Putting ScTGa5(T = Fe, Co, Ni) on the Map: How Electron Counts and Chemical Pressure Shape the Stability Range of the HoCoGa5Type

Abstract

We explore the factors stabilizing one member of the diverse structures encountered in Ln–T–E systems (Ln = lanthanide or similar early d-block element, T = transition metal, E = p-block element): the HoCoGa5 type, an arrangement of atoms associated with unconventional superconductivity. We first probe the boundaries of its stability range through the growth and characterization of ScTGa5 crystals (T = Fe, Co, Ni). After confirming that these compounds adopt the HoCoGa5 type, we analyze their electronic structure using density functional theory (DFT) and DFT-calibrated Hückel calculations. The observed valence electron count range of the HoCoGa5 type is explained in terms of the 18-n rule, with n = 6 for the Ln atoms and n = 2 for the T sites. The role of atomic sizes is investigated with DFT-chemical pressure (DFT-CP) analysis of ScNiGa5, which reveals negative pressures within the Ga sublattice as it stretches to accommodate the Sc and T atoms. This CP scheme is consistent with HoCoGa5-type gallides only being observed for relatively small Ln and T atoms. These conclusions account for the relative positions of the HoCoGa5, BaMg4Si3, and Ce2NiGa10 types in a structure map, demonstrating how combining the 18-n and CP schemes can guide our understanding of Ln–T–E systems.