Structural and bonding characteristics of Ga-containing polar intermetallics

High temperature solid-state synthesis and electronic structure calculations were used to characterize and analyze Ga containing polar intermetallic compounds, specifically systems with late transition metals (Co, Pt, Pd) and cations of alkaline-earth (Mg, Ca, Sr,) or rare-earth (Gd) metals. Total energy calculations, magnetic moment and structural optimizations were performed with the Vienna Ab initio Simulation Package (VASP) and density of states and crystal orbital Hamilton population curves as well as the integration of the COHP were calculated using the Stuttgart Tight-Binding Linear Muffin-Tin Orbital with the Atomic Sphere Approximation (TB-LMTO-ASA). These tools were used to provide insight into the structural and bonding characteristics of these Ga containing polar intermetallics. Two new fully ordered ternary Laves phases Ca2Pt3Ga and Ca2Pd3Ga were discovered and analyzed to shed light on the driving force behind the structural distortion of the cubic MgCu2-type CaPt2 and CaPd2 with the substitution of Ga. Nine symmetrically inequivalent coloring models, substituting one Ga on each tetrahedra of the cubic structure, were proposed and studied. Change in energy calculations for three theoretically proposed steps of this substitution show that it is energetically favorable to substitute Ga for Pd however, Ga for Pt is energetically unfavorable, demonstrating the effect of size differences and the differences in Mulliken electronegativities in these systems. An orthorhombic variant of the previously reported monoclinic Ca2Pd2Ga was synthesized and characterized. Structure comparisons show similar coordination environments for the atoms involved however, there is a change in the direction, stacking, and uniformity of the Pd−Pd linear chains in the structures. All electronic structure calculations indicate that the monoclinic structure is energetically more stable. Electronic structure calculations were performed on a series of GdCo2-xGax compounds to shed light on any driving forces of the structural changes with increased Ga content. Results indicate that the structure changes when the previous structure reaches a threshold of Ga−Ga homoatomic interactions. These structure changes happen with increased Ga content to optimize the Co−Ga heteroatomic interactions within the structures. The series of structures adopted also resembles the series of GdX2 binary (X = Co, Ni, Cu, Zn, Ga) compounds thus indicating that the valence electron count plays a significant role in the structure changes as well