Analysis of phase equilibria in RTX systems and analysis of the size factor in the RT 2X 2, RTX 3, RTX 2 (R 2T 3X 5), RTX and R 2XT 3 compounds (R = rare earths (Ce,Gd,Sm,Eu,Yb); T = transition metals (Fe,Co,Ni,Ru,Rh,Pd,Pt); X  Si,Ge)

Abstract

Specific regularities were found in the equilibrium diagrams of RTX systems (R = rare earths; T = transition metals; X  Si,Ge): Ce(Fe,Co,Ni,Ru,Rh,Pd,Pt)(Si,Ge), Gd(Fe,Ni)Si, Eu(Fe,Co,Ni)(Si,Ge), Sm(Ru,Rh,Pd,Pt)Si, Sm(Fe,Co,Ni,Ru,Rh,Pd,Pt)Ge, Yb(Pd,Pt)(Si,Ge). We found the following formal characteristics of equilibria. (a) The ‘main’ ternary compounds (RT 2X 2, RTX 3, RTX 2 (R 2T 3X 5), RTX and R 2TX 3) lie on the ‘main’ equilibria lines (the lines which connect two binary compounds (or one binary compound) with an element). Therefore, they can be considered as the result of a sort of chemical or structural interaction of the corresponding binary compounds. (b) Ternary compounds irrelevant to the ‘main’ ternary compounds, as a rule, are realized on the ‘main’ equilibrium lines and on the equilibrium lines' ‘main’ ternary compound-binary compound, which also makes it possible to consider them as a result of the transformation of the corresponding binary and ternary compounds. (c) Conodes of the phase equilibria in systems of any type, as a rule, are arranged along the equilibrium lines ‘binary compounds-binary compounds (or element)' and ‘main’ ternary compound-binary compound’. (d) It is of interest that in RTX systems no compounds are formed with X > 66.7 at.%, R > 63 at.%, and T > 77 at.%. A relationship between size and structure was found for the compounds RT 2X 2, RTX 3, RTX 2 (R 2T 3X 5), RTX and R 2TX 3. The dependence of V L R /V R on r str/ r cr for ternary compounds shows that the regions along the respective lines correspond to the compound structures. Here V L R = V L /n R , where V L is the lattice volume and n R is the number of R atoms in the lattice, r str is the crystallographic radius of an atom in the given compounds, and r cr is the crystallographic radius of the R element. V R is the volume occupied by an R atom in the structure of a given R element.