Metal ion displacements in noncentrosymmetric chalcogenides La{sub 3}Ga{sub 1.67}S{sub 7}, La{sub 3}Ag{sub 0.6}GaCh{sub 7} (Ch=S, Se), and La{sub 3}MGaSe{sub 7} (M=Zn, Cd)
- Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G2G2 (Canada)
- Department of Chemistry, Technical University of Munich, 85748 Garching b. München (Germany)
The quaternary Ga-containing chalcogenides La{sub 3}Ag{sub 0.6}GaS{sub 7}, La{sub 3}Ag{sub 0.6}GaSe{sub 7}, La{sub 3}ZnGaSe{sub 7}, and La{sub 3}CdGaSe{sub 7}, as well as the related ternary chalcogenide La{sub 3}Ga{sub 1.67}S{sub 7}, were prepared by reactions of the elements at 950 °C. They adopt noncentrosymmetric hexagonal structures (space group P6{sub 3}, Z=2) with cell parameters (a=10.2 Å, c=6.1 Å for the sulfides; a=10.6 Å, c=6.4 Å for the selenides) that are largely controlled by the geometrical requirements of one-dimensional stacks of Ga-centered tetrahedra separated by the La atoms. Among these compounds, which share the common formulation La{sub 3}M{sub 1–x}GaCh{sub 7} (M=Ga, Ag, Zn, Cd; Ch=S, Se), the M atoms occupy sites within a stacking of trigonal antiprisms formed by Ch atoms. The location of the M site varies between extremes with trigonal antiprismatic (CN6) and trigonal planar (CN3) geometry. Partial occupation of these sites and intermediate ones accounts for the considerable versatility of these structures and the occurrence of large metal displacement parameters. The site occupations can be understood in a simple way as being driven by the need to satisfy appropriate bond valence sums for both the M and Ch atoms. Band structure calculations rationalize the substoichiometry observed in the Ag-containing compounds (La{sub 3}Ag{sub 0.6}GaS{sub 7}, La{sub 3}Ag{sub 0.6}GaSe{sub 7}) as a response to overbonding. X-ray photoelectron spectroscopy supports the presence of monovalent Ag atoms in these compounds, which are not charge-balanced. - Graphical abstract: Partial occupation of metal atoms in multiple sites accounts for versatility in Ga-containing chalcogenides La{sub 3}M{sub 1–x}GaCh{sub 7} with noncentrosymmetric hexagonal structures. - Highlights: • La{sub 3}M{sub 1–x}GaCh{sub 7} (M =Ga, Ag, Zn, Cd; Ch =S, Se) adopt related hexagonal structures. • Large displacements of M atoms originate from partial occupation of multiple sites. • Bond valence sum arguments give a simple explanation for site preference. • XPS studies confirm presence of monovalent Ag in La{sub 3}Ag{sub 0.6}GaCh{sub 7}. • Substoichiometry in La{sub 3}Ag{sub 0.6}GaCh{sub 7} avoids occupation of Ag–Ch antibonding levels.
- OSTI ID:
- 22658103
- Journal Information:
- Journal of Solid State Chemistry, Vol. 243; Other Information: Copyright (c) 2016 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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