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  • Author: C. Calvo x
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Abstract

The crystal structure of Ca3(VO4)2 is rhombohedral with space group R3c and unit-cell parameters a = 14.124(1) Å and α = 44.96(2)°, with equivalent hexagonal axis lengths of a = 10.809(1) and c = 38.028(9) Å. The rhombohedral cell contains seven formula units. The structure was refined by full-matrix least-squares utilizing 1252 symmetry-independent reflections to a final R value of 0.049. One formula unit lies on the threefold axis with one of the calcium-ion sites on this axis half-occupied and another site octahedrally coordinated to oxygen atoms. The remaining calcium ions are coordinated to 8, 7 and 8 oxygen atoms with Ca–O bond lengths of 2.525, 2.442 and 2.570 Å respectively. The mean V–O bond length for the three orystallographically independent VO4 groups are within 0.01 Å of 1.69 Å. The structure is related to that of Ba3(VO4)2 but with a formula-unit density 7/8 as large, due to the loss of one formula unit per cell from chains along the threefold axis. This change in structure follows from the smaller average cation-oxygen bond length in the calcium salt. R values as low as 0.041 can be obtained with models that allow for differences in the environment of the half-occupied cation site, presumably dependent upon whether it is filled or empty.

Abstract

The trigonal form of selenourea has been examined by x-ray crystallographic methods both at room temperature and at −100°C. The lattice parameters reported by earlier workers have been refined by least squares to a = 15.285(5) Å, c = 13.007(5) Å at 20°C and a = 15.201(5) Å, c = 12.950(5) Å at −100°C. The space group has been confirmed to be P31 or P32, which requires nine crystallographically distinct SeC(NH2)2 molecules. The crystal structure was solved using Patterson techniques and difference Fourier syntheses, and was refined by least squares using three-dimensional data obtained at −100°C. The unit cell contains nine three-fold spiral chains which run parallel to c. The similarity of the crystal structure to the urea- and thiourea-hydrocarbon complexes in discussed. Evidence is presented of N–H⋯Se hydrogen bonds, supporting their recent discovery in chemically related compounds.

SUMMARIES

This study aims at evaluating the antifungal activity of three different products commonly used in Archives to prevent damage to documents by microbiological attack. Two commercial products were tested against a mixture of alcohol and water. From our results we assessed that greater effectiveness was reached using a solution of water:alcohol in the proportion 30:70. The advisable application method would be, as far as possible, a direct treatment over the substrate.

Abstract

The Nuclear and Energy Research Institute has produced, since 1998, iridium-192 wires used in low dose rate brachytherapy. In the paper the authors studied the influence of wire profile on the homogeneity distribution of radioactivity of iridium-192 along the wire. The authors propose the improvements in the quality control procedure that will provide more accurate measurement data and suggest changes in control devices.