VOLUME EFFECTS IN RARE-EARTH INTERMETALLIC COMPOUNDS

In the binary intermetallic phases formed by rare earths with other elements the experimental values of the average atomic volume VBAR are generally lower than the values calculated on the basis of the elemental atomic volumes. This follows from the analysis of crystallographic data of about 1800 compounds with known structure, occurring in 378 binary R-M systems, with R=Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Th, Dy, Ho, Er, Tm, Lu and M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Mg, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, Sb, Bi. The volume contractions can be described by the equation VBAR = (1-x)V(R)o+xV(M)o-0.5(n+1)(n+2)(n+3)x(n)(1-x)2\chi(R)-chi(M)\ where V(R)o and V(M)o are the elemental atomic volumes, x is the atomic fraction of the M element, n is a parameter related to the composition of the phase with the maximum volume contraction within a given system and chi is an empirical volume parameter. The chi quantity results correlated with the electronegativity for the transition elements, assuming mainly the role of a charge transfer parameter. For the non-transition elements chi resembles the periodic trend of the compressibility, indicating the greater influence of elastic effects.