Search Results - (Author, Cooperation:K. S. Knight)

2011-02-12

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-06-20

Cold Spring Harbor Laboratory Press

0890-9369

Biology

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] It has been recognized for some time that non-equilibrium cation ordering in minerals might provide a means to measure rock cooling rates, from early work on Mg/Fe ordering in amphi-boles6 and extensive studies of pyroxenes7'8 to more recent analysis of Al/Si ordering in alkali feldspars9. ...

Electronic Resource

1573-4803

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract With a view to the technological applications of ZnWO4 crystals, we have performed Rietveld profile refinement of medium-resolution, time-of-flight, neutron powder diffraction data of synthetic ZnWO4 to improve the quality of the current crystal structure derived from single crystal X-ray techniques that produced a final solution with an R factorof only 10%. The new structural data for ZnWO4, monoclinic space group P2/c, lattice parameters of 0.469263(5), 0.572129(7), 0.492805(5) nm for a, b and c respectively, and a β angle of 90.6321(9) ° with two formula units per unit cell. The ZnO6 octahedra contain 3 pairs of Zn-O bonds of 0.2026(2), 0.2090(2) and 0.2227(3) nm and the WO6 octahedra contain 3 pairs of W-O bonds 0.1789(2), 0.1914(2) and 0.2133(3) nm. These new data confirm the basic structure of ZnWO4 and provide accurate off-centring magnitudes for the Zn and W cations.

Electronic Resource

1432-2021

Key words gypsum ; thermal expansion tensor ; Rietveld analysis ; neutron powder diffraction

Springer Online Journal Archives 1860-2000

Chemistry and Pharmacology

Geosciences

Physics

Abstract The unit cell parameters, extracted from Rietveld analysis of neutron powder diffraction data collected between 4.2 K and 320 K, have been used to calculate the temperature evolution of the thermal expansion tensor for gypsum for 50 ≤ T ≤ 320 K. At 300 K the magnitudes of the principal axes are α 11  = 1.2(6) × 10−6 K−1, α 22  = 36.82(1) × 10−6 K−1 and α 33  = 25.1(5) × 10−6 K−1. The maximum axis, α 22 , is parallel to b, and using Institution of Radio Engineers (IRE) convention for the tensor orthonormal basis, the axes α 11 and α 33 have directions equal to (−0.979, 0, 0.201) and (0.201, 0, 0.979) respectively. The orientation and temperature dependent behaviour of the thermal expansion tensor is related to the crystal structure in the I2/a setting.

Electronic Resource

1432-2021

Key words Olivine ; Order/disorder ; High temperature cation partitioning ; Neutron diffraction (powder)

Springer Online Journal Archives 1860-2000

Chemistry and Pharmacology

Geosciences

Physics

Abstract  The partitioning of Fe and Mg between the M1 and M2 octahedral sites of olivine has been investigated by in situ time-of-flight neutron powder diffraction. The degree of M-cation order was determined from direct measurements of site occupancies in a synthetic sample of Fo50Fa50 heated to 1250 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site preferences cross over (T cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering behaviour is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that vibrational entropy, crystal field effects and changes in bond characteristics play a part in the cross-over of partitioning behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering of the type ΔG = −hQ + gTQ +   (T − T c)Q 2 +   Q 4, with a/h = 0.00406 K−1, b/h = 2.3, T c = 572 K and g/h = 0.00106 K−1. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite join will have a bearing on the activity-composition relations of this important rock-forming mineral, and indicate that Fe-Mg olivine solid solutions become less ideal as temperature increases.

Electronic Resource

0044-2313

Lithium permanganate, Li[MnO4] ; preparation ; crystal structure ; neutron powder data ; MAPLE calculation ; Chemistry ; Inorganic Chemistry

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

Coordination Number 4 or 6 for Lithium?: The Crystal Structure of Lithium Permanganate, Li[MnO4]Waterless 7Li[MnO4] was obtained by dehydration of 7Li[MnO4] · 3 D2O at 60°C above P4O10 in vacuum as violet powder.The structure determination by neutron diffraction measurements via pulsed polychromatic neutron source [time-of-flight powder diffractometer; profile-R-value = 2.1%, Bragg-R-value = 7.4%] confirms the space group Cmcm with a = 551.402(8), b = 839.754(15), c = 635.911(10) pm (Z = 4); parameters see text. The crystal structure of Li[MnO4] corresponds to the CrVO4-type with coordination number 6 for lithium and 4 for manganese.The Madelung Part of Lattice Energie, MAPLE, the Effective Coordination Numbers, ECoN and the charge distribution were calculated and discussed.

Durch Trocknung von 7Li[MnO4] · 3 D2O bei 60°C über P4O10 im Vakuum wurde wasserfreies 7Li[MnO4] als violettes Pulver erhalten.Die Strukturaufklärung mittels Neutronenbeugungsmessungen im gepulsten polychromatischen Neutronenstrahl [Flugzeitdiffraktometer; Profil-R-Wert = 2,1%, Bragg-R-Wert = 7,4%] belegt die Raumgruppe Cmcm mit a = 551,402(8), b = 839,754(15), c = 635,911(10)pm (Z = 4); Parameter siehe Text. Die Kristallstruktur von Li[MnO4] entspricht dem CrVO4-Typ mit der Koordinationszahl 6 für Lithium und 4 für Mangan.Der Madelunganteil der Gitterenergie, MAPLE, Effektive Koordinationszahlen, ECoN und die Ladungsverteilung wurden berechnet und diskutiert.

4 Ill.

Electronic Resource