Ordered phases in Ln-S systems (Ln = Tm, Yb)

The structures of known defect ordered phases present in Ln-X systems (50-60 at.% X) are analyzed and systematized according to the concentration of cation vacancies as cubic and trigonal phases. The structures of the defect phases present in Ln-X systems (X = S, Se, Te) are modeled by ordering the cation vacancies along (111) in the basic NaCl structure (a(0)similar to5.6 Angstrom), The following structures were derived: alpha -NaFeO2, and defect structures on its basis Sc-1.37[](0.63)S-2, CaHo2Se4 (a(hex) = 2a(0)root2/4, c(hex) = 2a(0)root3; a(R) approximate to 2a(0)root6/4 alpha (R) approximate to 33 degrees), CaY2Se4 (a(hex) = 2a(0)root2/2, c(hex) = 2a(0)root3; a(R) approximate to 2a(0)root2/2, alpha (R) approximate to 60 degrees), and EuEr2Te4 (a(hex) = a(0)root6/2, c(hex) = a(0)root3). All these phases except the last one may be obtained from NaCl by using direct rhombohedral distortion, and the last phase is a superstructure of a rhombohedral cell with parameters a(R) approximate to 2a(0)root2/2, alpha (R) approximate to 60 degrees The solution-melt method (eutectic mixture 1.86NaCl-NiCl2 with Tm(Yb):S from 1:3 to 1:10) was used to synthesize polycrystalline samples containing two new trigonal defect phases of possible composition Ln(1+x)[](1-x)S-2 with cell parameters a = 3.835(7), c = 18.89(4) Angstrom and a = 3.83(3), c = 18.78(1) Angstrom (I) and also a = 7.749(7), c = 18.46(3) Angstrom and a = 774(1), c = 18.41(3) Angstrom (II) in Tm-S and Yb-S systems, respectively. It is maintained that the crystal structure of phase I with x similar to 0.37 (space group R(3) over bar m) is derived from the NaCl structure and is close to Sc-1.37[](0.63)S-2 and the crystal structure of phase II with x similar to 0.34 (space group P3cl) is derived from the NiAs structure. Possible ordering of cations with different formal charges accompanied by variation of symmetry is discussed.