Structure and thermal stability of the RMgPb rare earth compounds, and the anomalous melting behaviour of SmMgPb

The synthesis, crystal structure and melting behaviour of the new family of ternary rare earth RMgPb compounds is reported in this work. All the rare earth elements (including Y and Sc) form the equiatomic phase 1:1:1 with Mg and Pb, similar to the recently investigated RMgSn compounds series. Unlike RMgSn, all the RMgPb phases (the lighter, as well as the heavier, trivalent lanthanides) crystallize with the same crystal structure: the tetragonal CeScSi-type (an ordered derivative of the La2Sb-type structure, tI12, space group I4/mmm). Both the observed unit cell volume (Vobs) and the mean atomic volume (Vobs/n, where n is the number of atoms in a unit cell) decrease linearly from LaMgPb [a = 4.598(1) Å, c = 16.512(2) Å] to LuMgPb [a = 4.356(1) Å, c = 15.783(2) Å] confirming the lanthanide contraction in the RMgPb series. On the other hand, the volume of formation (ΔV %) becomes more negative by a non-linear trend on going from La to Lu. A high temperature polymorph phase, orthorhombic TiNiSi-type, has been found for both YbMgSn and YbMgPb. Further work concerning the existence of the phase “ScMgSn” has been also performed. The relationships between the structural properties and formation thermodynamics of both the RMgSn and the RMgPb series of compounds have been examined in the present work.All the RMgPb phases form congruently (including YMgPb and ScMgPb) and their melting temperatures decrease non-linearly from LaMgPb to ScMgPb. YbMgSn and YbMgPb also form congruently with anomalously high melting points. A particular and interesting anomaly has been observed for SmMgPb and Sm containing pseudo-ternary compounds [Nd1−xSmxMgPb (x = 0.4, 0.6, 0.8) and Sm1−yGdyMgPb (y = 0.6, 0.4, 0.2)]; their melting temperature are lower than the ones expected from the trend established by the other RMgPb phases by as much as 70 °C. This anomalous behaviour led to an examination of the melting points of selected Sm-bearing materials, including Sm metal. The low melting points are thought to be due to a decrease in the valence of trivalent Sm phases at 25 °C as the materials are heated and thus a lower bonding strength.