George Jura

Semiconducting Region of Ytterbium

The resistivity of elemental ytterbium at room temperature rises, by a factor of 11, to a maximum at a pressure of 40 kilobars; a further increase in pressure causes a polymorphic transition; the new phase has a resistivity 80 percent of that of the metal at 1 atmosphere. In the temperature-pressure diagram, the phase boundary has a negative slope. The phase boundary, determined from -190� to 360�C, is a straight line that may be extrapolated nearly to the known α-β transition at 1 atmosphere. Between the transition pressure and 20 kbar, the lowest pressure at which the measurements were made, ytterbium behaved as a semiconductor. The temperature coefficient of resistance is negative; at constant pressure, the resistivity shows the exponential temperature dependence characteristic of a semiconductor. The parameter in the expontial would correspond to an energy gap 0.015 ev at 20 kbar, an increase with pressure to a maximum of 0.080 ev at 37 kbar, and then a decrease to 0.05 ev at 45 kbar.

Mössbauer Spectrum of Iron-57 in Iron Metal at Very High Pressures

The effect of pressure on the M�ssbauer spectrum of Fe57 in iron metal has been studied as the pressure was increased presumably to more than 140 kbar. At pressures up to 120 kbar, a six-line spectrum characteristic of α-iron was observed. At 140 kbar, a seventh line appeared in the spectrum at -0.12 � 0.06 mm/sec relative to stainless steel. This line was attributed to the appearance of the high-pressure phase of iron.

The relaxation energy of vacancies and impurities in a molecular lattice

Abstract The energy of relaxation and relaxation of surrounding atoms are computed for vacancies, large and small impurity atoms, vacancy impurity atom di-defects, and di-vacancies in a f.c.c. (argon) lattice, assuming a Lennard-Jones potential and neglecting all kinetic energy effects. It is found that at normal lattice spacing the energy correction is small for all types of defects considered. Under compression, relaxation energy increases rapidly. Relaxations are always small. It is found that formation of a vacancy next to an impurity is preferred to formation of an isolated vacancy. Concentration of di-vacancies is found to be very small except at temperatures approaching the melting point.

Self diffusion in solid argon: The activation energy

Abstract Using a Lennard-Jones 6–12 potential to represent the interactions in a f.c.c. molecular lattice (argon) we have calculated the “activation energy”, E , in the Arrhenius equation D = D 0 exp (− E RT ) for self diffusion to be 3811 cal assuming that diffusion occurs via a vacancy exchange mechanism. This compares well with the experimental value of 4120 cal. The relaxation of the nearest neighbors to the vacancy-diffusing atom pair were calculated and were found to lower the barrier by 50 per cent. We have also found that for a divacancy exchange self diffusion process the activation energy, 4847 cal, is in satisfactory agreement with experiment and the “frequency factor”, D 0 , is greater than that for monovacancy diffusion. We have calculated the activation energies for neon and krypton impurity diffusion via monovacancy exchange through an argon lattice to be 2182 and 3873 cal respectively.

Surface energy and configuration of inert gas crystals—I☆

Abstract An expression is derived for the average potential energy of a particle at any given distance above the plane face of a semi-infinite crystalline array of similar particles obeying an inverse power, pairwise additive potential function with spherical symmetry. This expression is then used with a Lennard-Jones potential function to discuss various models for the surface configuration of semi-finite crystals by comparing the minimum excess surface energy of the model with that of the undistorted half-crystal. The results indicate that the only likely distortion in the surface of the static lattice at0°K is a slight expansion normal to the surface as indicated by s huttleworth s calculations (1)

High Pressure: Effect on Dysprosium

The electrical resistance of dysprosium was studied in the temperature-pressure range 77� to 200�K and 15 to 120 kb. The variations of the N�el transition with pressure was found to good accuracy, the value of dTN/dP being-0.62�0.4 deg/kb. From this result, second-order thermodynamic equations yield values of -64 x 10-6 (�C)-1 and 4.0 x 10-14 cm2/dyne for the changes in the coefficient of linear expansion and the compressibility, respectively, at the 1-atm N�el point. The N�el transition disappears at 45 to 55 kb, and no magnetic transitions are seen at higher pressures in the temperature range indicated. The other boundaries in the region investigated have been determined but are very uncertain. The cause of the cusp in the isothermal compressions is not clear from the available data. Above the N�el boundary, the pressure dependence of the cusp with respect to temperature, dP/dT, is very small and may even be zero. Below the N�el temperature, dP/dT is uncertain. From isothermal measurements, the value is -1, while from the isobaric measurements a value of -0.25 is obtained.

Four-Lead Electrical Resistance Measurements in Bridgman Anvils

A geometry is described which permits four-lead electrical determinations of the pressure coefficient of resistance of metals in Bridgman anvils. It is also possible in this geometry to mount more than one sample aild to make independent measurements on each sample simultaneously.

Infrared Studies of Solutions Involving Aromatics and Halogens

Infrared studies were made in the region 640—1400 cm—1 to investigate the effect of adding iodine to benzene, toluene, mesitylene, hexamethylbenzene, naphthalene, cyclohexane, and methylcyclohexane. Spectral changes were observed for each of the aromatics except for naphthalene. The enhancement of the mesitylene band at 1300 cm—1 was observed for mesitylene purified in five distinct ways. The enhancement of the hexamethylbenzene band at 1298 cm—1 was studied as a function of the concentrations in carbon disulfide solutions and correlates with the product of hexamethylbenzene and iodine concentrations. The effect of bromine on the spectrum of benzene was noted also. The results can be discussed in terms of weak complex formation or strong solvent perturbations.

High pressure effects on the intensity of infrared spectra of molecular crystals

Abstract Through a unique design of the sample beam optics, the i.r. spectra of naphthalene, polyethylene and n -paraffins C 23 H 48 , C 24 H 50 , C 28 H 58 and C 29 H 60 have been measured in a Drickamer type sodium chloride high pressure cell to 40 kbar. Drastic changes of band intensities with pressure are observed in the vibrational spectra. It is noted that numerous types of intensity transfer among normal modes of a crystal arise from the perturbations of the crystalline potential and that pressure is a practical and useful parameter for observing such phenomena. The observed phenomena on the pressure induced changes of band intensities are presented. Attempts have been made to theorize these phenomena with simple arguments.

Negative Temperature Coefficient of Resistance in Bismuth I

Measurements of the electrical resistance have been made on bismuth 1 between 15 and 35 kilobars at temperatures between 77.4� and 120�K. Above about 150�K, the temperature coefficient of resistance is positive, as in a metal; below 150�K, the coefficient becomes negative, as is characteristic of semiconductors. On the basis that bismuth is a semiconductor, the energy gap, calculated by the exponential resistance formula, is 0.006 ev at 15 kb with a steady rise to 0.018 ev at 35 kb. At higher pressures, bismuth I is transformed into a metallic modification with the normal temperature dependence of the resistance. The energy gap in bismuth I is not visible at room temperature because thermal excitation populates the conduction band and metallic behavior is the result. From available evidence the observed behavior is due to an energy gap rather than to a decrease in carrier mobility.