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In this paper, a new version of the Stillinger–Weber (SW) potential for wurtzite GaN is presented, by which we systematically explore the structural and thermodynamical properties of native point defects and their complexes. In parallel, the semi-empirical Modified Embedded-Atom Method (MEAM) potential is selected for comparison. The SW and MEAM potentials are assessed by the reproduction of the fundamental properties of wurtzite GaN and by the ability to describe the inversion domain boundaries and the wurtzite–rocksalt phase transition. Then the structural search of native point defects and their complexes in GaN is implemented using both SW and MEAM potentials with the benchmark of Density Functional Theory (DFT) calculations. Besides vacancies and antisites, four N and five Ga interstitials are confirmed by refining the DFT calculations, among which two N split interstitials N + − N ⟨ 2 1 ̄ 1 ̄ 0 ⟩ and N + − Ga ⟨ 01 1 ̄ 0 ⟩ , and two Ga split interstitials, Ga + − Ga ⟨ 01 1 ̄ 0 ⟩ − g and Ga + − N ⟨ 01 1 ̄ 0 ⟩ , are observed for the first time. The SW potential correctly predicts the octahedral occupation GaOct to be the most stable Ga interstitial, while the MEAM potential predicts the ground state of the N + − N ⟨ 01 1 ̄ 0 ⟩ split interstitial ( N + − N ⟨ 01 1 ̄ 0 ⟩ − g) as the most stable N interstitial. However, neither of the two potentials could simultaneously generate the most stable configurations of N and Ga interstitials. The investigations of point defect complexes reveal that N octahedral Frenkel [FrenkelOct(N)] and paired antisite (NGaGaN) defects are unstable and get converted into V N ⊕ N + − N ⟨ 01 1 ̄ 0 ⟩ − g configurations with different separations between VN and N + − N ⟨ 01 1 ̄ 0 ⟩ − g point defects based on the DFT calculations. The formation energies calculated by the DFT and SW potential demonstrate that Schottky, Ga octahedral Frenkel [FrenkelOct(Ga)], and V N ⊕ N + − N ⟨ 01 1 ̄ 0 ⟩ − g point defect complexes are energetically feasible and that they should not dissociate into two isolated point defects. In contrast, the MEAM potential predicts the dissociation to be exothermic for Schottky and V N ⊕ N + − N ⟨ 01 1 ̄ 0 ⟩ − g. Overall, the structural features concerned with N–N or Ga–Ga bonds relaxed by the SW potential are more consistent with DFT calculations than the MEAM counterpart. [ABSTRACT FROM AUTHOR]

The magnetostructural phase transition in Fe50Al50 alloy with chemically ordered paramagnetic B2 and disordered ferromagnetic (FM) A2 phase has applications in spintronics such as phase-change magnetic memory and magnonic devices. We first conducted a systematic first-principles density functional theory study of the A2 and B2 phases in the Fe50Al50 alloy. A theoretical understanding of this equiatomic alloy's electronic and spin-dynamical properties leads us to the experimental exploration of the FM A2 phase. Therefore, we deposit the 50 nm Fe50Al50 alloy thin film using sputtering and investigate the influence of the Ta buffer layer on its structural and magnetic properties. Our results reveal that the film with a buffer layer exhibits the A2 phase with appreciably higher saturation magnetization (848 emu/cc) than the film without a buffer layer (576 emu/cc). However, the surface roughness and Gilbert damping (α) slightly increase with the presence of the buffer layer from 0.41 to 0.56 nm and 4.35 × 10−3–4.94 × 10−3, respectively. The enhancement in α is due to extrinsic contributions induced by surface inhomogeneities. [ABSTRACT FROM AUTHOR]

High pressure is an effective means to optimize the thermoelectric (TE) performance by sharply improving the electrical properties of materials. Studying the carrier effective mass (m*) is a feasible way to uncover the basic reason for superior electrical properties under high pressure. However, it is still difficult to obtain the m* under pressure in experiments. Thus, in this work, the m* of α-PbSe (F m 3 ̄ m) under high pressure is calculated by band dispersion relation based on the density functional theory. It is found that the high pressure decreases m* of α-PbSe, which is the cause for excellent electrical properties. Moreover, the isotropy of m* enhances with the increase in the pressure, which means the high pressure further optimizes the isotropy of the carrier migration in the structure. It is reveled that the higher the pressure, the more beneficial to improve the electrical properties of α-PbSe, thus optimizing the TE performance before the phase transition pressure (4.5 GPa). This work is of great significance for exploring the mechanism of in situ high-pressure TE properties in the future, as well as the prediction and selection of high-performance TE materials under high pressure. [ABSTRACT FROM AUTHOR]

Family of "breathing crystals" is the polymer-chain complexes of Cu(hfac)2 with nitroxides. The polymer chains consist of one-, two- or three-spin clusters. The "breathing crystals" experience simultaneous magnetic and Jahn-Teller type structural phase transitions with change of total cluster spin and drastic change of bond lengths (ca. 10-12%). For the first time the intra-cluster magnetic couplings in "breathing crystals" have been calculated both by band structure methods GGA+U and hybrid DFT (B3LYP and PBE0) for the isolated exchange clusters. The temperature dependence of the magnetic coupling constant was calculated for two polymer-chain compounds of the "breathing crystal" family - C21H19CuF12N4O6 with the chains containing two-spin clusters and C22H21CuF12N4O6 with the chains of alternating three-spin clusters and one-spin sites. It was found that adding a Hubbard-like parameter not only to the copper 3d electrons but also to the oxygen 2p electrons (GGA+Ud+Up approach) results in an improved description of exchange coupling in the "breathing crystal" compounds. At the same time treatment of the isolated clusters by a large basis hybrid DFT with high computational cost provides a similar quality fit of the experimental magneto-chemical data as that for the GGA+Ud+Up band structure calculation scheme. Our calculations also showed that in spite of the abrupt transformation of the magnetic coupling constant under the phase transition, the band gap in the "breathing crystals" remains about the same value with temperature decrease. [ABSTRACT FROM AUTHOR]