence System for Quantitative Pressure Measurement in the Diamond‐Anvil Cell

An optical system for rapid routine pressure measurement is described which utilizes a pressure shift in the sharp R‐line fluorescence spectrum of ruby or similar materials. The system, which consists of a standard polarizing microscope and a 1/4 m monochromator with associated photodetection system, is used with the diamond‐anvil pressure cell, but can be employed with suitable modification in any pressure system which has optical access. The precision of the pressure measurement in a hydrostatic environment up to 100 kilobar is 0.5 kilobar using ruby as the pressure sensor. This precision is better than the accuracy of the present pressure scale above 40 kilobar. The merits of fluorescent materials other than ruby as pressure sensors are also discussed. A description of a Waspaloy diamond cell with some modifications in design is given. This improved cell and associated techniques extends the pressure range in gasketed systems up to 200 kilobar at room temperature and to moderate pressures at 700° C.

Hydrostatic limits in liquids and solids to 100 kbar

The hydrostatic properties of the materials methanol, isopropyl alcohol, water, sodium chloride, silver chloride, and the binary mixtures pentane‐isopentane and methanol‐ethanol have been determined in the diamond‐anvil pressure cell up to 180 kbar by line‐broadening and line‐shift measurements of the sharp R1 ruby fluorescence line. A liquid mixture 4 : 1 by volume of methanol : ethanol remains hydrostatic to almost 100 kbar at room temperature. This mixture exceeds the hydrostatic limit of the previous generally accepted fluid, 1 : 1 pentane : isopentane which has a hydrostatic limit of about 70 kbar. Silver chloride and water (ice VII) are better than sodium chloride as pressure‐transmitting media, but do not even qualitatively approach hydrostatic conditions much above 70 kbar. The stress sensitivity level of the ruby limits the extent to which slight deviations from hydrostatic conditions can be determined in solid systems and suggests the qualitative nature of the method in characterization of quasi...

Ultrahigh pressure diamond‐anvil cell and several semiconductor phase transition pressures in relation to the fixed point pressure scale

A diamond‐anvil type optical cell of improved design has produced static pressures in gasketed samples up to 500 kilobar as measured by the ruby fluorescence technique. The ruby R1 line pressure shift is linear to 291 kilobar, and the maximum measured shift is extrapolated to 500 kilobar assuming continued linearity of the pressure dependence. The ultimate pressure capability of this diamond cell has not been established. Transition pressures in the semiconductors Si, ZnSe, ZnS, and GaP measured by the ruby method indicate that the revised 1970 fixed point scale and the ruby (NaCl) scale diverge above 135 kilobar and disagreement may be by as much as a factor of 2 in the 500 kilobar range with the ruby scale defining the lower pressure.

A high‐pressure phase of polyethylene and chain‐extended growth

Optical and x‐ray observations of polyethylene have been made at high pressures and temperatures using a gasketed diamond‐anvil cell. The experiments confirm the existence of the high‐pressure phase previously postulated by Bassett and Turner. The new phase is hexagonal, with orthohexagonal lattice parameters of a = 8.46 A and b = 4.88 A. Comparison with the previously measured volume change indicates that there is a decrease in the c dimension to 2.45 A per ethylene unit in transforming from orthorhombic to hexagonal structures. The likely implication is that the molecules in the hexagonal phase do not have an all‐trans conformation. Chain‐extended growth is the result of crystallization from the melt into the hexagonal phase, whereas chain‐folded growth is the familiar process of melt crystallization.into the orthorhombic phase. Chain‐extended lamellae are observed to grow outwards behind a growing edge with a permanent narrowed profile, showing that the lamellar thickness is determined in a region exte...

Crystal Structure of Benzene II at 25 Kilobars

Crystals of a high-pressure form of benzene (benzene 11) were grown in the diamond-anvil pressure cell at elevated temperature and pressure from the transition of solid I to solid II. X-ray precession data were obtained from a single-crystal in the high-pressure cell. At 21�C and about 25 kilobars, benzene II crystallizes in the monoclinic system with a = 5.417 � 0.005 angstroms (S.D.), b = 5.376 � 0.019 angstroms, c = 7.532 � 0.007 angstroms, β = 110.00� � 0.08�, space group P21/ c, Pc= 1.26 grams per cubic centimeter. The crystal structure was solved by generating all possible molecular packing configurations and calculating structure factors, reliability factors, and packing energies for each configuration. This procedure produced a unique solution for the molecular packing of benzene II.

Model line‐shape analysis for the ruby R lines used for pressure measurement

An empirical model spectral line shape has been considered for the R1 and R2 fluorescence lines of ruby to investigate the possibility of improvements in the precision and the reproducibility of pressure measurements at high temperature using the ruby fluorescence technique. Other advantages, such as using the ruby method to obtain a simultaneous measurement of pressure and temperature or to obtain quantitative estimates of pressure distributions under nonhydrostatic conditions, have also been considered.

Raman and x‐ray investigations of ice VII to 36.0 GPa

Raman data for ice VII to 30 GPa and x‐ray lattice parameters to 36 GPa, all at room temperature, are presented and discussed. Both the Raman OH‐stretching peak frequency (Δv) and the edge distance of the body‐centered cubic unit cell (a) decrease at a decreasing rate with pressure rise. The OH peak frequency is found to be virtually linear in the nearest‐neighbor O–O distance (rOO) between about 2.5 and 2.9 A. However, the data can also be fitted with a slight cubic dependence, which when extrapolated, gives rise to a minimum in Δv at rOO=2.35±0.06 A. This is close to the value of 2.4 A found for the symmetric O–H–O units in other materials. This minimum suggests, but does not prove, that a symmetric hydrogen bond structure O–H–O, and thus a new ice, may result at pressures of about 75±20 GPa. Treatment of the O–H⋅⋅⋅O units according to couplet oscillators using experimentally confirmed assumptions indicates that Δv≊A/rOH2 which when combined with present and previously published data yields, approxim...

Polymorphism in benzene, naphthalene, and anthracene at high pressure.

Optical observations, in which a microscope was used with the diamond-anvil pressure cell, were carried out on benzene, naphthalene, and anthracene up to temperatures of about 600�C and pressures of approximately 40 kilobars. New high-pressure phases of benzene (benzene III) and anthracene (anthracene II) were observed, and the existence of the high-pressure polymorph, naphthalene II, was verified. All three materials decompose initially to a reddish-orange liquid, and ultimately to amorphous carbon. The decomposition temperatures decrease with increasing molecular size.

Crystallography of Some High‐Pressure Forms of C6H6, CS2, Br2, CCl4, and KNO3

From single crystal x‐ray diffraction at high pressure and room temperature unit‐cell and space‐group data were obtained for the following materials: C6H6, I—orthorhombic, a = 7.17, b = 9.28, c = 6.65, Pbca, CS2—orthorhombic, a = 6.16, b = 5.38, c = 8.53, Cmca, Br2—orthorhombic, a = 8.54, b = 6.75, c = 8.63, Cmca, CCl4, I—rhombohedral, a = 14.27, α = 90°, CCl4, II—monoclinic, a = 22.10, b = 11.05, c = 25.0, β = 114°, Cc or C2 / c, CCl4, III—orthorhombic, a = 11.16, b = 14.32, c = 5.74, C2221, KNO3, III—rhombohedral, a = 4.31, α = 78°54′, KNO3, IV(?)—orthorhombic, a = 5.58, b = 7.52, c = 6.58, P2lnb or Pmnb. All unit‐cell dimensions are given in angstroms with estimated uncertainties of ± 2 in the last decimal place given and uncertainties of ± 0.5 deg in angles.