Volume of formation and ligand field effects in some intermetallics

The volume of formation Delta V-f/mol of the electropositive atom (B) for s everal close-packed structures with composition A(m) B, m greater than or e qual to 3 (AuCu3, Ni4Mo, Au4Zr, Cu3Ti, Ni3Ti and Ni Sn) is equal to ((V) ov er bar(B) - (V) over bar(B)) where (V) over bar(B) is the partial molar vol ume of solute B in the solvent A, and V-B is the elemental volume. This beh aviour indicates that the volume change is located on the electropositive a tom, and the volume of B atoms in these intermetallics is identical to that in A-rich solid solutions. This behaviour is attributed to similar environ ment of B atoms in solid solutions and in the close-packed structures of st oichiometry A(m)B, m greater than or equal to 3. Similar behaviour is obser ved in CsCl structures if B is a normal metal, Delta V-f being less negativ e when B is an early transition element. A comparison of Delta V-f for diff erent close packed structures of stoichiometry A(3)B with stoichiometry AB shows identical values if B is a normal metal, Delta V-f being less negativ e if B is an early transition element, or a rare earth. Ca and Sr behave li ke transition elements in the intermetallics examined. The less negative va lues of Delta V-f for CsCl and AuCu structures is attributed to ligand fiel d effects. Delta V-f for A(2)B stoichiometry (MoSi2 structures) shows more negative values (larger contraction) than that for CsCl structure, which is attributed to lower anisotropy of the ligand field. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.