Walter L. Roth

Neutron and Optical Studies of Domains in NiO

Antiferromagnetic domains in NiO have been studied by neutron diffraction and optical observation of single crystals. There are two kinds of antiferromagnetic domain walls, T (twin) and S (spin‐rotation). A T wall is produced by rotation of the antiferromagnetic pattern of magnetic moments and an S wall by rotation of the direction of the magnetic moments. The T walls can be observed with polarized light, and both T‐ and S‐type domains can be studied by neutron diffraction. Crystals with simple T‐wall structures are produced by annealing at high temperature. The crystallography of the T walls can be understood in terms of a small rhombohedral deformation which accompanies the antiferromagnetic ordering. The T walls are easily displaced by the application of small mechanical stresses or magnetic fields. A crystal without T walls may still contain S walls, and these S walls also are displaced or rotated by magnetic fields.

Nuclear Magnetic Resonance Diffusion Studies of 23Na in Beta Alumina, Effect of Water on the 23Na Quadrupole Interaction

Measurements of the 23Na nuclear magnetic resonance spectra in samples of sodium beta alumina are interpreted in terms of sodium diffusion and water absorption. Intense motional narrowing is observed below room temperature consistent with the low activation energy for diffusion. The 23Na spectra from finely powdered material show effects, similar to those observed in zeolites, due to absorbed water. The quadrupole interaction decreases markedly with increasing water content indicating the penetration of water into the beta alumina structure on a molecular scale.

Antiferromagnetic Structure and Domains in Single Crystal NiO

Single crystals of NiO have been annealed and stressed at room temperature to sweep out twin boundaries. Neutron diffraction and rotational torque studies have been made on a crystal which had only 1.3% of the twin orientation. The ferromagnetic sheet structure originally proposed in which the atomic moments lie in (111) planes is found to be correct. The anisotropy in the (111) plane is extremely small and at low fields the spins in the ferromagnetic sheet are locked in domains and the susceptibility is nearly isotropic. Rotation of the spins in (111) takes place at fields above 2400 oe.

Lithium-Sodium Beta Alumina: First of a Family of Co-ionic Conductors?

Lithium-sodium beta alumina having a lithium/sodium ratio greater than about I appears to be the first generally useful lithium superionic conductor that has been reported. It exhibits strikingly nonlinear ion exchange properties and may presage the discovery of similar co-ionic interactions in other superionic conductors. The properties of lithium-sodium beta alumina are discussed in relation to current concepts of ionic interaction and distribution in the beta alumina conduction plane.

NMR study of sodium ion motion and distribution in beta‐alumina

NMR studies of 23Na in single crystal beta‐alumina have been made which include (1) investigation of the quadrupole induced shifts of the 1/2↔−1/2 magnetic transition at 77 and 103 K, and (2) measurement of the width of the 23Na‐NMR line as a function of temperature for several crystal orientations. The low temperature rotation patterns show that sodium ions occupy several independent sites in the lattice. At temperatures above about 110 K, the jumping of sodium ions among these several sites is manifested by motional narrowing of the quadrupolar spectrum. At these higher temperatures, a single, symmetric resonance line is observed. Analysis of the narrowing of this resonance line with increasing temperature yields an activation energy of 0.1 eV.

Shock-Tube Study of Luminosity in Xenon

Shock waves in xenon producing temperatures in the range 6000–11 000° K have been found to result in emission of a visible continuum after an induction period. An activation energy for continuum emission has been found to correspond to the energy of the metastable state of the xenon atom. This energy can only be associated with visible continuum emission if ground state and metastable state atoms interact to form stable diatomic molecules in an excited state which subsequently make dissociative transitions to a repulsive ground state.

Heat Generation by Electric Sparks and Rate of Heat Loss to the Spark Electrodes

In a capacitance spark the gas between the electrodes is heated almost instantaneously; subsequently, the spark‐generated heat flows from the gas to the material of the electrodes. In the present experiments sparks of 0.1 to 2 millijoules were passed between Pt electrodes in a bulb containing helium or argon or xenon, and either the pressure change, Δp, at constant volume v or the volume change, Δv, at constant pressure p was recorded, the former by means of a sensitive diaphragm and the latter by the movement of a droplet in a capillary tube attached to the bulb. The spark‐generated heat H residing in the gas at any instant was computed from the equations H=1.5vΔp and H=2.5pΔv, which apply to constant volume and pressure, respectively, and are derived from the gas law and the energy equation, using the heat capacity of monoatomic gases. From the values of H and the discharge energy corresponding to the measured capacitance and breakdown voltage, the percentage of spark‐generated heat residing in the gas ...

Observation of antiferromagnetic domains in nickel oxide by neutron diffraction topography

The antiferromagnetic T‐domain structure of a single‐crystal plate of NiO was investigated by neutron diffraction topography. Two modes of observation were used. Topographs made using nuclear reflections provide images of the domain walls because of the change in lattice distortion associated with the antiferromagnetic ordering via exchange striction; the (non)visibility of walls depends on the Bragg reflection used, and is in good agreement with calculations. On topographs made using magnetic reflections, only one type of domain is imaged at a time, yielding unambiguous identification of the magnetic arrangement in each domain.

NMR study of sodium ion motion in single crystal beta‐alumina

The motion of sodium ions in single crystals of beta‐alumina has been studied via its effects on NMR linewidth and electric quadrupole interaction. A single line is seen between −160 and +220 °C at Larmor frequencies from 4 to 19 mHz, indicating rapid ionic motion. The average coupling constant remains almost constant over the temperaure range. The average asymmetry parameter is nonzero and decreases with temperature, reflecting the change in off‐axis site population. An activation energy of about 0.17 eV has been estimated from the linewidth changes in general agreement with that found from diffusion and conductivity measurements.

On the Nature of Defects in the Magnetic Structure of Wüstite

Neutron diffraction patterns from non‐stoichiometric FeO specimens obtained at 290°K and 4.2°K show the presence of defects which consist of cation vacancies on octahedral sites and interstitial cations in tetrahedral sites. The average magnetic moment per cation site is much smaller than expected. Magnetization studies show a ferromagnetic remanence is present, and the hysteresis loop is displaced along the H axis. A model is proposed in which clusters of vacancy‐interstitial defect complexes form superparamagnetic islands in exchange contact with the antiferromagnetic matrix.