Theoretical study of the phase stability and site preference for R3(Fe,T)29 (R=Nd, Sm; T=V, Ti, Cr, Cu, Nb, Mo, Ag)

Abstract

The phase stability of intermetallics R3(Fe,T)29 with Nd3(Fe,Ti)29 structure and site preference of some 3d or 4d transition elements T were investigated in molecular static and molecular dynamic methods with a series of ab initio pair potentials obtained though the lattice inversion method. Calculated results show that adding either Cr, Mo, Ti, V or Nb atoms makes the crystal cohesive energy of R3(Fe,T)29 decrease markedly, proving that these atoms can stabilize R3(Fe,T)29 with the structure of Nd3(Fe,Ti)29, even though the R3Fe29 crystal structure is itself metastable. The calculated lattice parameters are in good agreement with the experimental data. The degree of the decrease in cohesive energy corresponds with the species and occupation sites of the ternary atoms. The order of site preference of these stabilizing elements T is 4i2, 4i1 and 4g with the occupation of 4i2 corresponding to the greatest energy decrease. The calculated result further shows that the addition of Cu or Ag cannot play a role in stabilizing the structure. These calculated results correspond well to available experiments. Supported by the pair potentials, calculated structures are stable within a certain temperature range and the space group of the final structure remains unchanged with respect to a variety of initial deformations. So it was confirmed that there exist a series of R3(Fe,T)29 compounds with the stable structure of Nd3(Fe,Ti)29 in the R–Fe–T systems. The process of the evolution from the RFe5 structure to metastable R3Fe29 was well explained too with the pair potential in this paper. All these prove the effectiveness of ab initio pair potentials obtained through the lattice inversion method in the description of rare-earth materials.