A new hexagonal phase of Si1−xGex alloys have been successfully synthesized through efforts in recent reports. Utilizing the combined first-principle calculations and special quasi-random model, we precisely investigated the structural, electronic, optical, and vibrational properties of hexagonal Si and Ge and disordered hexagonal Si1−xGex random alloys. We found a large negative deviation between the calculated lattice constants within the revised Perdew–Burke–Ernzerhof for solids functional and the linear fitting results. The electronic structures obtained by using the Tran–Blaha modified Becke–Johnson exchange potential confirm that hexagonal Si1−xGex (x > 0.625) alloys present direct bandgaps. Through solving the Bethe–Salpeter equation, the linear optical spectra of hexagonal Si and Ge are demonstrated. We reveal that the peaks of complex dielectric functions are redshifted with the addition of Ge atoms. Also, the real and imaginary parts exhibit strong anisotropy, which makes hexagonal Si1−xGex alloys potentially useful as nonlinear crystals. The transition is allowed in the infrared region for the hexagonal Si1−xGex (x > 0.625) alloys, and the linear optical spectra can be continuously tuned over a wide range of frequency with Ge addition in the infrared region. Furthermore, density-functional perturbation theory calculations were carried out to predict the off-resonance Raman activity. The results suggest that the vibrational modes of the Si–Si bond exhibit a strong dependency on the compositions, which provides a useful way to identify the most probable atomic configurations of hexagonal Si1–xGex alloys in future experiments.

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Two-dimensional ice is a new type of atomic-scale material obtained by typical atomic manufacturing techniques. Its structure and nucleation growth play an essential role in many fields such as material science, tribology, biology, atmospheric science and planetary science. Although the structural properties of two-dimensional ice have been investigated extensively, little is known about its electronic and optical properties. In this paper, the main electronic, optical, dielectric properties and infrared spectra of two-dimensional ice I at zero temperature are calculated by density functional theory and linear response theory. The study reveals that the two-dimensional ice I is an indirect band gap and its optical properties show anisotropic lattice. And the absorption energy range for the two-dimensional ice I is in the ultraviolet region of the spectrum (> 3.2 eV) and the visible region of the spectrum (between 2 and 3.2 eV), respectively. Secondly, the radial distribution function and the vibrational density of states of the two-dimensional ice I at a finite temperature are simulated by <i>ab initio</i> molecular dynamics method. For the structure of the two-dimensional ice I, whether SCAN or PBE functional, after considering the vdW effect, there is almost no effect on the atomic distance, while by comparison, the SCAN functional and the PBE functional are quite different. Therefore, it can be seen that the main reason for affecting the distance between atoms in the structure is due to the consideration of the strong confinement effect of SCAN. In terms of the vibration characteristics of two-dimensional ice I, comparing with PBE and vdW-DF-ob86, the first two peaks of the IR spectrum of SCAN + rVV10 functional show blue shift, and the two peaks in the high frequency region present the red shift. Therefore, considering the strong confinement effect of SCAN, the intermolecular tensile vibration of two-dimensional ice I becomes stronger, while the intramolecular H—O—H bending vibration and O—H bond tensile vibration become weaker. The effect of van der Waals action on vibration properties is not obvious. Furthermore, we investigate the temperature effects on the vibration spectra of two-dimensional ice I. It is found that with the increase of temperature, the intermolecular librational mode weakens at a low frequency, the intramolecular bending and stretching bands gradually broaden, and the intramolecular O-H stretching peak presents the blue-shifts with temperature rising. The results of this paper reveal the electronic structure of atomic-scale two-dimensional ice I, and demonstrate its unique optical absorption mechanism, which is helpful in further experimentally characterizing and manipulating the two-dimensional ice on an atomic scale. Since the two-dimensional ice on the surface can promote or inhibit the formation of three-dimensional ice, it has potential applications in designing and developing the anti-icing materials. In addition, two-dimensional ice itself can also be used as a unique two-dimensional material, providing a brand-new standard material for high-temperature superconductivity, deep-ultraviolet detection, cryo-electron microscopy imaging.

Ta addition alters the microstructures, thermodynamic and mechanical properties of CoCrFeNi alloys.

Metallic uranium-based alloys, with d-transition metals such as Nb, Mo, and Zr, are promising candidates for actinide fuel. For this purpose, their behaviors under changing physical stimuli need to be understood. Here, we systematically investigate U–Nb intermetallic compounds and predict new compound formations under different pressures using the first-principles swarm-intelligence structure searching method. Two new compounds (U1Nb6 and U2Nb1) were identified to be thermodynamically stable at ambient and high pressures. U1Nb6 has a triclinic symmetry that is stable in the pressure range of 0–200 GPa, while U2Nb1 has a hexagonal closely packed structure at low pressure and transforms to a simple hexagonal lattice at 20 GPa. Other compounds, particularly U-rich ones (U3Nb1, U4Nb1, U5Nb1, and U6Nb1), are found metastable at ambient and high pressures, and all have orthorhombic structures. The structural, vibrational, electronic, and mechanical properties of predicted U-rich compounds were thoroughly studied using density-functional theory. The results of phonon spectra and elastic constant show that the predicted new structures are dynamically and mechanically stable in the corresponding pressure range. Also, these newly identified U-rich compounds exhibit strong composition dependence, and the pressure-induced enhancements of structural stability and mechanical performances are evident. These findings shall enrich the understanding of U-based alloys and serve as meaningful predictions for experimental research in the future.

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