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1.
Summary The chemical bond in the electron-deficient semiconductor series Mg2X (X=Si, Ge, Sn) is analysed in the pseudopotential self-consistent local-density scheme. Hard-core atomic potentials are used to investigate the physical properties of these compounds and to show the regularities peculiar to the periodic table. The ionic character of these materials is studied and related to the scaling electronegativities of their anions. Our results, in good agreement with the experiment, are discussed in order to explain the molecular behaviour of the bonding conduction states.
Riassunto Si studia il legame chimico nella serie dei semiconduttori mancanti di elettroni Mg2X, dove X=Si, Ge e Sn, con il metodo dello pseudopotenziale autocoerente nello schema del funzionale densità locale. Le proprietà fisiche dei composti in esame sono studiate utilizzando potenziali atomici che permettono di evidenziarne le caratteristiche di periodicità. Il carattere ionico di questi materiali è correlato all'elettronegatività degli anioni. I nostri risultati, in buon accordo con l'esperimento, sono discussi in modo da spiegare il comportamento molecolare degli stati di conduzione.
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2.
By means of density functional theory computations, we predicted two novel two-dimensional (2D) nanomaterials, namely P2X (X=C, Si) monolayers with pentagonal configurations. Their structures, stabilities, intrinsic electronic, and optical properties as well as the effect of external strain to the electronic properties have been systematically examined. Our computations showed that these P2C and P2Si monolayers have rather high thermodynamic, kinetic, and thermal stabilities, and are indirect semiconductors with wide bandgaps (2.76 eV and 2.69 eV, respectively) which can be tuned by an external strain. These monolayers exhibit high absorptions in the UV region, but behave as almost transparent layers for visible light in the electromagnetic spectrum. Their high stabilities and exceptional electronic and optical properties suggest them as promising candidates for future applications in UV-light shielding and antireflection layers in solar cells.  相似文献   

3.
This article demonstrates the first reported successful synthesis of Mg2SiO4 nanowires. We have thermally heated Au-coated Si substrates, using a quartz tube with its inner surface pre-coated with MgO nanostructures. We have characterized the sample morphologies by using scanning electron microscopy and transmission electron microscopy (TEM). X-ray diffraction analysis and high-resolution TEM observation coincidentally revealed that the nanowires were crystalline with an orthorhombic Mg2SiO4 structure. We have discussed the possible growth mechanism of Mg2SiO4 nanowires. PACS 81.07.-b; 81.05.Zx; 61.10.Nz; 68.37.Hk; 68.37.Lp  相似文献   

4.
The sample of Mg0. 5+y (Zr1-y Fey) 2 (PO4) 3 (0.0 ≤y ≤0.5) was synthesized using the sol-gel method. The structures of the samples were investigated using X-ray diffraction and Fourier transform infrared spectroscopy measurement. XRD studies showed that samples had a monoclinic structure which was iso-structured with the parent compound, Mg0.5Zr (PO4) 3. The complex impedance spectroscopy was carried out in the frequency range 1–6 MHz and temperature range 303 to 773 K to study the electrical properties of the electrolytes. The substitutions of Fe3+ with Zr4+ in the Mg0.5Zr (PO4) 3 structure was introduced as an extrainterstitial Mg2+ ion in the modified structured. The compound of Mg0.5+y (Zr1-y Fey)2(PO4)3 with y?=?0.4 gives a maximum conductivity value of 1.25?×?10?5 S cm?1 at room temperature and 7.18?×?10?5 S cm?1 at 773 K. Charge carrier concentration, mobile ion concentration, and ion hopping rate are calculated by fitting the conductance spectra to power law variation, σ ac (ω)?=?σ o ? +?Aω α . The charge carrier concentration and mobile ion concentration increases with increase of Fe3+ inclusion. This implies the increase in conductivity of the compounds was due to extra interstitial Mg2+ ions.  相似文献   

5.
The transport properties of Mg2 X 0.4Sn0.6 (X = Si, Ge) solid solutions are investigated. It is shown that these materials can be rendered p-type with a hole concentration of up to 4 × 1019 cm?3. The Hall coefficient, thermopower, and electrical conductivity are measured over a wide temperature range. The mobility of holes in these solid solutions is less than that of electrons by a factor of 2 for Mg2Si0.4Sn0.6 and by a factor of 1.5 for Mg2Ge0.4Sn0.6. Solid solutions in the Mg2Ge-Mg2Sn system appear more promising for thermoelectric applications.  相似文献   

6.
Gd-doped HfO2 films were deposited on p-type silicon substrates in a reducing atmosphere. Photoemission measurements indicate the n-type character of Gd-doped HfO2 due to overcompensation with oxygen vacancies. The Gd 4f photoexcitation peak at 5.5 eV below the valence band maximum is identified using both resonant photoemission and first-principles calculations of the f hole. The rectifying (diode-like) properties of Gd-doped HfO2 to silicon heterojunctions are demonstrated. PACS 79.60.-i; 68.55.Ln; 29.40.Wk; 81.05.Je  相似文献   

7.
We studied the formation energy and atomic structure of impurities in Mg2Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg2Sn and La-doped Mg2(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg2Sn.  相似文献   

8.
Emission spectra of three Cd0.6Mn0.4Te/Cd0.5Mg0.5Te superlattices with Cd0.6Mn0.4Te quantum-well (QW) widths of 7, 13, and 26 monolayers, respectively, and the same thickness (46 monolayers) of the Cd0.5Mg0.5Te barriers have been studied. The QW width affects the shape and spectral position of the Mn2+ intracenter luminescence (IL) band as a result of the crystal field being dependent on the position of the manganese ion with respect to the interface. Measured in identical experimental conditions, the exciton luminescence as compared to the IL is substantially higher in intensity in a QW than in a bulk CdMnTe crystal. Some samples of superlattices and bulk crystals exhibit, in addition to the conventional IL band near 2.0 eV, a weaker band at about 1.45 eV. This band apparently derives from intracenter transitions in the Mn2+ ions in the regions where the crystal lattice has the rock-salt rather than the conventional zinc blende structure.  相似文献   

9.
This paper reports the investigation results on directional growth of Al−Mg2Si eutectic alloy in the SZ2 unmanned spaceship. The analysis shows that microgravity has influence on the directional growth of Al−Mg2Si eutectic alloy. The eutectic colony in space-grown sample is larger than that in ground-grown sample, and the directional effect is not so good as that of terrestrial sample. But there is no obvious difference in the lamellar thickness between these samples, and lamellar interspacing in space sample is smaller than that grown on the ground. A simple discussion is done on the gravity effect on the directional growth of Al−Mg2Si eutectic alloy.  相似文献   

10.
The electronic structure and topological properties of the AIIMg2Bi2 (AII = Mg,Ca,Sr,Ba) compounds are theoretically studied with the use of exact exchange. It is found that the Mg3Bi2 compound in the equilibrium state is a semimetal, whereas three other compounds are semiconductors with a direct fundamental band gap. It is predicted that the uniaxial deformation of three-component compounds results in transitions to topologically nontrivial phases: topological insulator and topological and Dirac semimetals. Owing to such a rich variety of topologically nontrivial phases, these compounds may be of interest for further theoretical and experimental studies.  相似文献   

11.
A novel kind of nano-sized TiO2 (anatase) was obtained by high-temperature (400–700°C) dehydration of nanotube titanic acid (H2Ti2O4(OH)2, NTA). The high-temperature (400–700°C) dehydrated nanotube titanic acids (HD-NTAs) with a unique defect structure exhibited a p-type semiconductor behavior under visible-light irradiation ( nm, E photon=2.95 eV), whereas exhibited an n-type semiconductor behavior irradiated with UV light ( nm, E photon=3.40 eV).  相似文献   

12.
Zn2SnO4 (ZTO) is a stable semiconductor in ZnO–SnO2 system and important transparent conducting oxide (TCO) predominantly used in optoelectronic devices. ZTO thin films were prepared by RF magnetron sputtering using Zn2SnO4 ceramic target in this paper. The effects of annealing temperatures and oxygen contents on characterization of ZTO thin films were studied. The results show that ZTO thin films prepared by RF magnetron sputtering are amorphous with an optical band gap of 3.22 eV. After annealing at 650°C in Ar atmosphere for 40 min, ZTO films possess a spinel structure with an optical band gap of 3.62 eV. The atomic force microscope (AFM) data of morphology reveals that the surface roughness of films is about 2 nm. The results of energy dispersive spectrometer (EDS) show that the concentration ratio of Zn to Sn is in the range from 1.44 to 1.57. The results of Hall-effect-measurement system reveal that the resistivity of films varies from 102 to 10–1 Ωcm, carrier concentration is about 1017 cm–3, and mobility ranges from 100 to 101 cm2 v–1 s–1.  相似文献   

13.
The spectra of complex permittivity of a Ba2Mg2Fe12O22 single crystal belonging to the family of Y-type hexaferrites have been measured over a wide temperature range (10–300 K) with the aim of determining the dynamic parameters of the phonon and magnetic subsystems in the terahertz and infrared frequency ranges (3–4500 cm−1). A factor-group analysis of the vibrational modes has been performed, and the results obtained have been compared with the experimentally observed resonances. The oscillator parameters of all nineteen phonon modes of E u symmetry, which are allowed by the symmetry of the Ba2Mg2Fe12O22 crystal lattice, have been calculated. It has been found that, at temperatures below 195 and 50 K, the spectral response exhibits new absorption lines due to magnetic excitations.  相似文献   

14.
The objective of the present study was to investigate the effects of sintering temperature on the structure and electrochemical performance of Mg2SiO4 cathode materials using sol-gel method. X-ray diffraction and Fourier-transform infrared analysis were used to study the structural properties of the materials. The temperatures applied in the sintering process influenced the structure, morphology, as well as particle size distribution of the Mg2SiO4. All samples sintered at temperatures of 900, 1000, and 1100 °C yielded pure Mg2SiO4 compounds consisting of orthorhombic crystalline phase with a space group of Pbnm. Particle size and lattice parameters of Mg2SiO4 samples increased with the increases of sintering temperature due to an increase of the nucleation and crystal growth rates. The cyclic voltammetry analysis showed the presence of redox reaction. This result shows that the Mg2SiO4 material has potential to be used as cathode materials in magnesium rechargeable batteries.  相似文献   

15.
The relationship between structure and pressure for MgSiN2 has been simulated using a density functional approximation within the local density approximation. The low-pressure (LP-) phase of MgSiN2 with an orthorhombic structure transforms to a high-pressure (HP-) modification with a CsICl2-type structure at a pressure of about 16.5 GPa. HP-MgSiN2 , in which both Mg and Si are octahedrally coordinated by N, has a bulk modulus of about 238 GPa, much higher than that of the LP-modification (about 182 GPa) with tetrahedrally coordinated metal atoms. HP-MgSiN2 is a wide-gap semiconductor with an indirect energy gap of about 4.3 eV, similar to that of LP-MgSiN2. The direct gap at is about 5.8 eV. PACS 71.15.Mb; 61.50.Ks; 61.50.Ah  相似文献   

16.
We have studied the electronic structure, magnetic and transport properties of some Co based full Heusler alloys, namely Co2TiZ (Z=Si, Ge and Sn), in the frame work of first-principle calculations. The calculations show that Co2TiZ (X=Si, Ge and Sn) are to be half-metallic compounds with a magnetic moment of 2 μB, well consistent with the Slater-Pauling rule. The electronic structure results reveal that Co2TiZ has the high density of states at the Fermi energy in the majority-spin state and show 100% spin polarization. Our results also suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The origin of energy gap in the minority-spin states is discussed in terms of the electron splitting of Z (Z=Si, Ge and Sn) and 3d Co atoms and also the d-d hybridization between the Co and Ti atoms. The transport properties of these materials are discussed on the basis of Seebeck coefficients, electrical conductivity coefficients and thermal conductivity coefficients.  相似文献   

17.
A discussion of optical properties of mixed oxides In2O3—SnO2 system is presented. Film thickness, substrate temperature, composition (in molar %) and annealing have a profound effect on the structure and optical properties of these films. Initially the increase in band gap with the increase of SnO2 content in In2O3 is due to the increase in carrier density as a result of donor electrons from tin. The decrease in band gap above the critical Sn content is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in band gap with film thickness is caused by the increase in free carrier density which is generated by (i) Sn atom substitution of In atom, giving out one extra electron and (ii) oxygen vacancy acting as two electrons donor. The decrease in band gap with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorate the film properties and reduce the mobility of the carriers, or to the formation of indium species of lower oxidation state (In2+).  相似文献   

18.
The structural and electronic properties of neutral and negatively charged Ga12X (X=C, Si, Ge, Sn, and Pb) clusters are calculated by the first-principles method. The results show that the most stable symmetry depends on the doped atom rather than the geometry structure. However, the geometry symmetry plays an important role in calculating the energy gap. In addition, in the anionic clusters, the added electron would reduce the energy gap by about 0.4 eV. As for the density of states (DOS), clusters with the same symmetry show a similar trend of DOS. The major impact on DOS by adding an electron is the occurrence of relative energy shift.  相似文献   

19.
The structural parameters, density of states, electronic band structure, charge density, and optical properties of orthorhombic SrBi2Ta2O9 have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principle density functional theory (DFT). The calculated structural parameters were in agreement with the previous theoretical and experimental data. The band structure showed an indirect (S to Γ) band gap with 2.071 eV. The chemical bonding along with population analysis has been studied. The complex dielectric function, refractive index, and extinction coefficient were calculated to understand the optical properties of this compound, which showed an optical anisotropy in the components of polarization directions (100), (010), and (001).  相似文献   

20.
High resolution synchrotron radiation X-ray powder diffraction was used to investigate the melting of the cooperative Jahn-Teller distortion (cJTd) in a perovskite of composition KCu0.8Mg0.2F3. A first order phase transition relaxing the cJTd is observed at T ∼ 600 K. From the transition temperature, an estimation of kT is derived (kT = 0.05 eV) for the cJTd in the doped compound. This is the very first observation of cJTd melting in a compound of the series KCu1−x Mg x F3. A structural phase diagram for the Cu rich zone of the series is proposed. In principle, the extension of this experimental investigation could be used to disentangle the orbital order (OO) and cJTd energy scales in the parent compound KCuF3, a test system for OO theories.  相似文献   

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