The possible existence of compounds of bivalent or tervalent thorium has some current tpterest in relation to the position of thorium in the Periodic System, and the so-called Such compounds might indicate whether unutilised valency electrons are disposed in the cerium-like (5f) or the zirconium-hafnium-like ( 6 4 configuration.Thorium di-and tri-iodide have now been prepared and found to be deeply coloured compounds, analogous in every respect to the lower zirconium and hafnium iodides. They are formed by reduction of the tetraiodide with varying quantities of metallic thorium a t 450-550".The tri-iodide is perceptibly volatile ; it undergoes reversible dissociation t o tetra-and di-iodide

The tetrafluorides of uranium and plutonium form two hydrates, the higher one having 2-5 molecules of water per metal atom and the lower one 2.0 molecules of water or less. Methods for their formation have been investigated.The lower hydrate is pseudo-cubic, a = 5.69 kX for the uranium and 5.63 kX for the plutonium compound, and the proposed structure is similar to that of uranium dioxide, the water molecules occupying vacant uranium sites and being hydrogen-bonded to the fluorine. Very little change of cell dimension was observed over the range UF4,2H,0 to UF4,0.75H,0.The fluorination, at temperatures below loo", of the dioxides with gaseous hydrogen fluoride gives the lower fluoride hydrate and a second product which may be M(OH)F,,HF.

Thermal, metallographic, X-ray, and chemical analysis has unambiguously identified four phases in this binary system : (i) ThSe, which exists over a narrow solid-solution range, is face-centred cubic with a = 5.863 & 0-002 X.U. and has the NaC1-type structure ; (ii) Th,Se,, which again has a narrow solidsolution range, has an X-ray pattern which can be indexed in terms of an orthorhombic cell with a = 11-32 & 0.05, b = 11.55 f 0-05, and c = 4.26 f 0.01 X.U., and four molecules per unit cell, the extinctions being compatible with the Sb,S,-type structure; (iii) Th,Se,, also gave a complex pattern which was indexed as a hexagonal cell with a = 11-56, c = 4-35 f 0.05 X.U. ; (iv) ThSe,, which loses selenium when heated above lOOO", being degraded to ThSe,. ,, has a structure compatible with the orthorhombic PbC1,-type with a = 4-98, b = 7-50, c = 9.38 X.U. (all &O-Ol). A tentative phase diagram is given for the composition range ThSe,,.,.Above ThSe, the further compound Th,Se, has been identified by degradation experiments.The thorium used contained up to 0.5% of oxygen (Z 5% of Tho,).Consequently, ThOSe is found as a contaminant in the system. It has little effect on the phase diagram for, apart from the range ThSe,.,-ThSe,.,, where melting points were above 1660°, it occurred as a discrete phase.ThOSe is tetragonal, with a = 4.030 5 0.005, c = 7-005 f 0.005 X.U., and has two molecules per unit cell. The space group is DZ, -P4/nmm (PbFC1type structure) with the oxygen atoms on 2(a)-and Th and Se atoms on 2(c)-fold special positions. The variable parameters for Th and Se, determined by comparison of calculated and observed intensities, are ZTh = 0.18 f 0.005 and zSe = 0-63 & 0.01.THE recent research on the chemistry of the thorium-sulphur system by Eastman,

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By R. W. M. D'EYE.The crystal structures of ThSe, and Th,Se,, have been determined from powder photographs obtained by use of Guinier-type focusing cameras. ThSe, is orthorhombic with a = 4.411 & 0-002 kX, b = 7-595 5 0-002 kX, c = 9.046 f 0-002 kX. There are 4 molecules per unit cell and the space-group is Diz-Pmnb. Th,Se,, is hexagonal with a = 11-546 j--0.006 k X c = 4-22 f 0.01 k X . There is one molecule per unit cell and the space-group is CZh-PS3/m. Both compounds are thus isostructural with their corresponding sulphides.IN a recent report (D'Eye, Sellman, and Murray, J., 1952, 2555) of an investigation of the Th-Se system the compounds ThSe, and Th,Se,, were described. However, owing to the poor powder-diffraction data obtained from Debye-Schemer photographs it was then not possible to determine unambiguously the crystal structures of these two compounds. Through the courtesy of Professor G. Hagg in allowing me to work at his Institute in Uppsala University, Sweden, it has been possible to obtain good diffraction data by use of Guinier-type focusing cameras which necessarily eliminate the spacing errors caused by absorption in the specimen. It was tentatively suggested in the previous communication that Th,Se,, and ThSe, had hexagonal and orthorhombic lattices, respectively, and that they were isostructural with their corresponding sulphides. This has now been established. EXPERIMENTAL X-Ray photographs were obtained by using Guinier-type focusing cameras with a bent quartz-crystal monochromator and Cu-Ka radiation. The intensities of the resulting diffraction lines were visually estimated. The expression for the intensity of a diffraction line on a Guiniertype photograph is I,ocA ,A2pF2 ( 1 + c 0 s ~2 8 ~ cos2 28)/sin28 cos8, where I,, 8, Om, A A,, p , and