Inherent instabilities of some W5Si3-type binary phases with large electropositive metal atoms: six examples in the Ba-Pb, Ca-Sn-Mg,Cu,Zn, and La-Ga-Al,Zn systems

Ganguli, Ashok K. ; Gupta, Shalabh ; Zhao, Jing-Tae ; Alejandro Leon-Escamilla, E. ; Corbett, John D. (2005) Inherent instabilities of some W5Si3-type binary phases with large electropositive metal atoms: six examples in the Ba-Pb, Ca-Sn-Mg,Cu,Zn, and La-Ga-Al,Zn systems Journal of Solid State Chemistry, 178 (10). pp. 2959-2972. ISSN 0022-4596

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Official URL: http://www.sciencedirect.com/science/article/pii/S...

Related URL: http://dx.doi.org/10.1016/j.jssc.2005.06.044

Abstract

A particular pathology of certain W5Si3-type A5B3 structures (I4/mcm) appears to arise because of unduly close approaches of the A1-type atoms on the cell faces at ½, 0, (¼, 3/4) that occur with the larger and more electropositive A and/or in the presence of smaller B atoms. A structure refinement of binary Ba4.81Pb3 indicates such a marginal stability in that the Ba atoms in the facial Ba0.81 chains exhibit an extreme displacement ellipsoid along c̅→. Although Ca5Sn3 and La5Ga3 binaries are unknown in this structure type, five stable ternary derivatives of these have been synthesized via substitution reactions and characterized by single crystal X-ray diffraction means: Ca4Sn3.223(4)Mg0.777, Ca4Sn3Cu1.30(4), Ca4.66(6)Sn3Zn0.704(4), La4.81(1)Ga1.38(2)Al1.62, and La4.762(5)Ga1.5(1)Zn1.5. Only the Ca-Sn-Zn phase exhibits lower symmetry, P4/mbm. The problematic A1 sites exhibit diverse changes in these, whereas the surrounding B2 tetrahedra are largely unaltered. The Ca-Sn results are, respectively: direct Mg/Sn substitution at the Ca1 site; mixed fractional distribution of the smaller Cu at two sites around the A1 position with an unresolved disorder; a pair of apparently independent modes, fractional Ca in the normal position and fractional Zn rectangles thereabout. The two La-Sn phases contain normal Ga,Al (Ga,Zn) tetrahedral chains with pairs of fractional disordered La atoms along ½, 0, z. Each can be rationalized in terms of a reasonable incommensurate structure. Electronic effects may also be operable.

Item Type:Article
Source:Copyright of this article belongs to Elsevier Science.
Keywords:W5Si3 Structure; Crystal Structure; Structural Instability; Ba5Pb3 Problems; Ca5Sn3 Problems; La5Ga3 Problems
ID Code:62004
Deposited On:16 Sep 2011 03:51
Last Modified:16 Sep 2011 03:51

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