Effect of disorder on thermodynamic instability of binary Rare-earth – Nickel – Palladium compounds

Abstract

We have investigated the thermodynamic stability of disordered rare-earth phases SmX2 and Sm10X21 (X=Ni, Pd) using machine-learning based analytical descriptor and first-principles density functional theory methods. The absence of Laves phase compounds in R-Pd binary systems is a longstanding problem of rare earth science: even though Ni and Pd belong to the same group of the periodic table and have similar electronic structure, the Pd compound crystallizes in a monoclinic (C2/m) phase with 10:21 stoichiometry, i.e., Sm10Pd21, while the Ni compound adopts a cubic Laves phase (MgCu2) structure. To understand this contrasting phase stability, we performed thermodynamic convex hull analysis of SmxNi1-x and SmxPd1-x binary systems, which is experimentally validated using powder X-ray diffraction (PXRD) analyzes of polycrystalline Sm(NixPd1-x)2 samples with x=0, 0.5, and 1. A detailed electronic-structure (band-structure, charge density, and Fermi-surface) analysis of the differences between SmNi2/SmPd2 and Sm10Ni21/Sm10Pd21 compounds provides the quantum mechanical origin of the unfavorable mixing of Pd with Ni in cubic Laves phase. We show that the stability of Sm-Pd in 10:21 stoichiometry arises from improved intra-/inter-layer 5d-4d bonding compared to the 1:2 stoichiometry. Our work emphasizes the importance of ab-initio methods and computationally inexpensive analytical descriptors for the detailed analysis of thermodynamic and electronic properties of hard-to-prepare rare-earth compounds.