A number of lanthanide compounds LnX 2 (Ln Sc, Y, La-Lu; X S, Se, Te), LnY 2 (Y P, As, Sb), and LnZ 2 (Z Si, Ge) are known to crystallize with a structure derivable from only one aristotype, the ZrSSi type. This structure type contains square layers of silicon atoms (in general: square layers of the element of medium electronegativity). The structural deviations observed within the different compounds are usually small and principally affect this square layer. The space groups of the derivative structures under consideration are subgroups of that of the aristotype and the group-subgroup relations are outlined. Distortions of the square layers caused by electronic reasons can mostly be understood by applying simple electron counting rules. A number of compounds with chalcogen deficiency LnX 2± ±d is noted, too, in which the occurrence of vacancies in the square layers often leads to even larger superstructures.
The structure of samarium disulfide SmS 1.9, space group P4 2 /n; lattice constants a 19.717(2) A; c 15.928(1) A has been determined and refined by LS method to R 0.0345; R w 0.0419. The structure is a tenfold superstructure from the structure type ZrSSi.[S 2 ] 2± ± dumbbells with interatomic distances 2.146(1) A between S atoms were observed in the structure. Fully ordered vacancies and orientation of the [S 2 ] 2± ± dumbbells are responsible for the formation of the superstructure. The investigated sample was twinned by the reticular merohedral law, the twin symmetry is 4/mm H m H , the twinning symmetry elements are (310) and (120) mirror planes and the twin index is n 5.
The crystal structure of GdS(2-x) is determined by single-crystal X-ray diffraction as a 144-fold superstructure of the ZrSSi structure type. The superstructure is described as a two-dimensional, commensurately modulated structure with the superspace group P4/n(alphabeta 1/2)(00)(ss) and with alpha = 1/4 and beta = 1/3. Structure refinements within the classical approach, employing the 144-fold supercell, fail because most of the superlattice reflections have zero intensities within the experimental resolution. Within the superspace approach the absent superlattice reflections are systematically classified as higher-order satellite reflections. Accordingly, the superspace approach has been used to refine the structure model comprising the basic structure positions and the amplitudes of the modulation functions of the three crystallographically independent atoms. The quality of fit to the diffraction data and the values of the refined parameters are independent of the assumption on the true symmetry (incommensurate or a 12 x 12 x 2, I-centred superlattice with different symmetries). Arguments of structural plausibility then suggest that the true structure is a superstructure with space group I4, corresponding to sections of superspace given by (t1, t2) equal to [(4n - 1)/48, (4m - 3)/48] or [(4n - 3)/48, (4m - 1)/48] (n and m are integers). Analysis of the structure, employing both superspace techniques (t plots) and the supercell structure model all show that the superstructure corresponds to an ordering of vacancies and an orientational ordering of S2(2-) dimers within the square layers of the S2 atoms.
Synopsis: The magnetic alloy described in this paper is a composition of about 78.5% nickel and 21.5% iron and at magnetizing fields in the neighborhood of .04 gauss and with proper treatment has a permeability running as high as 90,000. This is about 200 times as great as the permeability of the best iron for these low magnetizing fields. This high permeability is attendant upon proper heat treatment and also upon other factors among which is freedom from elastic strain. The presence of other elements than iron or nickel and specially carbon, reduces the permeability, but slight variations in heat treatment produce large changes compared with those due to small quantities of impurities.
So far as discovered, other physical properties show no peculiarities at the composition which brings out the remarkable magnetic properties of permalloy. The equilibrium diagram, electric conductivity, crystal structure, mean spacing between adjacent atom centers and density are among the physical properties which have been studied.
To the engineer in electrical communication the development of permalloy is very significant. It assures a revolutionary change in submarine cable construction and operation and promises equally important advances in other fields.—Editor.
The new powder diffractometer installed on the beamline B2 at HASYLAB/DESY is described. It features a new design with a blocked double circle plus a third single circle which can be translated towards one another. The wide space between the two circle units conveniently holds complicated and spacious attachments and makes the instrument especially suitable for measurements under controlled environment conditions.
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