J. S. Olsen

X-ray Diffraction Investigations of CaF2 at High Pressure

Synchrotron-radiation X-ray diffraction studies of CaF2 at high pressures have been performed on a powder sample up to 45 GPa and on a single-crystal sample up to 9.4 GPa. The bulk modulus of the low-pressure phase was determined to be B o = 87 (5) GPa. A phase transition was observed at about 9.5 GPa. The transition is accompanied by a volume contraction of 11%. The high-pressure phase is orthorhombic PbC12 type (space group Pbnm). The sample only partially reverts to the low-pressure phase upon release of pressure.

Grain-size effect on pressure-induced semiconductor-to-metal transition in ZnS

The grain-size effect on the semiconductor-to-metal transition in ZnS has been investigated by in situ high-pressure electrical resistance and optical measurements. It is found that the grain-size effect can elevate the transition pressure of ZnS in a larger pressure range. On the basis of the results obtained and results reported in the literature, we demonstrate that the dangers of using the transition pressures of the II–VI compounds as pressure calibrators without a detailed knowledge of their grain-size effects on the transition pressures cannot be overstressed.

Pressure effect on crystallization of metallic glass Fe72P11C6Al5B4Ga2 alloy with wide supercooled liquid region

The effect of pressure on the crystallization behavior of metallic glass Fe72P11C6Al5B4Ga2 alloy with a wide supercooled liquid region has been investigated by in situ high-pressure and high-temperature x-ray diffraction measurements using synchrotron radiation. In the pressure range from 0 to 2.4 GPa, the crystallization temperature, Tx, increases with pressure, p, having a slope of 30 K/GPa while a minimum in Tx was detected in the range from 2.4 to 3.9 GPa. With a further increase of p, Tx is independent of p. The Tx vs p behavior can be qualitatively explained by the suppression of atomic mobility and changes of the Gibbs free energy of various phases with pressure.

Phase transformation and conductivity in nanocrystal PbS under pressure

The grain-size effect on the phase transition induced by pressure in PbS was studied by in situ high-pressure electrical resistance and synchrotron radiation x-ray powder diffraction measurements. The mean transition pressure of the B1-to-B16 phase transformation was found to be 6.3±1.3 GPa in 8±1 nm PbS while it is 3.1±0.7 GPa for 10 μm PbS. The resistivity of the B16 PbS phase decreases exponentially with pressure in both samples at ambient temperature. They follow R∝exp(−CP), where C=−0.64 GPa−1 for 10 μm PbS and C=−0.34 GPa−1 for 8±1 nm PbS. These results are discussed in terms of a decrease of energy band gap with increasing pressure.

Compressibility of nanostructured FeCu materials prepared by mechanical milling

Abstract The compressibility of nanostructured FeCu materials prepared by mechanical milling has been investigated by in-situ high-pressure x-ray diffraction using synchrotron radiation. It is found that the bulk modulus of both fcc-Cu and bcc-Fe phases decreases with decreasing grain sizes. The unstable ferromagnetic fcc-FeCu solid solution prepared by mechanical alloying has a bulk modulus of about 85 GPa, which is much smaller than the corresponding values for bulk fcc-Cu and bcc-Fe.

Crystal Structure and the Equation of State of Thorium Monophosphide for Pressures up to 50 GPa

High-pressure X-ray diffraction studies have been performed on ThP using synchrotron radiation and a diamond-anvil cell. The bulk modulus B0 and its pressure derivative B′0 have been determined (B0 = 137 GPa; B′0 = 5.1). A phase transition from the NaCl structure to the CsCl structure was observed at about 30 GPa.

A high-pressure study of thallium

High-pressure experiments on metallic thallium have been carried out using a diamond-anvil cell for pressures up to 32 GPa. A hexagonal close packed → face-centred cubic (f.c.c.) phase change takes place around 4 GPa. Fitting of the experimental data to the Murnaghan and Birch equations of state has been used to obtain values for the bulk moduli and their pressure derivatives for the two phases. Extrapolation of the high-pressure data for the f.c.c. phase to low pressure has been used to obtain an estimate of the pressure to about 0.23 GPa in nanosized metastable f.c.c. thallium inclusions formed in aluminium by ion implantation.

The pressure-induced transformation B1 to B2 in actinide compounds

The phase transformation from NaCl structure (B 1) to CsCl structure (B2) in actinide compounds has been studied using X-ray powder diffraction in the pressure range up to about 60 GPa. It is shown that the transition is sluggish, has a strong hysteresis and is accompanied by a volume change in the range 8–12%. These features are similar to those of the corresponding transition in the alkali halides and other B1 compounds, indicating a common mechanism for the transformation as concerns the lattice geometry.

Diffraction Study of Actinides under Pressure

Abstract Uranium and thorium have sufficiently low radioactive dose rates to allow their study at synchrotrons and neutron facilities. Correspondingly, numerous compounds of these two actinides have been studied under pressure by synchrotron x-ray diffraction. The maximum pressures reached were on the order of 60-80 GPa, and 300 GPa in one case. The situation is much more difficult for all other actinides. Their high level of radioactivity has up to now prevented their study at synchrotrons, except in a few special cases. In contrast, all actinide metals available in sufficient quantities, and a large number of compounds of highly radioactive actinides, have been studied in highpressure laboratory facilities. Recent examples of in situ high pressure x-ray diffraction work will be described.

Structural stability of binary CdCa quasicrystal under high pressure

The structural stability of a binary CdCa quasicrystal with a primitive icosahedral structure has been investigated by in situ high-pressure x-ray powder diffraction at an ambient temperature using synchrotron radiation. It is demonstrated that the icosahedral quasicrystalline structure of the sample is intrinsically stable up to 47 GPa. The bulk modulus at zero pressure and its pressure derivative of the icosahedral CdCa quasicrystal is 68.1±2.0 GPa and 4.3±0.2, respectively. The compression behavior of different Bragg peaks is isotropic, indicating no pressure-induced anisotropic elasticity in the stable binary icosahedral CdCa quasicrystals.