GdPS and Related New Rare Earth Compounds

Abstract

THE occurrence of such PbFCl-type phases as ThAs₂, ThSb₂ and UP₂ led me to suspect that analogous rare earth compounds LnYX (X = chalcogen, Y = pnigogen) might also exist. I tried to synthesize a number of these hypothetical compounds either by reacting the elements or by sintering pressed mixtures LnY + X at temperatures of between 600° and 1,000° C. PbFCl-type phases were obtained for LnSbTe (Ln = La … Sm, Gd … Tm, Y). The samples, however, appeared to be antimony-deficient. Sb- and As-deficient samples of LaAsSe, LnSbS (Ln = La … Nd) and LaSbSe also showed the X-ray pattern characteristic of the PbFCl structure. The compounds LnYX with the lighter anions, however, revealed slightly different X-ray patterns. The symmetry of the tetragonal PbFCl cell is lowered to orthorhombic because of a small difference between the a and b axes. The lattice constants of GdAsSe are: a = 3.99 Å, b = 3.95 Å, c = 8.77 Å. Knowledge of the the electrical conductivity might give information regarding the distortion of the lattice; thus non-metallic properties are possible only if two electrons per formula unit are involved in anion–anion bonds and these bonds would give rise to specific distortions. Undistorted PbFCl-type LnYX compounds must be metallic. Unfortunately, the quality of the samples was not high enough to decide unequivocally whether these phases, if stoichiometric, are semiconductors or metals. Seebeck coefficients are of the order of 10–30 μV/°C and the resistivities are rather low. GdAsSe is antiferromagnetic with a Néel temperature of 10° K.