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,
