The Unique Electronic Structure of Mg2 Si: Shaping the Conduction Bands of Semiconductors with Multicenter Bonding

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4

The new phase Mg₇Pt₄Ge₄ (≡Mg₈□₁Pt₄Ge₄; □ = vacancy) was prepared by reacting a mixture of the corresponding elements at high temperatures. According to single crystal X-ray diffraction data, it adopts a defect variant of the lighter analogue Mg₂PtSi (≡Mg₈Pt₄Si₄), reported in the Li₂CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg₇Pt₄Ge₄. However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg₂PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg₂PtGe" reveal potential electronic instabilities at E_F in the band structure and significant occupancy of states with an antibonding character resulting from unfavorable Pt-Ge interactions. These antibonding interactions can be eliminated through introduction of Mg defects, which reduce the valence electron count, leaving the antibonding states empty. Mg itself does not participate in these interactions. Instead, the Mg contribution to the overall bonding comes from electron back-donation from the (Pt, Ge) anionic network to Mg cations. These findings may help to understand how the interplay of structural and electronic factors leads to the "hydrogen pump effect" observed in the closely related Mg₃Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.

Machine-learned model Hamiltonian and strength of spin-orbit interaction in strained Mg2X (X = Si, Ge, Sn, Pb)

Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds Mg₂Si, Mg₂Ge, Mg₂Sn, and Mg₂Pb subject to strain. The MMTB models incorporate spin-orbital mediated interactions and they are calibrated to the electronic band structures calculated via density functional theory by a massively parallelized multi-dimensional Monte-Carlo search algorithm. The results show that a machine-learned five-band tight-binding (TB) model reproduces the key aspects of the valence band structures in the entire Brillouin zone. The five-band model reveals that compressive strain localizes the contribution of the 3sorbital of Mg to the conduction bands and the outer shellporbitals of X (X = Si, Ge, Sn, Pb) to the valence bands. In contrast, tensile strain has a reversed effect as it weakens the contribution of the 3sorbital of Mg and the outer shellporbitals of X to the conduction bands and valence bands, respectively. Theπbonding in the Mg₂X compounds is negligible compared to theσbonding components, which follow the hierarchy|σsp|>|σpp|>|σss|, and the largest variation against strain belongs toσ_pp. The five-band model allows for estimating the strength of spin-orbit coupling (SOC) in Mg₂X and obtaining its dependence on the atomic number of X and strain. Further, the band structure calculations demonstrate a significant band gap tuning and band splitting due to strain. A compressive strain of-10%can open a band gap at the Γ point in metallic Mg₂Pb, whereas a tensile strain of+10%closes the semiconducting band gap of Mg₂Si. A tensile strain of+5%removes the three-fold degeneracy of valence bands at the Γ point in semiconducting Mg₂Ge. The presented MMTB models can be extended for various materials and simulations (band structure, transport, classical molecular dynamics), and the obtained results can help in designing devices made of Mg₂X.

Unique Conduction Band Minimum of Semiconductors Possessing a Zincblende-Type Framework

Tetrahedral semiconductors such as Si adopt a diamond-type crystal structure with low packing density arising from open cavities in the crystallographic space. By taking LiAlGe as an example, we propose a zincblende-type framework as a platform for semiconductors possessing electroactive cavities. LiAlGe adopts a half-Heusler-type crystal structure including an ordered diamond-type sublattice (zincblende-type) (AlGe) and is an indirect semiconductor with a band gap of ∼0.1 eV. The conduction band minimum (CBM) is uniquely located at the cavity space surrounded by four cations (Al₄) in real space. The bond ionicity and cation (Al) p orbitals located around the Fermi energy are requisite for the CBM to float in the cavity space. DFT calculations indicate the conversion of the semiconductor to a semimetallic electride under a pressure of ∼8 GPa, which is accompanied by band gap collapse due to electron transfer from valence band maximum to the cavity space. The high-pressure electride of LiAlGe formed under a very small critical pressure is derived from the presence of inherent crystallographic cavities having deep orbital levels energetically. This finding suggests the possible utilization of electroactive cavity spaces in tetrahedral semiconductors, which are widely used in modern electronic devices.

Sc2C, a 2D Semiconducting Electride

Electrides are exotic materials that typically have electrons present in well-defined lattice sites rather than within atoms. Although all known electrides have an electropositive metal cation adjacent to the electride site, the effect of cation electronegativity on the properties of electrides is not yet known. Here, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize Sc₂C. Our studies identify the material as a two-dimensional (2D) electride, even though Sc is more electronegative than any metal previously found adjacent to an electride site. Further, by exploring Sc₂C and Al₂C computationally, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals, which opens a band gap in these materials. Sc₂C is the first 2D electride semiconductor, and we propose a design rule that cation electronegativity drives the change in its band structure.

Rapid, Energy-Efficient and Pseudomorphic Microwave-Induced-Metal-Plasma (MIMP) Synthesis of Mg2Si and Mg2Ge

Polycrystalline magnesium silicide, Mg2Si and magnesium germanide, Mg2Ge were synthesised from the elemental powders via the microwave-induced-metal-plasma (MIMP) approach at 200 W within 1 min in vacuo for the first...

On the Origin of the Negative Thermal Expansion Behavior of YCu

Among the intermetallics and alloys, YCu is an unusual material because it displays negative thermal expansion without spin ordering. The mechanism behind this behavior that is caused by the structural phase transition of YCu has yet to be fully understood. To gain insight into this mechanism, we experimentally examined the crystal structure of the low-temperature phase of YCu and discuss the origin of the phase transition with the aid of thermodynamics calculations. The result shows that the high-temperature (cubic CsCl-type) to low-temperature (orthorhombic FeB-type) structural phase transition is driven by the rearrangement of three covalent bonds, namely, Y-Cu, Y-Y, and Cu-Cu, which compete for the bonding energy and phonon entropy. At low temperatures, the mixing of Y and Cu does not take place easily because of the weak attractive force between these atoms expected from the small negative mixing enthalpy. This causes all three interactions to take part in the bonding, and Y and Cu are segregated to form an FeB-type structure, which is stabilized by internal energy. At higher temperatures, Cu ions are bound loosely with Y ions due to the large Y-Cu distance (3.01 Å), which results in large vibration entropy and stabilizes a CsCl-type crystal structure. In addition, the CsCl-type structure is reinforced by the Y-Y interaction between next-nearest neighbors, resulting in a smaller unit cell volume. The crystal structure has the simple cubic framework of Y containing Cu ions bound loosely at the cavity sites. The calculated frequency of the Y-like phonon modes is much higher than that of the Cu-like modes, indicating the presence of Y-Y covalent interactions in the CsCl-type phase.

V2Se: a novel antifluorite-type cubic phase with a metal-metal bonding

A new antifluorite-type (Li₂O-type) cubic compound, V₂Se, has been synthesized for the first time by changing the amount of selenium in chemical vapor transport. The vanadium-based cubic phase studied here reveals a metal-metal bonding feature in the electronic band structure. This compound is the first example of an antifluorite-type cubic structure in a V-Se system.

Transition metal silicides: fundamentals, preparation and catalytic applications

Transition metal silicides as low-cost and earth-abundant inorganic materials are becoming indispensable constituents in catalytic systems for a variety of applications and exhibit excellent properties for sustainable industrial process.

Cubic Fluorite-Type CaH2 with a Small Bandgap

A cubic variant of CaH₂ adopting a fluorite-type crystal structure was synthesized by cationic substitution with La or Y, yielding the first alkaline earth hydride-based with fluorite-type framework. The material has a bandgap of ∼2.5 eV (greenish yellow in color), which is much smaller than that of orthorhombic PbCl₂-type CaH₂ (4.4 eV) and is, in fact, the smallest among alkaline or alkaline earth metal hydrides reported to date. Analysis of the density functional theory band structure of cubic-CaH₂ indicates that its conduction band minimum is formed mainly by the interaction between the Ca 3d e_g orbitals around the crystallographic cavity defined by cubes of H^- ions. The use of such cavities in the creation of low-lying conduction band minima by semiconductors is extremely rare, and has similarities to inorganic electrides.