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A first-principles-based lattice model is applied to investigate the ordered phases of mixed group-III nitride ternary alloys. The model surveys the atomistic configurations with the lowest formation enthalpy for a wide range of compositions. We found novel ordered phases in wurtzite structures having specific compositions of three- and four-sevenths molar fractions of group-III cations. The configurations of group–III atoms on cation sites in those phases consist of a characteristic fragment of the ordered phases of one-third and one-half ordered phases that were previously reported. The simulation results indicate that group-III cations in ternary nitrides follow spatial positioning "atomistic distancing rules" that can be described by the pairwise interaction energy of group-III cations to realize the stabilities of the ordered structures. To minimize the formation enthalpy of a mixed crystal, the minor B, Al, Ga, and In atoms on cation sites remain neither too close to nor too distant from each other, allowing those ordered phases to be realized. [ABSTRACT FROM AUTHOR]

Some fluctuations in composition are commonly observed in epitaxial-grown III-V multinary alloys. These fluctuations are attributed to compositional pulling effects, and an insight into their atomistic origin is necessary to improve current epitaxial growth techniques. In addition, the crystallinity of III-V multinary alloys varies widely depending on the constituent atoms. Using first-principles calculations, we then investigated different geometric configurations of gallium nitride (GaN)-based ternary alloy, X0.125Ga0.875N where X is the minority atom which is boron (B), aluminum (Al), or indium (In). The minority atoms are presented as two atoms in the simulation cell, and the energetics of five geometric configurations are analyzed to estimate the most stable configuration. For the B0.125Ga0.875N alloy, the most stable configuration is the one where the minority atoms occupy gallium (Ga) sites in a collinear orientation along the c-axis. On the contrary, the configurations along the in-plane direction result in a higher energy state. In0.125Ga0.875N and Al0.125Ga0.875N also show the same trend with a small relative energy difference. These preferential sites of minority atoms are consistent with composition pulling effects in wurtzite nitride phases. Moreover, the degree of crystallinity for wurtzite nitride alloys can be well described by the order of calculated relative energy. [ABSTRACT FROM AUTHOR]

A quasibinary system of Ga2O3-Al2O3 offers a range of applications in wide bandgap semiconductor engineering. Different polymorphs and concentrations of (Al x Ga1− x )2O3 manifest a variety of structural and electronic properties, paving the way for tunability of (Al x Ga1− x )2O3 for specific functions. In this work, we investigate the energetics of alpha (α) and beta (β) polymorphs of Ga2O3 and Al2O3 by considering all possible configurations in a conventional unit cell. Using density functional theory, we show that the formation energies of (Al x Ga1− x )2O3 in α and β configurations start to coincide at 50% concentration (Al0.5Ga0.5)2O3. The corundum configuration then becomes more dominant (lower in energy) than its monoclinic counterpart at around 80% Al concentration. The lowest formation energy configurations for 50% concentration in both α and β polymorphs also manifest a preference towards an ordered phase. These show that the stability of Ga2O3-Al2O3 and its phase transitions are significantly influenced by the relative arrangements of Ga and Al within the quasibinary semiconducting crystal. [ABSTRACT FROM AUTHOR]

We identify using ab initio calculations new types of three-dimensional carbon allotrope constructed by inserting acetylenic or diacetylenic bonds into a body-centered cubic C8 lattice. The resulting sp + sp3-hybridized cubane-yne and cubane-diyne structures consisting of C8 cubes can be characterized as a cubic crystalline modification of linear carbon chains, but energetically more favorable than the simplest linear carbyne chain and the cubic tetrahedral diamond and yne-diamond consisting of C4 tetrahedrons. Electronic band calculations indicate that these new carbon allotropes are semiconductors with an indirect band gap of 3.08 eV for cubane-yne and 2.53 eV for cubane-diyne. The present results establish new types of carbon phases consisting of C8 cubes and offer insights into their outstanding structural and electronic properties. [ABSTRACT FROM AUTHOR]

Synthesis of pristine MXene sheets from MAX phase is one of the foremost challenges in getting a complete understanding of the properties of this new technologically important 2D-material. Efforts to exfoliate Nb4AlC3 MAX phase always lead to Nb4C3 MXene sheets, which are functionalized and have several Al atoms attached. Using the first-principles calculations, we perform an intensive study on the chemical transformation of MAX phase into MXene sheets by inserting HF, alkali atoms and LiF in Nb4AlC3 MAX phase. Calculated bond-dissociation energy (BDE) shows that the presence of HF in MAX phase always results in functionalized MXene, as the binding of H with MXene is quite strong while that with F is weak. Insertion of alkali atoms does not facilitate pristine MXene isolation due to the presence of chemical bonds of almost equal strength. In contrast, weak Li–MXene and strong Li–F bonding in Nb4AlC3 with LiF ensured strong anisotropy in BDE, which will result in the dissociation of the Li–MXene bond. Ab initio molecular dynamics calculations capture these features and show that at 500–650 K, the Li–MXene bond indeed breaks leaving a pristine MXene sheet behind. The approach and insights developed here for chemical exfoliation of layered materials bonded by chemical bonds instead of van der Waals can promote their experimental realization. [ABSTRACT FROM AUTHOR]

Metastability of Al n /12 Ga 1- n /12 N ( n = 2-10: integer) with the 1-2 monolayer (ML) in-plane configuration towards the c [0001] direction has been demonstrated recently. To theoretically explain the existence of these metastable structures, relatively large calculation cells are needed. However, previous calculations were limited to the use of small calculation cell sizes to estimate the local potential depth (Δ σ ) of ordered Al 1/2 Ga 1/2 N models. In this work, we were able to evaluate large calculation cells based on the interaction energies between proximate Al atoms ( δE Al-Al ) in AlGaN alloys. To do this, δE Al-Al values were estimated by first-principles calculations (FPCs) using a (5 a 1 × 5 a 2 × 5 c ) cell. Next, a survey of the possible ordered configurations using various large calculation cell models was performed using the estimated δE Al-Al values and the Monte-Carlo method. Then, various Δ σ values were estimated by FPCs and compared with the configurations previously reported by other research groups. We found that the ordered configuration obtained from the (4 a 1 × 2 a 2 × 1 c ) calculation cell ( C 42 ) has the lowest Δ σ of -9.3 meV/cation and exhibited an in-plane configuration at the c (0001) plane having (-Al-Al-Ga-Ga-) and (-Al-Ga-) sequence arrangements observed along them11-00planes. Hence, we found consistencies between the morphology obtained from experiment and the shape of the primitive cell based on our numerical calculations., (© 2023 IOP Publishing Ltd.)

Correction for 'New carbon allotropes in sp + sp3 bonding networks consisting of C8 cubes' by Jian-Tao Wang et al., Phys. Chem. Chem. Phys., 2018, 20, 7962-7967.

We identify using ab initio calculations new types of three-dimensional carbon allotrope constructed by inserting acetylenic or diacetylenic bonds into a body-centered cubic C 8 lattice. The resulting sp + sp 3 -hybridized cubane-yne and cubane-diyne structures consisting of C 8 cubes can be characterized as a cubic crystalline modification of linear carbon chains, but energetically more favorable than the simplest linear carbyne chain and the cubic tetrahedral diamond and yne-diamond consisting of C 4 tetrahedrons. Electronic band calculations indicate that these new carbon allotropes are semiconductors with an indirect band gap of 3.08 eV for cubane-yne and 2.53 eV for cubane-diyne. The present results establish new types of carbon phases consisting of C 8 cubes and offer insights into their outstanding structural and electronic properties.