No abstract

No abstract

Specific heat, thermal expansion and electrical resistivity measurements on PrB6 single crystals show that there are two low temperature phase transitions at 6. 9 K and 4. 2 K, respectively, the latter temperature varying somewhat among different crystals.Neutron diffraction measurements were made on both single and polycrystalline samples of PrB6. The neutron data indicate a spontaneous incommensurate magnetic ordering at 6. 9 K with 0 = (0. 23, 0. 23, 0. 5) 21T/a0. At 4. 2 K a commensurate magnetic phase is seen with 0 (0. 25, 0. 25, 0. 5) ZTT/a0 coexisting with the incommensurate phase At 1.74 1<, only the commensurate phase remains.A model is proposed for the commensurate antiferromagnetic structure and a profile analysis based on that model yields a magnetic moment of 1.77 Bohr magnetons per praseodymium ion at 1.74 K.

No abstract

X-ray diffraction patterns were obtained, using improved technique, for liquid sulfur at 80°C (supercooled), 120°C, 165°C, 200°C, 240°C, and 300°C; and for amorphous sulfur at 4°C and for powdered crystalline orthorhombic sulfur at 23°C, confirming reported new details at small angles and extending observations to nearly twice the range of (sinθ)/λ heretofore reported. Atomic distribution curves are presented for all these cases. Nearest neighbor distances and numbers remain constant throughout at 2.07 A and 2 atoms, respectively, but other features of the distribution curves change with temperature. New results with neutron diffraction at 120°C are compared with these x-ray results.

A linear position-sensitive detector has been constructed with an array of three proportional counters and installed on the neutron powder diffractometer at the University of Missouri Research Reactor. The event position is determined by the charge division method using digital arithmetic. Consideration has been given to the corrections necessary to account for the differences in detection efficiency, solid angle, dead time and resolution for different points along the detector, caused by the finite thickness and tangential orientation of the detectors. An important feature of the instrument is an oscillating radial collimator with high transmission placed between the sample and detector which enables the system to have low background and a high signal-to-noise ratio. By considering the angular rate of accumulating data, the peak intensity per channel and the resolution of a Bragg diffraction peak, the enchancement factor for the positionsensitive detector is estimated to be about 50 compared to a single detector with a step scan. This allows experiments to be performed with better statistics, better resolution and smaller samples, and enables marginal experiments to be worthwhile. A number of structures have been refined with this detector using Rietveld analysis including a new rareearth-transition-metal phase whose structure has been solved from the powder diffraction data.

The characteristic temperatures and thermal expansion coefficients of MgO, CaO, MgS, CaS, and BaTe have been measured by observing Bragg reflections as functions of temperature in the temperature range from 100° to about 1000°K. The compounds selected were those for which the simple cubic approximation should be most applicable, and interpretation of the data was based upon the Debye approximation for a solid. For at least one of these materials, evidence was found at high temperatures for a temperature dependence of ΘM nearly equal to that predicted by the explicit dependence of ΘM on volume, while at low temperatures ΘM was a much more rapid function of temperature. For MgO the absolute value of ΘM was found to agree fairly well with the published specific heat values, after correction for the difference in averaging over the vibrational spectrum, although it was found to be somewhat larger than a value obtained from earlier x-ray measurements. Expansion coefficients for MgO and CaO were in agreement with other determinations. Neither characteristic temperatures nor expansion coefficients were found in the literature for the other materials.

No abstract