Ga 4p States Research Articles

A first-principles study on Ga stabilized δ-Pu phase stability based on structural and electronic properties

The high temperature face-centered-cubic delta phase of plutonium (FCC δ-Pu) is metallurgically important in industrial applications and can be stabilized at room temperature by alloying with small amounts of gallium (Ga). Therefore, it is of fundamental interest to elucidate the factors that govern the δ-Pu-Ga interactions via the electronic structure to understand the stabilization mechanism of the FCC phase. We employed density functional theory to systematically model the change in the structural and electronic properties with increasing Ga concentrations of δ-Pu-Ga alloys. The results indicate that structural optimizations with an applied cubic symmetry constraint are sufficient to describe the alloy properties. The variations in Pu-Pu and Pu-Ga bond lengths and the lattice disorder induced by the Ga substitution at Pu sites are in qualitative agreement with previously published experimental EXAFS data. Additionally, in accordance with the experimental results, the atomic volume decreases as the Ga concentration increases. The alloyformation energydecreasesas the Ga concentrationincreases, indicating an increase in stability and an exothermic alloying reaction. The Pu-Ga electronic interaction is governed principally by hybridization between the Pu 6d and Ga 4p states, with decreasing Pu 5f states at the Fermi energy with increasing Ga content due to the increase of Ga-Pu-Ga bonds within the lattice.

Infrared nonlinear optical performances of a new sulfide β-PbGa2S4

Nonlinear optical (NLO) crystals can produce tunable lasers through the direct second-harmonic generation (SHG) process, which is of great significance in modern laser technology. Herein, a new chalcogenide β-PbGa2S4 (1) crystallized in space group of Pna21 was synthesized via high-temperature solid-state reactions. Compound 1 is composed of a three-dimensional framework [Ga8S16]8− with hexagonal voids filled by Pb atoms via covalent Pb–S interactions. Importantly, it exhibits promising NLO performances, such as phase-matchable SHG efficiency (0.6×AgGaS2@1200 nm and 0.1×AgGaS2@1910 nm), moderate band-gap (2.46 eV), suitable laser-induced damage threshold (3.0×AgGaS2), and a broad IR transparency window (0.5–25.0 µm). SHG response of 1 is mainly attributable to the occupied S–3p and Ga–4p states and unoccupied S–3p and Pb–6p states.

Interplay between magnetic moment and magnetocrystalline anisotropy in tetragonally distorted galfenol films

Magnetic properties of Fe100−xGax (x = 15 and 30) epitaxial thin films on GaAs(001) substrates were studied experimentally and theoretically. The samples grown by molecular beam epitaxy under certain conditions adopt body-centered structures with tetragonal distortion along the film normal with a c/a ratio varying from 1.014 to 1.036. The density functional theory (DFT) calculations were performed for different (c/a) ratio values and Ga contents. DFT results showed that Fe atoms placed at the first neighborhood of Ga positions present a much stronger distortion in the local exchange-correlation field than second neighbor Fe atoms. The formation of non-bonding Fe 3d states in the minority spin sub-band and the predominance of hybridization of Ga 4p states with Fe 4p states due to Ga content cause this magnetic behavior. The in-plane magnetic anisotropies are explained in terms of the exchange-correlation field isosurfaces. As a result of that, the global magnetic anisotropy can be described as cubic magnetocrystalline mixed with sixfold anisotropy.

Ab Initio Study of the Effect of Mono-Vacancies on the Metastability of Ga-Stabilized δ-Pu

Most experimental studies on metallic Pu are on the room temperature monoclinic α-phase or the fcc Ga stabilized δ-phase. Stabilized δ-phase Pu-Ga alloys are metastable and exhibit a martensitic phase transformation to α’-phase at low temperatures, or applied shear, with concentrations lower than three atomic percent Ga. By using first principles, we explore the metastability of δ-phase by investigating the structural and electronic behavior induced by Ga alloying and by a mono-vacancy point defect. We find that a site substitutional Ga induces a tetragonal distortion in the lattice affected by hybridization of Ga 4p and Pu 6d states. With the addition of a mono-vacancy, a monoclinic or tetragonal distortion forms locally (dependent on its distance from Ga), and decoupling of the Pu 5f and 6d states and broadening of the 6d states occurs. This response enables hybridization of Pu 6d with the Ga 4p states affecting the mono-vacancy formation energy. Thus, stabilization of the fcc lattice correlates with hybridization of Pu 6d states with Ga 4p states, and this becomes more evident in the presence of a mono-vacancy.

Comprehensive first-principles calculations on physical properties of ScV2Ga4 and ZrV2Ga4 in comparison with superconducting HfV2Ga4

We use first-principles method for a comprehensive investigation of structural, elastic, electronic, Mulliken atomic population based bonding, phonon, thermodynamic and optical properties of ScV2Ga4 and ZrV2Ga4 in comparison with the isostructural HfV2Ga4 superconductor. The single and polycrystalline elastic constants are all evaluated and discussed. Cell stability employing elastic constants criteria, formation enthalpy and phonon dispersion are verified. Brittleness and hardness features are also discussed. Elastic anisotropies are evaluated and discussed to unravel some of the macroscopic behaviors of the studied compounds. The significant overlapping between conduction and valence bands in the vicinity of Fermi level (EF) confirms the metallic characteristics for MV2Ga4 (M = Hf, Sc, Zr). It is predicted that the TDOS around the EF arises mostly from V–3d with a smaller contribution of Ga–4p states. The Fermi surfaces show multiband nature but that of ScV2Ga4 has different and complex shape in contrast to ZrV2Ga4 and superconducting HfV2Ga4. The bonding characteristics are analyzed using the energy band structure, charge density distribution and Mulliken atomic and bond overlap population. Such analysis exhibits directional electronic density-channel, highly populated 1D V leashed in (100) plane in the lattices. The formation of such V-chains which accumulate the highest electronic charge might generate Cooper pairs to exhibit superconducting behavior in the yet unobserved Sc and Zr containing compounds. The phonon dispersion curves, phonon density of states and optical phonon frequencies, Raman and infrared activities of the materials are discussed for the first time. The temperature dependent thermodynamic properties including heat capacity at constant pressure are calculated as a function of temperature and compared with the available data. Various optical properties are calculated and their features are analyzed. The potentials of the compounds for practical uses as coating materials to reduce solar heating and enhance radiative cooling are also mentioned.

Synthesis, spectroscopic and luminescence properties of Ga–doped γ–Al2O3

Gallium-doped aluminum oxide (Al1-xGax)2O3 with γ-Al2O3 (spinel) structure has been synthesized by the precursor method using the formate Al1-xGax(OH)(HCOO)2 as a precursor. The examination of Al1-xGax(OH)(HCOO)2 (x = 0.0, 0.1, 0.2, 0.3, 0.4) was carried out by X-ray powder diffraction, Infrared, Raman spectroscopy and differential-thermal methods. The solid solutions γ-(Al1-xGax)2O3 with х≤0.2 have been synthesized by thermolysis of precursors in helium atmosphere at 700 °C; they exhibit white-blue emission under UV excitation, whose intensity lowers with increasing dopant concentration. As an independent method, the DFT calculations confirmed thermodynamically the stability field of γ-(Al1-xGax)2O3 solid solutions and the NMR data on relative abundance of Al and Ga within the tetrahedral and octahedral sites in the metal sublattice. Furthermore, the structural and thermodynamic properties of carbon-containing impurities within these compounds were suggested theoretically as possible models of luminescence emission centers. The experimentally observed Ga-dependent quenching of luminescence was explained using the competition between C2p and Ga4p states within the band gap of γ-Al2O3.

Electronic structure and optical properties of (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals, novel light-converting materials

Electronic structure and optical properties were studied for novel (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals synthesized by solution-melt technique. In particular, X-ray photoelectron spectroscopy (XPS) was used to measure core-level binding energies and valence-band spectra for as-synthesized and Ar+ ion-irradiated surfaces of these crystals. Presented XPS measurements show that the (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals are rather stable in relation to Ar+ ion-irradiation. X-ray emission (XE) S Kβ1,3 and Ga Kβ2 bands were measured for the (Ga70La30)2S300 crystal giving information on the energy distribution of the S 3p and Ga 4p states, respectively. A comparison of these XE bands on a common energy scale with the XPS valence-band spectrum of (Ga70La30)2S300 indicates that the principal contribution of the S 3p and Ga 4p states occurs mainly at the top and in the central part of the valence band, respectively. In addition, optical absorption and photoluminescence spectra of the crystals were explored. Energy band gap values are estimated as 2.01 and 1.99 eV at room temperature for the (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 crystals, respectively. Observed high-intensity green photoluminescence band when excited by a laser emitting at 810 nm suggests that the (Ga69.75La29.75Er0.5)2S300 crystal is a very attractive material for infrared to visible light conversion.

PbGa2GeS6 crystal as a novel nonlinear optical material: Band structure aspects

New quaternary sulfide PbGa2GeS6 crystal was synthesized from co-melting high-purity elements The studies of second harmonic generation and the third harmonic generation for new quaternary sulfide PbGa2GeS6 crystal have shown that its nonlinear optical response is higher with respect to other similar compounds. The band structure analysis performed by X-ray spectroscopy methods and first principles DFT band structure calculations indicate that the main contributions of the S 3p states are located at the top of valence band, while those of the Ga 4p states give contribution to the central and upper portions of the valence band of the PbGa2GeS6 compound. The calculations reveal that the band gap Eg = 2.445 eV is indirect and is formed between the valence Γ-point and the conduction X-point of Brillouin zone. The theoretically evaluated band gap energy is close to the experimental value, namely Eg = 2.37 eV at 300 K The present results allow recommending PbGa2GeS6 for nonlinear optical application in the near IR spectral range. At the same time, the crystal possesses a good transparency in the mid-IR spectral range.

Electronic structure and optical properties of defect chalcopyrite HgGa2Se4

We report on studies from an experimental and theoretical viewpoint of the electronic structure of mercury digallium selenide, HgGa2Se4, a very promising optoelectronic material. In particular, the method of X-ray photoelectron spectroscopy (XPS) was used to evaluate binding energies of the constituent element core electrons and the shape of the valence band for pristine and Ar+-ion bombarded surfaces of HgGa2Se4 single crystal. First principles band-structure calculations were performed in the present work using the augmented plane wave + local orbitals (APW+lo). These calculations indicate that the Se 4p states are the main contributors at the top and in the upper portion of the valence band with slightly smaller contributions of the Ga 4p states in the upper portion of the band as well. Further, the central portion of the valence band is determined mainly by contributions of the Ga 4s states, and the Hg 5d states are the principal contributors to the bottom of the valence band. These theoretical data are in fair agreement when matching on a common energy scale of the X-ray emission bands giving information on the energy distribution of the Se 4p and Ga 4p states and the XPS valence-band spectrum of the HgGa2Se4 crystal. The principal optical constants are elucidated from the DFT calculations.

AgGaSiSe4: Growth, crystal and band electronic structure, optoelectronic and piezoelectric properties

Growth of AgGaSiSe4 single crystals, determination of their crystal structure, studies of electronic structure and optical properties are presented. The X-ray photoelectron core-level and valence-band spectra were measured. We recorded the X-ray emission Se Kβ2 and Ga Kβ2 bands, giving information on the energy distribution of the Se4p and Ga4p states, respectively, and compared with the X-ray photoelectron valence-band spectrum. The main contributions of the Se4p and Ga4p states have been found in the upper and central parts of the valence band, respectively, with their substantive contributions in other parts of the band. The spectral distribution of the absorption coefficient in the temperature range 100–300K was investigated and band gap variation coefficient was evaluated (dEg/dT=−4.5×10−4eV/K). The activation energies of dark conductivity were calculated as follows: Ea1=0.03eV, Ea2=0.24eV. The IR transmission curve and the relaxation kinetics of photoinduced piezoelectric module were described.

Electrical and optical properties of Si-doped Ga2O3

The charge densities, band structure, density of states, dielectric functions of Si-doped [Formula: see text]-Ga2O3 have been investigated based on the density functional theory (DFT) within the hybrid functional HSE06. The heavy doping makes conduction band split out more bands and further influences the band structure. It decreases the band gap and changes from a direct gap to an indirect gap. After doping, the top of the valence bands is mainly composed by the O-2p states, Si-3p states and Ga-4p states and the bottom of the conduction bands is almost formed by the Si-3s, Si-3p and Ga-4s orbits. The anisotropic optical properties have been investigated by means of the complex dielectric function. After the heavy Si doping, the position of absorption band edges did not change much. The slope of the absorption curve descends and indicates that the absorption became more slow for Si-doped [Formula: see text]-Ga2O3 than undoped one due to the indirect gap of Si-doped [Formula: see text]-Ga2O3.

Electronic structure and optical properties of noncentrosymmetric LiGaSe2: Experimental measurements and DFT band structure calculations

We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.

Modification of spin electronic properties of Fen/GaSe monolayer adsorption system

Group-ⅢA metal-monochalcogenides have been extensively studied due to their unique optoelectronic and spin electronic properties. To realize the device applications, modifying their magnetic properties is desirable. Atomic doping and vacancy defects have been proven to produce itinerant ferromagnetism and half-metallicity in GaSe monolayer. Relatively, the magnetic modification by adsorbing foreign atoms is rarely reported. Traditional ferromagnetic material, Fe element, possessing high electronic polarizability and high Curie temperature, becomes the best option of adsorbate. In this work, Fen(n=1, 2) atoms adsorbed GaSe monolayer systems are constructed, and the spin electronic properties are systematically studied through the density function theory. Based on the geometric configuration of fully relaxed 33 GaSe supercell, three highly symmetrical sites, i.e., the hollow site, the top site of Se atom, and the top site of Ga atom are inspected to search for the stable absorption positions of Fen atoms. Computation results of adsorption energies indicate that the top site of Ga atom is preferred for single Fe atom, and the hollow site near the first Fe adatom is the most stable site serving as adsorbing the second Fe atom. Based on the most stable configuration, the spin electronic properties are studied. For the single Fe adsorbed system, the valence band maximum moves to point, resulting in a direct-band-gap. The strong orbit coupling effect between Fe adatom and its nearest Ga and Se atoms causes un-coincident majority and minority spin channels. Two impurity bands are located near the Fermi level and contribute only to the minority spin channel, producing a half-metallicity with a 100% spin polarization in the system. Bader charge analysis and spin-resolved partial density of states suggest that the spin polarization is mainly attributed to the transfer of Fe-3d electrons, and the hybridizations of Fe-3d, Se-4p, and Ga-4p states. Charge transfer from the Fe adatom to GaSe generates an n-type doping and an antiferromagnetic coupling between Fe and vicinal Ga and Se atoms. For the two-Fe-atoms adsorbed GaSe monolayer, the spin electronic states are found to be mainly located between the two Fe adatoms, leading to the reduction of the charge transfer from Fe to GaSe ML. As the original single spin channel turns into two spin channels (majority spin channel and minority spin channel) near the Fermi level, the ferromagnetic coupling between Fe atom and the vicinal Se atoms turn into antiferromagnetic coupling and the spin polarization falls to 0%. Therefore, the spin properties of GaSe monolayer can be controlled by modifying the number of adsorbed Fe atoms. These results reveal the formation and transform of the spin electronic properties of typical ferromagnetic/GaSe adsorption system, which offers some advice for designing and constructing the two-dimensional spin nanostructures.

Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag2Ga2SiS6 compound

For the first time phase equilibria and phase diagram of the AgGaS2–SiS2 system were successfully explored by differential thermal and X-ray phase analysis methods. Crystal structure of low-temperature (LT) modification of Ag2Ga2SiS6 (LТ- Ag2Ga2SiS6) was studied by X-ray powder method and it belongs to tetragonal space group I-42d, with unit cell parameters a=5.7164(4) Å, c=9.8023(7) Å, V=320.32(7) Å3. Additional details regarding the crystal structure exploration are available at the web page Fachinformationszentrum Karlsruhe. X-ray photoelectron core-level and valence-band spectra were measured for pristine LТ- Ag2Ga2SiS6 crystal surface. In addition, the X-ray photoelectron valence-band spectrum of LТ-Ag2Ga2SiS6 was matched on a common energy scale with the X-ray emission S Kβ1,3 and Ga Kβ2 bands, which give information on the energy distribution of the S 3p and Ga 4p states, respectively. The presented X-ray spectroscopy results indicate that the valence S p and Ga p atomic states contribute mainly to the upper and central parts of the valence band of LТ-Ag2Ga2SiS6, respectively, with a less significant contribution also to other valence-band regions. Band gap energy was estimated by measuring the quantum energy in the spectral range of the fundamental absorption. We have found that energy gap Eg is equal to 2.35eV at 300K. LT-Ag2Ga2SiS6 is a photosensitive material and reveals two spectral maxima on the curve of spectral photoconductivity spectra at λmax1 =590nm and λmax2 =860nm. Additionally, linear electro-optical effect of LT-Ag2Ga2SiS6 for the wavelengths of a cw He-Ne laser at 1150nm was explored.

First-Principles Study of CuGaO2 Polymorphs: Delafossite α-CuGaO2 and Wurtzite β-CuGaO2.

The electronic structures of delafossite α-CuGaO2 and wurtzite β-CuGaO2 were calculated based on density functional theory using the local density approximation functional including the Hubbard correction (LDA+U). The differences in the electronic structure and physical properties between the two polymorphs were investigated in terms of their crystal structures. Three major structural features were found to influence the electronic structure. The first feature is the atomic arrangements of cations. In the conduction band of α-CuGaO2 with a layered structure of Cu2O and Ga2O3, Cu and Ga states do not mix well; the lower part of the conduction band mainly consists of Cu 4s and 4p states, and the upper part consists of Ga 4s and 4p states. By contrast, in β-CuGaO2, which is composed of CuO4 and GaO4 tetrahedra, Cu and Ga states are well-mixed. The second feature is the coordination environment of Cu atoms; the breaking of degeneracy of Cu 3d orbitals is determined by the crystal field. Dispersion of the Cu 3d valence band of β-CuGaO2, in which Cu atoms are tetrahedrally coordinated to oxygen atoms, is smaller than those in α-CuGaO2, in which Cu atoms are linearly coordinated to oxygen atoms; this results in a larger absorption coefficient and larger hole effective mass in β-CuGaO2 than in α-CuGaO2. The interatomic distance between Cu atoms-the third feature-also influences the dispersion of the Cu 3d valence band (i.e., the effective hole mass); the effective hole mass decreases with decreasing interatomic distance between Cu atoms in each structure. The results obtained are valuable for understanding the physical properties of oxide semiconductors containing monovalent copper and silver.

Electronic structure of (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals

We report on studies of the X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion bombarded surfaces of (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals grown by solution-melt technique. The (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals were found to be very rigid with respect to Ar+-ion bombardment. In particular, such a treatment did not cause any significant changes of values of the binding energies of core-level electrons as well as of the shapes of the XPS core-level and valence-band spectra of (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystal surfaces. We measured for the (Ga55In45)2S300 compound the X-ray emission bands representing the energy distribution of mainly the valence S p and Ga p states and compared them on a common energy scale with the XPS valence-band spectrum. The comparison allows us for concluding that the S 3p and Ga 4p states contribute mainly at the top and in the central portion of the valence band of (Ga55In45)2S300, respectively. In addition, we have also detected at ambient conditions a strong photoluminescence of the (Ga54.59In44.66Er0.75)2S300 single crystal with a maximum at 810 nm for the excitation wavelength of 532 nm. Our results indicate that the (Ga54.59In44.66Er0.75)2S300 single crystal is a very promising material for its application in optoelectronic technologies.

Understanding oxygen adsorption on 9.375 at. % Ga-stabilized δ-Pu (111) surface: A DFT study

Plutonium (Pu) metal reacts rapidly in the presence of oxygen (O), resulting in an oxide layer that will eventually have an olive green rust appearance over time. Recent experimental work suggested that the incorporation of gallium (Ga) as an alloying impurity to stabilize the highly symmetric high temperature δ-phase lattice may also provide resistance against corrosion/oxidation of plutonium. In this paper, we modeled a 9.375 at. % Ga stabilized δ-Pu (111) surface and investigated adsorption of atomic O using all-electron density functional theory. Key findings revealed that the O bonded strongly to a Pu-rich threefold hollow fcc site with a chemisorption energy of −5.06 eV. Migration of the O atom to a Pu-rich environment was also highly sensitive to the surface chemistry of the Pu–Ga surface; when the initial on-surface O adsorption site included a bond to a nearest neighboring Ga atom, the O atom relaxed to a Ga deficient environment, thus affirming the O preference for Pu. Only one calculated final on-surface O adsorption site included a Ga–O bond, but this chemisorption energy was energetically unfavorable. Chemisorption energies for interstitial adsorption sites that included a Pu or Pu–Ga environment suggested that over-coordination of the O atom was energetically unfavorable as well. Electronic structure properties of the on-surface sites, illustrated by the partial density of states, implied that the Ga 4p states indirectly but strongly influenced the Pu 6d states to hybridize with the O 2p states. The Ga 4p states also weakly influenced the Pu 5f states to hybridize with the O 2p states, even though Ga was not participating in bonding with O.

Surface analysis of Ga-doped 3D urchin-like ZnO structure and density functional theory investigation of the quenching in photoluminescence property

Ga-doped 3D urchin-like ZnO structures with different morphologies have been synthesized by thermal evaporation with different molar ratio gallium oxide as the dopant source. The morphology, microstructure and chemical state were determined by field emission scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). The investigation confirmed that the morphologies of the 3D urchin-like ZnO structure had a significant transform with the increase of the dopant. The XPS result indicates that Ga is incorporated into the ZnO lattice. photoluminescence emission peak has been found to be quenched remarkably caused by 5mol% Ga2O3 dopant. The reason of the emission quench is discussed by means of density functional theory calculations. It is found that O-2p states slightly hybridize with Zn-3d and Ga-4s, Ga-4p states giving rise to the conduction band downward and introduction of the correlative impurity levels, which will result in the increase of non-radiative transition.

Syntheses, Structures, and Nonlinear Optical Properties of Quaternary Chalcogenides: Pb4Ga4GeQ12 (Q = S, Se)

Two noncentrosymmetric isostructural compounds Pb4Ga4GeQ12 (Q = S, Se) with their own structure type have been synthesized by solid-state reactions at high temperature. They crystallize in the tetragonal space group P42(1)c (No. 114) with a = 12.673(2) Å and c = 6.128(2) Å, and a = 13.064(7) Å and c = 6.310(5) Å, respectively, and Z = 2. The major structure motif features a three-dimensional framework constructed by chains of GaQ4 tetrahedra that are interconnected by separated GeQ4 tetrahedra at regular intervals. Interestingly, such a [Ga4GeQ12](8-) framework is flexible to allow the addition of Ag(+) or Li(+) to occupy the embedded A- or B-type of vacancies to generate the previously reported [AgGa5Q12](7-) or [LiGa5Q12](7-) interstitial compounds without symmetry breaking. The title compounds (Q = S, Se) have optical band gaps of 2.35 and 1.91 eV, respectively, and wide IR transparent regions of 0.80-22.5 and 0.75-22.5 μm, respectively. Significantly, the powder Pb4Ga4GeSe12 sample exhibits a strong second-harmonic-generation (SHG) response that is ∼2 times that of the benchmark AgGaS2 at a laser radiation of 2.05 μm with a non phase-matchable behavior. The calculated d36 coefficient agrees well with the experimental observation. The density functional theory (DFT) calculations suggest that the SHG response originates from the electronic transitions from Se 4p states to Pb 6p, Ga 4p, and Ge 4p states.

Electronic properties on GaN(0001) surface – ab initio investigation

Density Functional Theory (DFT) calculations were used to determine physical properties of bulk GaN and GaN(0001), i.e. N-terminated surface. The bulk data are in agreement with the theoretical and experimental data reported by Magnuson et al. [M. Magnuson et al., Phys. Rev. B 81 (2010) 085125], demonstrating that bonding in GaN is of a mixed nature in which Ga 4s, Ga 4p, Ga 3d, and N 2s, N 2p orbitals are involved. Ga 4s and Ga 4p states are sp3 hybridized while the N states are involved independently in bonding. Ga sp3 states overlap with the N 2s states creating the upper subband extending down to 6 eV below the valence band maximum (VBM). The lower valence band (VB) subband is composed of Ga 3d states overlapping with N 2s states whose energy levels range from 13 to 17 eV below VBM. GaN bulk bonding enforces creation of the two surface states at clean GaN(0001) surface: the lower energy state, located 15 eV below VBM is composed of N 2s states. The upper energy surface state, located below VBM, is composed of N 2pz nonsaturated state, i.e. it is of the broken bond type. The Fermi energy is pinned by this dispersionless acceptor state. Ga adsorption leads to emergence of donor state, which pins the Fermi level at 1.1 eV above VBM which is compatible with the Fermi level pinned at the surface for p-type GaN and is inconsistent with n-type bulk. The total energy of slab has the energy lower by about 2 eV for the adsorbed than for the separate Ga atom. The inconsistent data obtained from thermodynamic stability and band diagram analyses suggest further investigations to explain inconsistencies between these two pictures.