VOLUME EFFECTS IN RARE-EARTH INTERMETALLIC COMPOUNDS

In the binary intermetallic phases formed by rare earths with other el ements the experimental values of the average atomic volume VBAR are g enerally lower than the values calculated on the basis of the elementa l atomic volumes. This follows from the analysis of crystallographic d ata of about 1800 compounds with known structure, occurring in 378 bin ary 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 contr actions can be described by the equation VBAR = 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 a tomic volumes, x is the atomic fraction of the M element, n is a param eter related to the composition of the phase with the maximum volume c ontraction within a given system and chi is an empirical volume parame ter. The chi quantity results correlated with the electronegativity fo r the transition elements, assuming mainly the role of a charge transf er parameter. For the non-transition elements chi resembles the period ic trend of the compressibility, indicating the greater influence of e lastic effects.