X-ray structural proof of the stoichiometry of samarium polysulfide SmS1.9 Sm10S19

The crystal structure of a samarium polysulfide single crystal, which is a phase with a composition SmS1.9 was determined (Enraf-Nonius CAD-4 automati c diffractometer, lambda MoKalpha, graphite monochromator, an absorption co rrection applied according to transmission curves and habit data, R-aniso = 0.0467 for 1963 independent I-hkl > 2 sigma(I) of 7817 I-hkl measured in t he range 2.56 less than or equal to theta less than or equal to 30.95 degre es). The crystal is tetragonal, space group P4(2)/n, a = 8796(1), c = 15.88 9(3) Angstrom, V = 1229.3(3)Angstrom(3) for the composition SmS1.9 and Z = 20, d(calc) = 5.707 g/cm(3), d(exp) = 5.71+/-0.02 g/cm(3). It is shown that the crystal is isostructural to polyselenide crystals of analogous composi tion LnSe(1.9) (Ln = La, Ce, Pr, Nd); the stoichiometric formula of the com pound is suggested to be Ln(10)X(19) (Ln = La, Ce, Pr, Na: Sm; X = S,Se). T he crystals are of PbFCl structural type (P4/nmm, a(o), c(o)) with the unit cell parameters related as (a) over bar = (a) over bar(o) + 2 (b) over bar (o), (b) over bar = -2 (a) over bar(o) + (b) over bar(o), (c) over bar = 2 (c) over bar(o) or a = b = a(o)root 5, c = 2c. The phase individuality of t he polysulfide is confirmed by the form of the P-T projection corresponding to the P-T-X diagram of the Sm-S system.