On the RMgSn rare earth compounds

A new family of ternary rare earth compounds, RMgSn, has been synthesized and their crystal structures, formation thermodynamics and melting behavior have been studied. All of the rare earth elements (including Y) form the 1:1:1 equiatomic phase with Mg and Sn. These compounds crystallize with two different structure types: the RMgSn phases with the light R (R = La, Ce and Pr) adopt the orthorhombic TiNiSi structure type (an ordered derivative of the Co2Si-type structure, oP12, space group Pnma), while the ones formed by the heavier R (R = Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu, plus Y) have the tetragonal CeScSi-type structure (an ordered derivative of the La2Sb-type structure, tI12, space group I4/mmm). The observed unit cell volume Vobs and the mean atomic volume Vobs/n (where n is the number of atoms in a unit cell) both decrease as expected due to the lanthanide contraction, but following different trends. The volume of formation (ΔV%) becomes more negative on going from La to Lu along the series.All phases have been found to form congruently (including YMgSn and probably LuMgSn). Their melting temperatures decrease from La to Lu, but with different slopes for the two different structure types.Relationships, between the volume of formation and also the melting points with the lanthanide contraction have been examined. The relationship between the former is anomalous compared to that observed for other RxMy series of compounds, while the latter relationship is consistent with previously published results.