The behavior of a number of commonly used pressure media, including nitrogen, argon, 2-propanol, a 4:1 methanol–ethanol mixture, glycerol and various grades of silicone oil, has been examined by measuring the X-ray diffraction maxima from quartz single crystals loaded in a diamond-anvil cell with each of these pressure media in turn. In all cases, the onset of non-hydrostatic stresses within the medium is detectable as the broadening of the rocking curves of X-ray diffraction peaks from the single crystals. The onset of broadening of the rocking curves of quartz is detected at ∼9.8 GPa in a 4:1 mixture of methanol and ethanol and at ∼4.2 GPa in 2-propanol, essentially at the same pressures as the previously reported hydrostatic limits determined by other techniques. Gigahertz ultrasonic interferometry was also used to detect the onset of the glass transition in 4:1 methanol–ethanol and 16:3:1 methanol–ethanol–water, which were observed to support shear waves above ∼9.2 and ∼10.5 GPa, respectively, at 0.8–1.2 GHz. By contrast, peak broadening is first detected at ∼3 GPa in nitrogen, ∼1.9 GPa in argon, ∼1.4 GPa in glycerol and ∼0.9 GPa in various grades of silicone oil. These pressures, which are significantly lower than hydrostatic limits quoted in the literature, should be considered as the practical maximum limits to the hydrostatic behavior of these pressure media at room temperature.

Effective hydrostatic limits of pressure media for high-pressure crystallographic studies

The ruby luminescence method is widely used for pressure measurement in the diamond anvil cell and other optically transparent pressure cells. With this application in mind, we briefly review the ground-state physical properties of corundum (α-Al2O3) with some emphasis on its behavior under high pressure, survey the effects of temperature and stress on the R-line luminescence of ruby (Cr-doped corundum), and address the recent efforts towards an improved calibration of the R-line shift under hydrostatic pressures beyond the 50 GPa mark.

Ruby under pressure

Cr K-edge XANES spectra were obtained for a variety of Cr-bearing model compounds containing Cr(II), Cr(III), Cr(IV), Cr(V) and Cr(VI), in which the Cr-site symmetry is D4h, Oh and Td. The centroid position of the pre-edge feature is a better indicator of the Cr valence than the edge position. In Cr-rich oxides, higher-energy transitions must be excluded in order to refine a robust valence for Cr. The pre-edge for chromates is not unique and varies as a function of the CrO4
 2− moiety distortion, which is often related to Cr-polymerization (monochromate vs. dichromate). Both the analogy with the Mn K-pre-edge information and ab initio FEFF calculations of the pre-edge feature for Cr(III) and Cr(VI) confirm the experimental trends. This methodology is applied to the Cr K-edge pre-edge feature collected in gems (emerald, spinel and ruby), the layered minerals fuchsite and kammererite, two Cr-bearing aqueous solutions and a set of sodo-calcic silicate glasses used for bottling sparkling white wine. In emerald and fuchsite, the Cr-site is differently distorted than its ruby or spinel counterpart. In a Cr(III)-bearing aqueous solution and sodo-calcic glass, no evidence for Cr(III) with Td and C3v symmetry is detected. However, minor amounts of chromate moieties (most likely monomeric) are detected in a glass synthesized in air. Preliminary spectra for the wine bottle glass suggest that only trace amounts of chromates might possibly be present in these glasses.

Chromium speciation in oxide-type compounds: application to minerals, gems, aqueous solutions and silicate glasses

Pyrene as emitting chromophore in organic-inorganic lead halide-based layered perovskites with different halides

Using the ultrasonic pulse-echo technique and a newly developed method to treat the data, all the elastic compliances of α-quartz have been obtained as a function of hydrostatic pressure up to 1 GPa with great accuracy. Fifteen independent measurements were performed in five directions of high symmetry (X, Y, Z, Y' and Z'). The pressure dependence of the six second-order elastic and two piezoelectric constants were deduced. The pressure derivatives of c14 and c44 are positive, whereas that of c66 is negative, in contradiction to previously published results. Under ambient conditions, the linear and bulk moduli Ba0 = 103.4(0.5) GPa, Bc0 = 137.0(0.5) GPa and B0 = 37.5(0.2) GPa calculated from our determination of the elastic tensor are in good agreement with the published values. However, we find a clear discrepancy with published values for the pressure derivative of the bulk modulus B' = 4.7(0.5). From these results, the Born stability criteria have been calculated as a function of pressure and compared with previous results. Their behavior can help to explain the structural instability of α-quartz at around 18 GPa.

/pdf/complete-determination-of-the-elastic-moduli-of-a-quartz-4j79nycw5t.pdf

Complete determination of the elastic moduli of α-quartz under hydrostatic pressure up to 1 GPa: an ultrasonic study

The high pressure behavior of ruby (α-Al 2 O 3 doped with Cr 3+ ) was investigated to pressures up to 31 GPa. Whereas data on compressibility has been obtained at pressures up to 175 GPa, no structure determination exists above 9 GPa. Results show that the highly incompressible material has a bulk modulus of K= 242(8) GPa and its pressure derivative K' is calculated to be 6(1). In order to compensate for the applied forces, the MO 6 octahedra (M = Al, Cr) volume decreases with increasing pressure. At pressures above 20 GPa, a distinct change in the oxygen positional parameter is observed. The reason might be a short M-O distance of 1.814(2) A which stays constant throughout the pressure range investigated. A comparable effect of a considerable change of the internal structure under high pressure despite a very low compressibility was recently observed for the mineral spinel [1].

High pressure single crystal X-ray diffraction study on ruby up to 31 GPa

Single crystal X-ray diffraction experiments on α-quartz (trigonal, space group P3121) were performed to pressures up to 19.3 GPa using synthetic samples. Volumina of the unit cell were determined to 13.1 GPa, by combining the data with data taken from the literature; the bulk modulus and pressure derivative calculate to 38.7(3) GPa and 5.2(1) respectively according to a Birch-Murnaghan equation of state. Intensity data were collected at 10.9(1), 12.0(1), 12.1(1), 12.6(1) and 13.1(1) GPa. From the five intensity data sets, one was collected using synchrotron radiation at HASYLAB/DESY and the other four using a conventional X-ray tube. Results show that the SiO 4 tetrahedra, the building blocks of the structure, become increasingly distorted as pressure is increased but no significant change in the polyhedral volume is observed. The volume reduction is mainly compensated by the decrease in the inter-tetrahedral Si-O-Si-angle, which is correlated to the increase in the tilt angle φ. In addition to the five intensity data sets collected, X-ray diffraction experiments were carried out up to 19.3 GPa in order to address the question of when the structure undergoes amorphization. It was observed that up to 19.3 GPa, no pressure-induced amorphization takes place. This result is in agreement with studies carried out on powder quartz and theoretical studies.

S. Werner

Papers

High pressure single crystal X-ray diffraction study on ruby up to 31 GPa

High pressure single crystal X-ray diffraction study on α-quartz