M. H. Mueller

Defect characterization in CeO2−x at elevated temperatures—I: X-Ray diffraction

Abstract The atomic defect structure of nonstoichiometric ceria was studied by means of X-ray diffraction. Polycrystalline samples of CeO 2−x (0⩽ x ⩽0.21) have been examined at 900 and 1000°C, with the stoichiometry controlled by adjusting the oxygen partial pressure between 1 and 10 −21 atm. It was observed that the lattice expands as a function of increasing defect concentration and exhibits only fluorite-like diffraction peaks. The integrated intensities of the Bragg reflections were analyzed for CeO 2 and CeO 1.91 , at 900°C by difference electron-density techniques. It was concluded that the cation sublattice is essentially intact, and that the oxygen sublattice must be defective in nonstoichiometric ceria. Least-squares analyses on CeO 2−x (0⩽ x ⩽ 0.21) at 900 and 1000°C supported the electron-density results and also showed that the temperature factors of both cations and anions increase with an increase in defect concentration, implying greater mean-square displacement of the atoms from their equilibrium positions.

A neutron diffraction determination of the coherent scattering amplitude of NP and the possible antiferromagnetism of neptunium dioxide

Neptunium dioxide has been reported to be antiferromagnetic with a Neel temperature of 25°K based on magnetic susceptibility and specific heat measurements. Neutron diffraction patterns were obtained at 5.4°K and 78°K from two neptunium dioxide samples with oxygen to metal ratios of 1.93 and 2.034. Since the scattering amplitude of Np was unknown, it was determined to have a value of l.055±0.010 × 10−12 cm. No significant difference was observed between the 5.4°K and 78°K neutron diffraction patterns of either sample. Assuming that neptunium dioxide is magnetically isostructural with uranium dioxide the magnetic moment of each neptunium atom is less than 0.5 Bohr magnetons.

Germanium as a Neutron Monochromator

A comparison has been made of several ways of introducing different amounts of imperfection into neutron transmission monochromators of germanium in order to control their efficiency. The recommended technique is uniaxial plastic compression along the [110] direction at 650°C to a reduction in thickness equal to or slightly under 2.5%. Tests indicate an improvement in efficiency in both single‐crystal and power diffractometry of a factor of 25 to 40 over that of an undeformed slab.

Quantitative Pole Figures for Sheet Material by the Reflection Technique

A method is described for obtaining quantitative pole figures for sheet material by the use of the reflection technique only. Seven different sections of the sample are used for obtaining the data for one quadrant of the pole figure which makes it possible to obtain adequate resolution of very close reflections. One section is prepared from a single piece of the sheet. The other six sections are made up of a number of small pieces of the sheet mounted in a jig in order to form a plane surface at an angle of 45° or 90° to the surface of the sheet. These jigs are mounted on a special table for the modified Schulz reflection specimen holder. Intensity data are obtained automatically for a number of different positions on the pole figure defined by φ and α. The method of plotting this intensity data is shown which results in quantitative pole figures with contour intensity lines.

AN AUTOMATIC X-RAY REFLECTION SPECIMEN HOLDER FOR THE QUANTITATIVE DETERMINATION OF PREFERRED ORIENTATION

A reflection specimen holder has been developed, which has several new features. A removable jig, which fits into the inner ring, is used for the rapid alignment of the specimen. Two different specimen scanning devices are described, which allow the primary beam to cover successively a large number of grains in the specimen surface while the diffracted intensity is being measured. Since many such time consuming measurements are required for an accurate pole figure, the operation of the instrument and the recording of the data have been made automatic.

Neutron diffraction study of NpC

Abstract A neutron diffraction investigation ofpolycrystalline NpC has confirmed the measurements of magnetic susceptibility which suggested that the compound is antiferromagnetic between 310° and 220°K, and ferromagnetic below 220°K. Neptunium carbide has the NaCl type structure (a = 4.992 A ) ; the antiferromagnetic structure consists of ferromagnetic layers perpendicular to [001] coupled antiferromagnetically with the spin direction along the [001] axis. The change in ordered moment in the antiferromagnetic region below 310°K does not follow a Brillouin function. The maximum moment in the antiferromagnetic state is 1.3 ± 0.1 μB and the magnetic form factor for the neptunium atom has been determined at low angles. The transition to ferromagnetism is probably first order, and the initial moment at 218°K is 1.6 μB. The moment increases to a maximum value of 2.1 ± 0.1 μB at ∼150°K and remains constant to 6°K.

Temperature dependence of the x-ray and neutron diffraction from a metallic glass

Abstract The amorphous alloy Be 0.4 Ti 0.5 Zr 0.1 has one of the largest negative temperature coefficients of resistivity known in a metallic glass and thus presents a demanding test case for any theory that would explain this phenomenon. By measuring the temperature dependence of the x-ray and neutron diffraction profiles we have attempted to test the extended Ziman theory for the resistivity of amorphous metals. The observed change in the structure factor is large enough to explain the observed resistivity data in terms of this theory.

STEPPING MECHANISM FOR X-RAY AND NEUTRON DIFFRACTOMETERS AND SPECTROMETERS

This motor is a permanent magnet type a-c motor with two field windings. As an a-c motor it has a speed of 72 rpm; however, when direct voltage is applied to the field windings, the motor will lock into a magnetic hold position, and if the voltage is then switched, the motor will advance pending upon the switching used. There is no need for braking since the accuracy of positioning each step is extremely great, and any inaccuracy of positioning is nonaccumulative. To control the amount of rotation, a transistonized electronic stepping motor control consisting of a pre-set counter and motor drive switohing circuit is used. The pre-set counter counts the number of drive pulses delivered to the motor by the switching circuit, and hence the amount of rotation of the motor shaft. After reaching the pre-set amount of rotation, the control automatically stops itself and resets the pre-set counter to zero. This control can be started and stopped manually or automatically and can also drive continuously. (N.W.R.)

Magnetic and lattice properties of CeBi

The magnetic structures of CeBi have been examined as a function of field with neutron diffraction. At low fields unusual domain reorientation effects are observed. For H≳18 kOe a 3+, 1‐ spin configuration is found. A first‐order transition to induced ferro‐magnetic behavior occurs at 47.5 kOe. X‐ray experiments at low temperature show that no lattice distortion occurs at either TN=25 K or the I‐IA transition at 12.5 K. A volume discontinuity is observed at the low temperature transition.

Lattice shear distortions in fluorite structure oxides

Crystallographic shear distortions have been observed in fluorite structure, single crystals of UO2 And Zr(Ca)O2−x by neutron‐diffraction techniques. These distortions localize on the oxygen sublattice and do not require the presence of an external strain. The internal rearrangement mode in UO2 is a transverse, zone boundary q↘=2π/a (0.5, 0,0) deformation with amplitude 0.014 A. In Zr(Ca)O2−x, the mode is a longitudinal, q↘=2π/a (0,0,0.5) deformation with amplitude 0.23 A. Cation‐anion elastic interaction dominate in selecting the nature of the internal distortion.