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 (ao~5.6 A). The following structures were derived: α-NaFeO₂ and defect structures on its basis Sc_I.37[]_0.62S₂, CaHo₂Se₄ (a_hex = 2a₀√2/4, Chex = 2a₀√3 a_R = 2a₀√6/4, α_R = 33?), CaY₂Se₄ ( a_hex = 2a₀√2/2, c_hex = 2a₀√3: a_R = 2a₀√2/2, α_R = 60?), and EuEr₂Te₄ ( a_hex = a₀√6/2, c_hex = a₀√3 ). 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 ~ 2a₀√2/2, α_R ~ 60?. The solutionmelt method (eutectic mixture 1.86NaCl-NiCl₂ with Tm(Yb):S from 1:3 to 1:10) was used to synthesize poly crystalline samples containing two new trigonal defect phases of possible composition Ln_1+x+[]_1-xS² with cell parameters a = 3.835(7), c = 18.89(4) Å and a = 3.83(3), c= 18.78(1) A (I) and also a = 7.749(7), c = 18.46(3) Å and a = 7.74(1), c = 18.41(3) A (II) in Tm-S and Yb-S systems, respectively. It is maintained that the crystal structure of phase I with x ~ 0.37 (space group R3m) is derived from the NaCl structure and is close to Sc_1.37[]_0.63S₂ and the crystal structure of phase II with x~0.34 (space group P3cl) is derived from the Ni As structure. Possible ordering of cations with different formal charges accompanied by variation of symmetry is discussed.