Order and disorder in rocksalt and spinel structures in the MgS-Yb ₂S₃ system

The MgS-Yb₂S₃ solid solution, which can be formulated as Mg_1-xYb_(2/3)x□_(1/3)xS (□, cation vacancy), was studied in a wide composition range (0 ≤ x ≤ 0.75). The structure and microstructure characterizations of the samples were mainly performed via transmission electron microscopy (TEM) and associated techniques, such as X-ray energy dispersive spectroscopy (XEDS) and electron energy-loss spectroscopy (EELS). At low Yb³⁺ contents (x ≤ 0.30), the system presents an average NaCl-type structure. The selected area electron diffraction (SAED) patterns corresponding to these crystals show diffuse scattered intensity, which is related to short-range order in the cation sublattice. EELS studies indicate the existence of Mg²⁺ in tetrahedral coordination, in addition to the octahedral coordination of the basic NaCl-type structure. Extended defects were observed perpendicular to 〈111〉 in the NaCl matrix for the x = 0.30 sample. When x ≥ 0.35, a phase with spinel-type structure is observed and characterized. Besides, different defects with different structure were also observed perpendicular to 〈111〉 in the spinel matrix for 0.35 ≤ x ≤ 0.45 samples.