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The topological properties of layered β-GaS and ∈-GaSe under strain are systematically investigated by ab initio calculations with the electronic exchange-correlation interactions treated beyond the generalized gradient approximation (GGA). Based on the GW method and the Tran-Blaha modified Becke-Johnson potential approach, we find that while ∈-GaSe can be strain-engineered to become a topological insulator, β-GaS remains a trivial one even under strong strain, which is different from the prediction based on GGA. The reliability of the fixed volume assumption rooted in nearly all the previous calculations is discussed. By comparing to strain calculations with optimized inter-layer distance, we find that the fixed volume assumption is qualitatively valid for β-GaS and ∈-GaSe, but there are quantitative differences between the results from the fixed volume treatment and those from more realistic treatments. This work indicates that it is risky to use theoretical approaches like GGA that suffer from the band gap problem to address physical properties, including, in particular, the topological nature of band structures, for which the band gap plays a crucial role. In the latter case, careful calibration against more reliable methods like the GW approach is strongly recommended. [ABSTRACT FROM AUTHOR]

The preferential site substitution of the Fe by Si, Ga and Zr in the Y(Fe,M) 9.2 and Y(Fe,M) 9.2 C compounds, and the doping effects on the magnetic properties have been studied by the first-principles calculations. It is found that the doping of the Si or Zr can improve the thermodynamic stability of the 1:9 phase, while the substitution of the Fe by Ga makes it unstable. Si atom tends to enter the 3g crystal site and Zr prefers to occupy the 2e site when Y(Fe,M) 9.2 and their carbides are synthesized. Although the substitution of the Fe by Si and Zr will reduce the total magnetic moments of the YFe 9.2 and their carbides, the volumetric and the d-band narrowing effects caused by the doping can still modify the electron density distributions of the Fe near the Fermi level, improving the magnetic ordering temperature of the non-carbonated compound YFe 9.2 . The calculated magnetic ordering temperatures of Y(Fe,M) 9.2 C decrease with the increasing content of the doping elements M due to the stronger hybridization of the d bands in the carbides. For the rare-earth(RE) iron based intermetallics REFe 9.2 with the TbCu 7 -type structure, it is suggested that Zr is able to stabilize the phase and enhance the magnetic ordering temperature, indicating the possible further application in the field of permanent magnets, which has not been reported before. [ABSTRACT FROM AUTHOR]

The site preference of N atom for R2Fe14BNx (R= Pr, Nd) and the interstitial nitrogen effect on the magnetic properties have been studied by the first-principles method. It was found that the nitrogen is more likely to occupy the 4e site for Pr2Fe14BNx compound, while 4f site for Nd2Fe14BNx. When N atoms entering some specific crystal sites (such as 2a and 4f), the total magnetic moments of these compounds are not reduced, but slightly increased. Although the doping of N may reduce the total magnetic moments of some R2Fe14B compounds in the cases of optimal occupancy, the volumetric effect caused by N doping can still change the electron density distributions of Fe near the Fermi level, improving the magnetic ordering temperature of such compounds. [ABSTRACT FROM AUTHOR]

Using GGA+ U approach based on the density functional theory, the on-site correlation and the electronic structure of wurtzite zinc oxide are investigated. By atomic population analysis, the zinc 3 d orbital is found to be almost fully filled, but the oxygen 2 p orbital is partially filled. So, the zinc oxide can be approximately regarded as a sort of d10 semiconductor. Similar to that of d0 magnets found previously, the on-site Coulomb repulsion of 2 p orbital in such a d10 system is shown to be also indispensable in addition to that of 3 d orbital. As the unconventional on-site Coulomb repulsion of O 2 p is considered, the theoretical results agree with the previous experimental observations satisfactorily. The partial density of states shows significant OZn hybridization in the valence band, but it decreases a little as the on-site correlations are taken into account. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Deep-red-emitting Mn4+ activated oxide phosphors are the hot research topic due to their potential applications in indoor plant cultivation LEDs. In this paper, a novel double-perovskite LiLaMgTeO 6 :Mn4+ phosphor was firstly prepared by high-temperature solid-state reaction method and its structure of the phosphor was determined by X-ray diffraction (XRD) and Rietveld refinement. Density functional theory (DFT) calculations using VASP provide an insight into the bandgap and electronic structures of the LiLaMgTeO 6 host. Under UV or blue excitation, LiLaMgTeO 6: Mn4+ shows a good stability and bright far-red light in the 540–800 nm range with the maximum at 708 nm, which matches well with the absorption band of phytochrome P FR , indicating its potential application in improving plant growth. The optimal Mn4+ doping concentration was approximately 0.6 mol%. Furthermore, a white-LEDs device made of a 365 nm chip coated with present phosphor and commercial green, blue phosphors was designed for indoor plant cultivation. The results indicate that the LiLaMgTeO 6 : Mn4+ phosphor possesses great application prospect in white-LEDs for plant cultivation. Image 1 • Deep-red-emitting Mn4+ activated double perovskite LiLaMgTeO 6 phosphors were firstly synthesized. • The bandgap and electronic structures of the LiLaMgTeO 6 host were performed. • LiLaMgTeO 6 : Mn4+ phosphors exhibited far red light centered on 708 nm excited by ultraviolet light and blue-light. • The crystal field strength and nephelauxetic effect influencing the Mn4+ emission energy were also analyzed. • A white-LEDs device for indoor plant cultivation was manufactured. [ABSTRACT FROM AUTHOR]

A series of red-emitting phosphors SrLaLiTe 1-x O 6 : xMn4+ were prepared via high-temperature solid-state reaction technique. The structure of SrLaLiTeO 6 (abbreviated as SLLT) is a typical cubic double-perovskite structure with the space group of Fm-3m (225). Based on the density functional theory (DFT) calculation, the SrLaLiTeO 6 presents a direct band gap property with the band gap energy (E g) of 3.22eV. The estimated E g of SrLaLiTeO 6 and SrLaLiTe 0.988 O 6 : 0.012Mn4+ were evaluated to be 3.08 eV and 2.98 eV derived from diffuse reflectance spectra, respectively. The emission spectra show a narrow emission band from 650 nm (15384 cm−1) to 770 nm (12987 cm−1) peaking at 696 nm and 708 nm, which exhibits a greater overlap with the phytochrome (P R and P FR) absorption. Furthermore, the nephelauxetic effect of Mn4+ in the SLLT host were estimated in detail by using the crystal field strength (Dq) and Racah parameters (B, C). Finally, the intensity remained at 79.01% and 81.44% of the original intensity when the temperature was elevated at 423 K, respectively, which illustrates good thermal stability. All these results manifest that SrLaLiTeO 6 : Mn4+ is expected to be a candidate for indoor plant cultivation LED. • Red-emitting Mn4+ activated double perovskite SrLaLiTeO 6 phosphors were firstly prepared. • The bandgap and electronic structures of the SrLaLiTeO 6 host were investigated. • SrLaLiTeO 6 : Mn4+ phosphors presented far red light peaking at 696 nm and 708 nm under different excitations. • The nephelauxetic effect of Mn4+ in the SrLaLiTeO 6 host were estimated. • A red-LEDs device for indoor plant cultivation was made. [ABSTRACT FROM AUTHOR]