Inherent instabilities of some W 5Si 3-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

Abstract

A particular pathology of certain W 5Si 3-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 Ba 4.81Pb 3 indicates such a marginal stability in that the Ba atoms in the facial Ba 0.81 chains exhibit an extreme displacement ellipsoid along c ⇒ . Although Ca 5Sn 3 and La 5Ga 3 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: Ca 4Sn 3.223(4)Mg 0.777, Ca 4Sn 3Cu 1.30(4), Ca 4.66(6)Sn 3Zn 0.704(4), La 4.81(1)Ga 1.38(2)Al 1.62, and La 4.762(5)Ga 1.5(1)Zn 1.5. 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 1 2 , 0, z. Each can be rationalized in terms of a reasonable incommensurate structure. Electronic effects may also be operable.