Polar metals are an intriguing class of materials that simultaneously host free carriers and polar structural distortions. Despite the name "polar metal," however, most well-studied polar metals are poor electrical conductors. Here, we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe, two polar metal compounds whose electrical resistivity is an order of magnitude lower than the well studied oxide polar metals. These materials belong to a broad family of ABC intermetallics adopting the stuffed wurtzite structure, also known as hexagonal Heusler compounds. Scanning transmission electron microscopy (STEM) reveals a polar structure with unidirectionally buckled BC (PtSb, AuGe) planes. Magnetotransport measurements demonstrate good metallic behavior with low residual resistivity (ρLaAuGe = 59.05 µΩ·cm and ρ LaAP tSb = 27.81 µΩ·cm at 2K) and high carrier density (n h ∼ 10 21 cm −3 ). Photoemission spectroscopy measurements confirm the band metallicity and are in quantitative agreement with density functional theory (DFT) calculations. Through DFT-Chemical Pressure and Crystal Orbital Hamilton Population analyses, the atomic packing factor is found to support the polar buckling of the structure, though the degree of direct interlayer B − C bonding is limited by repulsion at the A − C contacts. When combined with insulating hexagonal Heuslers, these materials provide a new platform for fully epitaxial, multiferroic heterostructures. arXiv:1910.07685v1 [cond-mat.mtrl-sci]
Atomic packing and electronic structure are key factors underlying the crystal structures adopted by solid-state compounds. In cases where these factors conflict, structural complexity often arises. Such is born in the series of REAl 3 (RE = Sc, Y, lanthanides), which adopt structures with varied stacking patterns of face-centered cubic close packed (FCC, AuCu 3 type) and hexagonal close packed (HCP, Ni 3 Sn type) layers. The percentage of the hexagonal stacking in the structures is correlated with the size of the rare earth atom, but the mechanism by which changes in atomic size drive these large-scale shifts is unclear. In this Article, we reveal this mechanism through DFT-Chemical Pressure (CP) and reversed approximation Molecular Orbital (raMO) analyses. CP analysis illustrates that the Ni 3 Sn structure type is preferable from the viewpoint of atomic packing as it offers relief to packing issues in the AuCu 3 type by consolidating Al octahedra into columns, which shortens Al−Al contacts while simultaneously expanding the RE atom's coordination environment. On the other hand, the AuCu 3 type offers more electronic stability with an 18-n closed-shell configuration that is not available in the Ni 3 Sn type (due to electron transfer from the RE d z 2 atomic orbitals into Al-based states). Based on these results, we then turn to a schematic analysis of how the energetic contributions from atomic packing and the electronic structure vary as a function of the ratio of FCC and HCP stacking configurations within the structure and the RE atomic radius. The minima on the atomic packing and electronic surfaces are non-overlapping, creating frustration. However, when their contributions are added, new minima can emerge from their combination for specific RE radii representing intergrowth structures in the REAl 3 series. Based on this picture, we propose the concept of emergent transitions, within the framework of the Frustrated and Allowed Structural Transitions principle, for tracing the connection between competing energetic factors and complexity in intermetallic structures.
Materials either have a high hardness or excellent ductility, but rarely both at the same time. Mo2BC is one of the only crystalline materials that simultaneously has a high Vickers hardness and is also moderately ductile. The origin of this unique balance is revealed here using stress-strain calculations. The results show an anisotropic non-linear elastic response including an intermediate tensile strainstiffening behavior and a two-step sequential failure under shear strain that resembles the behavior of soft materials like biological systems or polymer networks rather than hard, refractory metals. The mechanism of the unusual non-linear elasticity is established by analyzing changes in the electronic structure and the chemical bonding environments under mechanical perturbation. The optimized structure under extreme strain shows the formation of a pseudogap in DOS and dimerization of the structure's boron-boron zigzag chain. These mechanisms delay the ultimate failure establishing a new pathway for developing the next generation of structural materials with high hardness and ductility.
As with other electron counting rules, the 18-n rule of transition metal–main group (T–E) intermetallics offers a variety of potential interatomic connectivity patterns for any given electron count. What leads a compound to prefer one structure over others that satisfy this rule? Herein, we investigate this question as it relates to the two polymorphs of IrIn3: the high-temperature CoGa3-type and the low-temperature IrIn3-type forms. DFT-reversed approximation Molecular Orbital analysis reveals that both structures can be interpreted in terms of the 18-n rule but with different electron configurations. In the IrIn3 type, the Ir atoms obtain largely independent 18-electron configurations, while in the CoGa3 type, Ir–Ir isolobal bonds form as 1 electron/Ir atom is transferred to In–In interactions. The presence of a deep pseudogap for the CoGa3 type, but not for the IrIn3 type, suggests that it is electronically preferred. DFT-Chemical Pressure (CP) analysis shows that atomic packing provides another distinction between the structures. While both involve tensions between positive Ir–In CPs and negative In–In CPs, which call for the expansion and contraction of the structures, respectively, their distinct spatial arrangements create very different situations. In the CoGa3 type, the positive CPs create a framework that holds open large void spaces for In-based electrons (a scenario suitable for relatively small T atoms), while in the IrIn3 type the pressures are more homogenously distributed (a better solution for relatively large T atoms). The open spaces in the CoGa3 type result in quadrupolar CP features, a hallmark of low-frequency phonon modes and suggestive of higher vibrational entropy. Indeed, phonon band structure calculations for the two IrIn3 polymorphs indicate that the phase transition between them can largely be attributed to the entropic stabilization of the CoGa3-type phase due to soft motions associated with its CP quadrupoles. These CP-driven effects illustrate how the competition between global and local packing can shape how a structure realizes the 18-n rule and how the temperature can influence this balance.
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