Allen C. Larson

Magnetic and crystallographic order in α‐manganese

We have made time‐of‐flight neutron diffraction measurements on α‐manganese metal. Powder diffraction measurements were made at 14 temperatures between 15 and 305 K, and single crystal measurements were made at 15 and 300 K. We found that the crystal structure of α‐Mn is tetragonal below its Neel point of 100 K, with crystal symmetry I42m and magnetic (Shubnikov) symmetry PI421c. In agreement with the earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight changes in the atomic positions and in the average atomic elastic constant.We have made time‐of‐flight neutron diffraction measurements on α‐manganese metal. Powder diffraction measurements were made at 14 temperatures between 15 and 305 K, and single crystal measurements were made at 15 and 300 K. We found that the crystal structure of α‐Mn is tetragonal below its Neel point of 100 K, with crystal symmetry I42m and magnetic (Shubnikov) symmetry PI421c. In agreement with the earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight changes in the atomic positions and in the average atomic elastic constant.

A neutron scattering investigation of cubic stabilised zirconia (CSZ). I: The average structure of Y-CSZ

Abstract Single crystal neutron diffraction has been used to study the relaxations of the O-atom from its ideal fluorite position in yttria stabilised cubic zirconia (Y-CSZ). The samples used in this study cover a compositional range of 9.5–24 mol% Y 2 O 3 ZrO 2 . We find that the O-atom is predominantly displaced in the 〈100〉 direction and to a lesser extent in the 〈111〉, while appreciable amounts remain at the ideal fluorite site. Further the average ZrO distances decrease with Y 2 O 3 content to values similar to that of tetragonal zirconia for compositions close to the cubic-tetragonal zirconia phase boundary. The controversy in the literature over the direction of relaxation of the O-atom is resolved with the aid of probability density functions. We demonstrate that there is more than one way to describe the distribution of the O-atom about its ideal site. A probability maximum is always produced at the ideal site, while probability density in the 〈100〉 directions is strong.

Crystal structure refinements with generalized scattering factors. III. Refinement of 1, 1′‐azobiscarbamide and melamine, 2,4,6‐triamino‐s‐ triazine, at the octopole level

Single crystal x‐ray diffraction data for 1,1′‐azobiscarbamide, C2H4N4O2, and melamine (2,4,6‐triamino‐s‐triazine), C3N6H6, have been refined using one‐center generalized x‐ray scattering factors through the octopole level. This work is an extension of previous work in which the crystal structures of these compounds were refined using one‐center dipole and quadrupole population parameters and two‐center monopole paramaters to account for the bonding electron density. The one‐center octopole model accounts for bonding electron density more efficiently than the two‐center model. Radial parameters in the Slater‐type functions used to compute scattering factors were also refined. In both compounds, radial parameters about 7% smaller than molecular optimized values were found. Interatomic distances are not very sensitive to the model. In melamine, all C–N distances increase by about 0.004 A for models with octopoles. Also, if octopoles are used on the nitrogen atoms, all N–H distances become approximately equa...

Temperature dependence of the structure of YBa2Cu3O7: The role of the axial oxygens

Abstract It is now generally accepted that the CuO 2 planes in YBa 2 Cu 3 O 7 are responsible for carrying the supercurrents while the CuO chains provide a charge reservoir. The role of the axial oxygens in coupling the two is less clear. Within the last year there have been suggestions from both experiment and theory for the existence of temperature dependent structural anomalies at and below T c , but their nature has not been clearly elucidated. Using Rietveld refinement of structural models from pulsed-neutron powder-diffraction data, we have performed a careful study of the temperature dependence of the structure of YBa 2 Cu 3 O 7 in order to search for possible anomalies involving the axial oxygens in the vicinity of the transition temperature. We find that the axial oxygen gradually moves closer to the CuO 2 planes as the temperature is lowered from 305 to 13 K, indicating a small redistribution in electronic charge from the planes to the chains; however, we find no evidence for anomalous changes in bond lengths involving the axial oxygens near T c , the existence of a double-well potential for the axial oxygens, or evidence that these bonds become either more harmonic or softer as the sample temperature is lowered past T c .

SELF-CONSISTENT-FIELD CALCULATIONS FOR THE TRANSLAWRENCIUM ELEMENTS: BEGINNING OF A 5g TRANSITION SERIES.

The preferred electronic configurations of the very heavy synthetic elements in the vicinity of atomic number 126 have been deduced through nonrelativistic self‐consistent Hartree calculations made for many configurations. The resulting wavefunctions were used to compute the total average Hartree–Fock energy of each configuration. The configuration 8s26f15gν emerges as more stable than either 8s27d15gν or 8s25gν+1. The principal chemical valences are 3+ and 2+; oxidation to 4+ would probably be difficult.

Magnetic phases in UNiGe

Original measurements of 4.2 K magnetization curves and temperature dependence of magnetic susceptibility were completed by an extended study of magnetization, specific heat, and electric resistivity as a function of temperature and magnetic field. Neutron diffraction experiments were also performed on powder and single crystal. The specific heat shows a sharp peak at 41.5 K, corresponding to the first-order phase transition, and a broad anomaly around 50 K. The magnetic structures in UNiGe confirm that the strong bonding of 5f orbitals along the a axis leads to a huge magnetic anisotropy with U magnetic moments perpendicular to this direction.

Rietveld refinement of magnetic structures from pulsed-neutron-source powder-diffraction data

The General Structure Analysis System, GSAS, has recently been modified to include magnetic neutron-scattering cross-sections. Low-temperature diffraction data have been taken on the hexagonal noncollinear antiferromagnet UPdSn on both the HIPD and the NPD powder diffractometers at LANSCE. The low-resolution data reveal that the magnetic structure has orthorhombic symmetry between 23 K and 37 K, and monoclinic symmetry below 23 K. The high-resolution data reveal that there are structural distortions with corresponding symmetry changes in each of these phases, while the paramagnetic phase above 37 K remains hexagonal. Using GSAS, we have refined data sets from both diffractometers simultaneously, including both magnetic and structural cross-sections. The magnetic results from Rietveld refinement are in good agreement with model fitting to the integrated intensities of seven independent magnetic reflections and these, in turn, agree with measurements made on the same sample using the constant-wavelength reactor technique.

Crystallographic and Magnetic-Properties of UPdSn

A single crystal of the intermetallic compound UPdSn has been studied by means of neutron diffraction using the white‐beam Laue diffractometer (single‐crystal diffractometer) at the Los Alamos spallation neutron source. Data were taken in the paramagnetic phase at 55 K and just below the Neel point at 37 K. The crystallographic data refine well within the space group P63mc and confirm the results obtained previously on powders, namely that the Pd and Sn atoms are ordered. At low temperature, UPdSn undergoes two phase transitions at 40 and 25 K to complicated noncollinear antiferromagnetic structures. All three pairs of magnetic domains are observed, in roughly equal populations, in the intermediate‐temperature orthorhombic magnetic structure (phase I). A uranium moment of 0.863μB and canting angles φ=56.51° and θ=21.42° were obtained, in reasonable agreement with previous powder neutron data for this temperature. The observed form factor yields an orbital‐to‐spin moment ratio −μL/μS=2.6 which is character...

Crystal structure refinements with generalized scattering factors. I. 1,1′‐Azobiscarbamide

1,1′‐Azobiscarbamide, C2H4N4O2, is monoclinic, space group P21/c, with a = 3.593, b = 9.095, c = 7.635, β = 113°15′, and Z = 2. Data were collected with an automatic diffractometer using crystal monochromated MoK α radiation. The usual spherical free atom model with anisotropic thermal parameters was used in making the least‐squares refinement. In addition, refinements were made with generalized scattering factors, and the scalar, dipole, and quadrupole one‐center terms and scalar two‐center terms were successively added. The aspherical distortions found are reasonable and produce a significant improvement between measured and calculated intensities. No significant difference in bond distances was observed.

CIF applications. XIV. Reporting of Rietveld results using pdCIF: GSAS2CIF

A discussion of the process of creating powder diffraction CIF documents (pdCIF) from Rietveld results is presented, with particular focus on the computer program GSAS2CIF. The data structures used within GSAS2CIF are described, as well as how the program implements template files for descriptive information. Two graphical user interface utilities are also discussed.