Order and Disorder in Rocksalt and Spinel Structures in the MgS-Yb 2 S 3 System Esteban Urones-Garrote,* ,² Adria ´n Go ´mez-Herrero, ‡ A Ä ngel R. Landa-Ca ´novas, § Ray L. Withers, | and L. Carlos Otero-Dı ´az ²,‡ Departamento de Quı ´mica Inorga ´ nica, Facultad de Ciencias Quı ´micas, UniVersidad Complutense de Madrid, E-28040, Madrid, Spain, Centro de Microscopı ´a, UniVersidad Complutense de Madrid, E-28040, Madrid, Spain, Inst. Ciencia de Materiales de Madrid, CSIC, E-28049, Madrid, Spain, and Research School of Chemistry, Australian National UniVersity, 02000, Canberra, Australia ReceiVed March 16, 2005. ReVised Manuscript ReceiVed April 22, 2005 The MgS-Yb 2 S 3 solid solution, which can be formulated as Mg 1-x Yb (2/3)x 0 (1/3)x S(0, cation vacancy), was studied in a wide composition range (0 e x e 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 Yb 3+ contents (x e 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 Mg 2+ 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 g 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 e x e 0.45 samples. Introduction In solid state chemistry, smooth variations of stoichiometry can take place according to different mechanisms, involving extended defects, long-range or short-range ordering, and/ or structural modulations. 1 In this work we present an interesting case of nonstoichiometry accommodation and structural flexibility. 2 The formation of MS-RE 2 S 3 (M, divalent metal; RE, rare earth element) solid solutions is well documented. 3,4 In the case that M ) Mg, Mn, and Ca, extensive NaCl-type solid solutions are generated for several rare earth elements. 5 When M ) Ca, these solutions exist for heavy lanthanides (from Dy to Lu). X-ray studies on the CaS-Y 2 S 3 system 6,7 showed the existence of a wide nonstoichiometric solid solution region with NaCl-type structure, and the variation of parameters with Y concentration was not linear. Transmission electron microscopy (TEM) studies on this system 8 allowed observation of diffuse scattered intensity in the selected area electron diffraction (SAED) patterns, as well as additional reflections at high Y contents, which indicated the presence of a rhombohedral NaCl-derived superstructure. In the CaS- Yb 2 S 3 system, a cubic superstructure has been reported, 9 with a s ) 2a NaCl , when increasing the Yb concentration. The ordering of the Ca and Yb cations in the {111} planes accounts for this 2-fold superstructure. Besides, this system has been further studied due to its possible application as an ecological inorganic pigment for plastics. 10 When M ) Mg, the NaCl-type solid solutions are extensive, especially for heavy lanthanides and for Y. 5 Besides, spinel-type compounds MgRE 2 S 4 were reported for RE ) Tm, Yb, Lu and Sc, 11 and they have been characterized only by means of X-ray powder diffraction (XRPD) data. A high-pressure (55 kbar) transition to Th 3 P 4 -type structure was reported by Hirota et al. 12 in these compounds. The introduction of trivalent metal cations in a MS matrix generates cation vacancies according to the following charge balance: Thus, cation-deficient solid solutions with NaCl-type struc- * Corresponding author. Tel: +34 913944995. Fax: +34 913944352. E-mail: esteban@brunilda.sme.ucm.es. ² Departamento de Quı ´mica Inorga ´nica, Universidad Complutense de Madrid. ‡ Centro de Microscopı ´a, Universidad Complutense de Madrid. § Inst. Ciencia de Materiales de Madrid, CSIC. | Australian National University. (1) Hyde, B. G.; Andersson, S. Inorganic Crystal Structures; Wiley: New York, 1989. (2) Withers, R. L. Prog. Solid State Chem. 1998, 26, 1. (3) Flahaut, J. Sulfides, Selenides and Tellurides. In Handbook on the Physics and Chemistry of Rare Earths; Gschneider, K. L., Eyring, L., Eds.; North-Holland: Amsterdam, 1979; vol. 4. (4) Burdett, J. K., Mitchell, J. F. Prog. 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