James W. Richardson

The reversible transition of the molecular sieve VPI-5 into ALPO−84 and the structure of ALPO−84

In this letter the authors describe the reversibility of the recrystallization of the aluminophosphate molecular sieve VPI-5 to ALPO{sub 4}{sup {minus}8}, and the structure of ALPO{sub 4}{sup {minus}8}. VPI-5 is known to contain one-dimensional circular pores circumscribed by 18 tetrahedral atoms. Upon heating in the presence of moisture it is known to recrystallize to ALPO{sub 4}{sup {minus}8}. This transition now proves to be reversible. Although phase transitions of aluminophosphate molecular sieves have been described, this is the first report of such a transition being reversible. ALPO{sub 4}{sup {minus}8} is found to contain one-dimensional elliptical pores, with free diameters of 7.5 to 9.5 {angstrom} circumscribed by 14 tetrahedral atoms. This new pore system thus contains one of the largest openings described so far, second only to VPI-5.

Crystal structure of (LixK1 − x)2CO3 (x = 0, 0.43, 0.5, 0.62, 1) by neutron powder diffraction analysis

Abstract The crystal structures of (LixK1 − x)2CO3 (x = 0, 0.43, 0.5, 0.62, 1) were determined by neutron powder diffraction at room temperature. These samples were obtained by a melt and quench process. (LixK1 − x)2CO3 (x = 0, 0.5, 1) formed as single phases and (LixK1 − x2CO3 (x = 0.43, 0.62) contained two phases at room temperature. In the Rietveld refinements, the improvement from 7Li enrichment is due to the lower absorption coefficient of 7Li than of 6Li. A final weighted R-factor of 4.54% was obtained for the refinement of 2373 reflections by the Rietveld method from a (Li0.5K0.5)2CO3 synthesized using 7Li2CO3 and K2CO3 (99.9% pure). Slight distortion of the CO32− units in the monoclinic cell was observed; the O(1)ue5f8Cue5f8O(2) angle and Cue5f8O(3) length are larger than those for the other Cue5f8O bonds and Oue5f8Cue5f8O angles. These local structure characteristics can be explained by the difference in the ionic size of Li+ and K+ and the different electrostatic interactions between the cations and CO32− units.

Formation of the Fe23Zr6 phase in an Fe-Zr alloy

Abstract This study confirms the existence of Fe 23 Zr 6 in an Fe-9.8 at.% Zr alloy. This phase forms from the α-Fe + Fe 2 Zr eutectic during high temperature annealing. Previously, Fe 23 Zr 6 formation was explained by simple diffusion of iron atoms into the Laves intermetallic; however, this mechanism does not explain the presence of α-Zr within the Fe 2 Zr 6 phase. We have proposed a two-step mechanism for Fe 23 Zr 6 formation: Fe 2 Zr decomposes into Fe 23 Zr 6 + α-Zr, and α-Zr reacts with α-Fe to form more Fe 23 Zr 6 . The slow kinetics of Fe 23 Zr 6 formation is due to its dependence on atomic diffusion through the intermetallic phases.

Correcting time-of-flight neutron diffraction data for chopped delayed neutron backgrounds: the action of a drum chopper in a pulsed neutron beam

Abstract Delayed neutron choppers in pulsed source neutron beams serve to reduce the background caused by delayed neutrons in pulsed source instruments. We analyze the effect of a drum chopper placed in an incident pulsed beam which contains delayed neutrons and compute its influence on the detector counting rate. Expressions are found for the time and wavelength dependence of the counting rates for both prompt and delayed neutrons, in both the monitor and the scattered neutron detectors. On the basis of these results, an exact method for determining the delayed neutron background is suggested for use in measuring and correcting pulsed-source diffractometer data, by operating the chopper in random phase with respect to the source. Other approximate methods are investigated. We report measurements which confirm the analysis.