Arthur L. Ruoff

Pressure‐induced rocksalt phase of aluminum nitride: A metastable structure at ambient condition

Energy‐dispersive x‐ray diffraction studies were carried out on the III‐V compound aluminum nitride to 65 GPa using a synchrotron x‐ray source. A pressure‐induced first‐order phase transition from a wurtzite structure to a rocksalt structure was observed. The first peaks of the high‐pressure phase appeared at 14 GPa. On further loading of the diamond anvil cell, the wurtzite peaks disappeared by 20 GPa. After unloading the cell to atmospheric pressure, the rocksalt structure persisted. The equilibrium transformation pressure lies on the interval 0–14 GPa.

Solid hydrogen at 342 GPa: no evidence for an alkali metal

Solid hydrogen, an electrical insulator, is predicted to become an alkali metal under extreme compression, although controversy surrounds the pressure required to achieve this. The electrical conductivity of hydrogen as a function of pressure and temperature is of both fundamental and practical interest—metallic hydrogen may be of relevance to planetary interiors, and has been suggested as a potential high-temperature superconductor. Calculations, suggest that depairing (destruction of the molecular bond) should occur around 340 GPa, accompanied by the formation of an alkali metal at this pressure, or at substantially higher pressures,. Here we report that solid hydrogen does not become an alkali metal at pressures of up to 342 ± 10 GPa, achieved using a diamond anvil cell. This pressure (which is almost comparable to that at the centre of the Earth) significantly exceeds those reached in earlier experiments—216 GPa (ref. 6) and 191 GPa (ref. 7)—at which hydrogen was found to be non-metallic. The failure of solid hydrogen to become an alkali metal at the extreme pressures reported here has implications for our current theoretical understanding of the solid-state phase.

Linear Shock‐Velocity‐Particle‐Velocity Relationship

An equation of state, based on a bulk modulus variation with pressure of the form BS=B0S+B0S′P+B0S″P2/2, where B0S, B0S′, and B0S″ are constants, is developed in this analysis. The resultant equation of state is combined with the Rankine‐Hugoniot conservation relations to obtain a Maclaurin series expansion for the shock velocity vs the particle velocity us=c+sup+s′up2+⋯. The coefficients c, s, and s′ are given in terms of the unshocked density and quantities available from ultrasonic elastic constant measurements at high pressures. Using new experimental data for sodium, it is shown that s′ is nearly zero. For ionic crystals such as KBr a theoretical expression is given for B0S″ (in terms of B0S′). In the case of KBr, the value of s′ is also very close to zero. The smallness of s′ depends on the cancellation of a number of terms brought about by the fact that B0S″ is negative. CsI and xenon are also discussed.

Miniaturization techniques for obtaining static pressures comparable to the pressure at the center of the earth: X‐ray diffraction at 416 GPa

X‐ray diffraction studies on tungsten and molybdenum were performed, using a 4‐μm‐diam x‐ray beam, to very high pressures, with the pressures being obtained from the measured lattice parameters and isothermal equations of state of tungsten and molybdenum deduced from shock data. The bcc structure persists to the highest pressure, 378 GPa in tungsten, and 416 GPa in molybdenum. The static pressures generated and measured here exceed the pressure of 361 GPa at the center of the earth, the first time that this has been achieved and measured with a calibrated pressure scale. The details of the pressure profile at 335 GPa are shown and are of great use in designing anvils for future research. It is noted that the maximum pressures attained by x‐ray diffraction with beveled anvils varies linearly with D−1/2 where D is the diameter of the flat suggesting that, perhaps, even higher pressures are possible with further miniaturization. A scaling law is used to calculate the minimum correction due to the presence of...

Static compression of silicon in the [100] and in the [111] directions

High pressures were generated using a diamond indentor technique. The phase transition in silicon loaded in the [100] direction and silicon loaded in the [111] direction under high pressure was studied. From the behavior of electrical resistance of silicon loaded in the [100] direction, it was found that it becomes metallic at about 12.0 GPa in agreement with the previously reported results on silicon using different techniques. However, silicon loaded in the [111] direction becomes metallic at a lower pressure. The silicon transition is highly sensitive to the type of stress applied.

Isothermal equation of state for sodium chloride by the length‐change‐measurement technique

The change in length of a 1‐m‐long NaCl single crystal has been determined as a function of hydrostatic pressure up to 7.5 kbar and at temperatures of 29.5 and 40.4 °C, to an accuracy of 500 A using a Fabry‐Perot–type He‐Ne laser interferometer. The best values of the isothermal bulk modulus and its pressure derivative at atmospheric pressure and at 29.5 °C are B0=237.7±0.3 kbar, B′0 =5.71±0.25, and B″0=−0.10±0.05 kbar−1, respectively. These are the averages of the values obtained by a least‐squares fit of several different equations of state to the present isothermal data. From these low‐pressure measurements along, it is not possible to conclude which one of these equations provides a better fit to the data than the others. However, when other high‐pressure data are taken into account, it appears that Keane’s equation best represents the measurements. When Keane’s equation is fitted to the data, and the published lattice parameter of NaCl at the Bi‐III‐V transition and at the NaCl B1‐B2 transition are u...

The bulk modulus of C60 molecules and crystals: A molecular mechanics approach

In this letter, the bulk modulus of an individual C60 molecule is calculated in terms of the C,C bond force constant. A range of values for the bulk modulus is obtained with literature values for the force constant. The values obtained all exceed the bulk modulus (441 GPa) of diamond. With a C,C bond force constant equal to that between adjacent carbon atoms in graphite, 7.08 mdyn/A, a bulk modulus of 903 GPa is obtained. On the basis of a simple composite model it is calculated that single closest‐packed C60 crystals of C60 will have a bulk modulus of roughly 668 GPa under hydrostatic pressures. The calculated bulk modulus for a single C60 ‘‘buckyball’’ therefore suggests the possibility that a C60 crystal could be the most incompressible material known, at a pressure above about 50 GPa.

Strain‐Enhanced Diffusion in Metals. II. Dislocation and Grain‐Boundary Short‐Circuiting Models

Rapid diffusion along dislocation cores generally enhances the average bulk diffusion in a dislocated crystal. If each diffusing atom has at least several opportunities to make rapid excursions along various dislocation cores, Hart has shown that DT/DT0 = 1+fDP/DT0. DT is the average bulk diffusivity, DT0 is the diffusivity in the dislocation‐free lattice, f is the fraction of atoms in dislocation pipes, and DP is the diffusivity along dislocation pipes. Under these conditions, the diffusion‐penetration is increased by the short‐circuiting but the general shape of the penetration curve is unaffected. When the dislocations are static, each diffusing atom must visit a number of dislocations, and a necessary condition for the Hart relation to hold is 2(DT0t)½>ld, where t is the diffusion time, and ld is the dislocation spacing. When this condition fails (for example, at lower temperatures), and when the dislocations are static, it is demonstrated that the amount of short‐circuiting is greatly reduced and tha...

The effect of a tapered aperture on x‐ray diffraction from a sample with a pressure gradient: Studies on three samples with a maximum pressure of 560 GPa

X‐ray diffraction patterns were obtained using a tapered lead aperture. A detailed analysis of the effect of the tapered aperture is given. Experimental studies include results on W (423 GPa), on Pt (440 GPa, obtained using type IIa diamonds), and Mo (560 GPa). These results extend earlier studies in which we achieved for the first time static pressures greater than the pressure at the center of the earth.

Etching of diamond with argon and oxygen ion beams

The angular dependences of the sputter yield of (100) single crystal diamond under argon and oxygen ion beams were investigated. For argon ion beams a large increase in the sputter yield was observed as the angle of incidence was increased from the normal, with a maximum occurring at approximately 60° from the normal. The magnitude of the increase and the angle at which the maximum was observed were dependent on the ion energy and species. The shape of the yield versus angle of incidence curve indicated the presence of a highly damaged surface layer. Large deviations from this curve were observed where ion incidence was along channeling directions. Studies with oxygen ion beams showed almost no angular dependence for 500 eV ions while at an energy of 1000 eV the angular dependence was similar to that for argon ions. At normal incidence the yield for 500 eV oxygen ions was seven times larger than that for 500 eV argon ions. For 1000 eV oxygen and argon ions the corresponding ratio is only 2.5. Implications...