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

A particular pathology of certain W5Si3-type A(5)B(3) structures (I4/mcm) appears to arise because of unduly close approaches of the A1-type atoms on the cell faces at 1/2, 0, (1/4, 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 Ba-0.81, chains exhibit an extreme displacement ellipsoid along (c) over right arrow. 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 A 1 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 Cal site; mixed fractional distribution of the smaller Cu at two sites around the A 1 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 1/2, 0, z. Each can be rationalized in terms of a reasonable incommensurate structure. Electronic effects may also be operable. (c) 2005 Elsevier Inc. All rights reserved.