Electrical conduction behaviors of isovalent and acceptor dopants on B site of (La 0.8Ca 0.2)CrO 3− δ perovskites

The density of states of a particle in a 2D area is independent both of the energy and form of the area only at the region of large values of energy. If energy is small, the density of states in the rectangular potential well essentially depends on the form of the area. If the bottom of the potential well has a potential relief, it can define the small eigenvalues as the discrete levels. In this case, dimensions and form of the area would not have any importance. If the conservation of zero value of the angular momentum is taken into account, the effective one-particle Hamiltonian for the 2D electron gas in the magnetic field in the circle is the Hamiltonian with the parabolic potential and the reflecting bounds. It is supposed that in the square, the Hamiltonian has the same view. The 2D density of states in the square can be computed as the convolution of 1D densities. The density of one-particle states for 2D electron gas in the magnetic field is obtained. It consists of three regions. There is a discrete spectrum at the smallest energy. In the intervening region the density of states is the sum of the piecewise continuous function and the density of the discrete spectrum. At great energies, the density of states is a continuous function. The Fermi energy dependence on the magnetic field is obtained when the field is small and the Fermi energy is located in the region of continuous spectrum. The Fermi energy has the oscillating correction and in the average it increases proportionally to the square of the magnetic induction. Total energy of electron gas in magnetic field also oscillates and increases when the magnetic field increases monotonously.

A concise procedure to determine the self-consistent Fermi energy and defect and carrier concentrations in an extended crystalline system is presented. It is assumed that the formation enthalpies of a set of variously charged point defects in thermodynamic equilibrium are known, as well as the density of electronic states in the defect-free system. By applying the constraint of overall charge neutrality, the self-consistent Fermi energy is determined using an iterative searching routine. The procedure is incorporated within a Fortran code ‘SC-FERMI’: the input consists of the defect formation energies, density of sites where they can form, and the degeneracy of each charge state; the material band gap; and the calculated density of states of the pristine system. The output is the self-consistent Fermi energy, the total concentrations of each defect as well as the concentration of its individual charge states, and the free carrier concentrations. Furthermore, the procedure facilitates fixing the concentration of one or more defects and determining the resulting self-consistent Fermi energy and concentrations of other defects (performed using the related code ‘FROZEN-SC-FERMI’), thus modelling ‘frozen-in’ defects which may form by kinetic, rather than thermodynamic, processes. One can fix the total concentration or the concentration of a particular charge state; it is also possible to introduce new defects with a fixed concentration, but here the charge state must be specified. The background theory is discussed in some detail, and the operation of the program is demonstrated by some examples. Program summaryProgram Title:SC-FERMIProgram Files doi:http://dx.doi.org/10.17632/dh3hjdf4fc.1Licensing provisions: MIT licenseProgramming language:FORTRAN 90Nature of problem: To determine the self-consistent Fermi energy and equilibrium defect and carrier concentrations given a set of point defect formation energies in a crystalline system, assuming the constraint of charge neutrality.Solution method: The concentrations of each defect in each charge state are calculated, as are the free carrier concentrations. These concentrations are functions of the Fermi energy. The code, using an iterative search algorithm, determines the Fermi energy that satisfies the charge neutrality constraint (the self-consistent Fermi energy). The defect and carrier concentrations at that Fermi energy are then reported, as well as the Fermi energy itself.Restrictions: Thermodynamic equilibrium is assumed. The defect formation enthalpies and electronic density of states of the pristine system must be known.Additional comments: The concentrations of defects can be fixed to a particular value, thus modelling ‘frozen-in’ defects formed by e.g. kinetic processes. This procedure is facilitated by the related program, FROZEN-SC-FERMI, which is identical to SC-FERMI apart from the additional defect concentration fixing routine.

Electrical conduction behaviors and mechanical properties of Cu doping on B-site of (La 0.8Ca 0.2)(Cr 0.9Co 0.1)O 3− δ interconnect materials for SOFCs

(K0.95Na0.05)1−3xLaxTa0.60Nb0.40O3 ceramics (KNTN-La-x; x = 0, 0.003, 0.007, and 0.01) were fabricated by the conventional solid-state method. The charge compensation mechanism was investigated by analyzing the current–voltage characteristics of the KNTN ceramics with various La doping contents. It was found that La doping induced semiconductivity in KNTN ceramics. This indicates the presence of the electronic compensation mechanism in La-doped KNTN ceramics, in addition to the well-studied ionic compensation mechanism. Further investigation revealed that the charge compensation mechanism could be altered by annealing, and that La was converted to become ionically compensated.

An approach is considered that allows one to make a decision on the choice of compositions of composite materials that have been exposed to temperature changes for a long time. The alternatives are considered – compositions of materials and special criteria, the analysis of which underlies the optimal choice of composition from the group under consideration. Proposed the formation of criteria by which the decision is made to assess the quality of building composite materials. In this article, the criteria are formed on the basis of such properties of building materials as hardness and modulus of elasticity on the surface of the test samples, which were subject to negative and positive temperature extremes. Criteria for each property of the sample are formed on the basis of the given values relative to the starting point of exposure. To make a decision about the quality of composite materials, it is proposed to form three criteria for each of the considered properties of materials. The first criterion is calculated as a change in the corresponding property relative to the initial exposure point. The second criterion is determined sequentially with respect to each exposure point by a change in each of the considered properties. The third criterion is represented as the sum of the first two criteria. After that, some provisions of the ELECTRE methods are applied for ranking the studied compositions of composite materials.

An efficient carrier compensation mechanism in semiconductor layers by Fermi-level engineering is demonstrated using the modulation-doping of a deep acceptor and a shallow donor. The punch-through of the depletion region across the whole stack of modulation-doped layers shifts the Fermi level closer toward the midgap position, resulting in the compensation of residual background free carriers. The method represents an alternative to achieve semi-insulating properties in semiconductor materials where a suitable deep acceptor or donor state at the midgap position is not available. We demonstrate the applicability of the concept with a commercially important GaN case study using carbon (deep acceptor) and Si (shallow donor) doping. A strong enhancement of breakdown field strength and reduced charge pileup effects are observed due to the efficient pinning of the Fermi level.

We have performed ab-initio electronic structure calculations to investigate the ground state properties of Pd1−xNixTe (x = 0.0–0.20) alloys. The PdTe and all of its alloys are paramagnetic metals. For low concentrations, the band structure remains almost unchanged and at higher concentrations, a strong redistribution of spectral weights is observed. The most striking feature of the band structure is that the bands around the Fermi energy remain almost unchanged. The calculated Fermi surfaces are remarkably robust against disorder, strongly three-dimensional and have no or negligible nesting. The density of states at Fermi energy increases monotonically with concentration (x). Although the contribution of Ni to the density of states at Fermi energy is increasing continuously yet, Pd and Te dominate the density of states at Fermi energy. The density of states at Fermi energy and superconducting transition temperature Tc show opposite trends with respect to Ni concentration. So, density of states at Fermi level alone is not sufficient to discern the trends in Tc. We need to know the phonons and electron-phonon interactions as well, which at the moment are not available.

We investigate the distribution of occupied band-gap states in undoped, B-doped, and P-doped a-Si:H within the first \ensuremath{\sim}100 A\r{} of the surface using total-yield photoelectron spectroscopy in combination with the Kelvin probe. In clean, undoped a-Si:H the occupied density of states extracted from the measured yield spectrum consists of a linear valence-band edge, an exponential valence-band tail decreasing into the gap, and a broad band of deep defect states superposed on this tail. The deep-defect density of undoped a-Si:H measured by total yield is 2 orders of magnitude larger than that measured by photothermal deflection spectroscopy (PDS) on simultaneously prepared 1-\ensuremath{\mu}m-thick samples, from which we infer the existence of an excess density of deep defect states near the surface corresponding to an equivalent surface-state density of 3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$.The addition of diborane (phosphine) to the glow-discharge plasma decreases (increases) the density of excess occupied near-surface defect states in the gap and shifts the Fermi energy towards the valence- (conduction-) band edge. A 0.5-eV increase in the work function over that in undoped a-Si:H accompanies the \ensuremath{\sim}98% reduction of these excess occupied near-surface deep defect states by ${10}^{\mathrm{\ensuremath{-}}5}$ B doping, from which we infer an (0.4--0.5)-eV downward band bending at the clean, undoped a-Si:H surface. The inverse logarithmic slope of the exponential intrinsic valence-band tail observed in B-doped a-Si:H is 45 meV, in excellent agreement with that inferred from dispersive-transport, electron-spin-resonance (ESR), and traveling-wave data. Dividing the occupied density of states of ${10}^{\mathrm{\ensuremath{-}}3}$ P-doped a-Si:H by the Fermi-Dirac distribution function, we observe an exponential conduction-band tail extending over more than 3 orders of magnitude in the density of states. Its inverse logarithmic slope of \ensuremath{\sim}35 meV agrees with that inferred from ESR and traveling-wave data on P-doped a-Si:H, but is slightly larger than the slope inferred from dispersive-transport measurements on undoped a-Si:H. This discrepancy arises from the presence of a large concentration of P donors which affects the deeper tail-state distribution observed in heavily-P-doped a-Si:H. The average density of occupied states at the Fermi energy ${E}_{F}$ for the 30 doped samples we have studied is 8(\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ states/eV ${\mathrm{cm}}^{3}$, in good agreement with the thermodynamic minimum density of defect states permitted in undoped a-Si:H. This implies that the Fermi level lies at a minimum in the a-Si:H density of states for all doping levels.From this result we infer that the position of the ${D}^{+}$ defect level in B-doped a-Si:H lies more than 0.5 eV above the ${D}^{\mathrm{\ensuremath{-}}}$ level in P-doped a-Si:H; an arrangement in conflict with a fixed energy distribution of deep defect states and with the generally accepted positive value of the defect correlation energy in a-Si:H. We resolve this seeming discrepancy by postulating that the amorphous network responds to changes in ${E}_{F}$ by changing its defect structure so as to minimize the total energy of the system. This postulate leads to a variable energy distribution of deep defect states that depends only on the position of ${E}_{F}$ and the defect-formation energies; intimate pairing of dopants and defects, which has been suggested to account for this discrepancy, is not required. The relation between the surface and bulk distributions of localized gap states in a-Si:H is discussed.

The mechanism of doping BaTiO3 with La has been investigated by a combination of X-ray diffraction, electron probe microanalysis, scanning and transmission electron microscopy and impedance measurements. Phase diagram results confirm that the principal doping mechanism involves ionic compensation through the creation of titanium vacancies. All samples heated in oxygen at 1350–1400°C are electrical insulators, consistent with an ionic compensation mechanism. Samples heated in air or atmospheres of low oxygen partial pressure, at similar temperatures, lose a small amount of oxygen and this gives rise to a second, electronic compensation mechanism in addition to the main, ionic compensation mechanism; as a result, samples are dark-coloured and semiconducting. The change from insulating to semiconducting behaviour is reversible, by changing the atmosphere on heating at 1350–1400°C. We find no evidence for any changes in cationic composition of the BaTiO3 solid solutions arising from changes in oxygen content.

Moiré is More: Access to New Properties of Two-Dimensional Layered Materials

First-principles calculations of the electronic energy-band structures of an isolated one-dimensional ${(\mathrm{SN})}_{x}$ chain and of a three-dimensional ${(\mathrm{SN})}_{x}$ crystal have been made by using the method of linear combinations of atomic orbitals (LCAO). The crystal potential is taken as a superposition of the atomic potentials at each site with a Slater-type exchange approximation. The basis functions consist of the Bloch sums of the nitrogen $1s$, $2s$, $2p$ orbitals and sulfur $1s$, $2s$, $2p$, $3s$, $3p$ orbitals and single-Gaussian Bloch sums. The technique of orthogonalization is used so that the core-state Bloch sums can be deleted from the basis set. All the multicenter integrals occurring in the Hamiltonian matrix elements are evaluated exactly by means of the Gaussian technique and the summation of the multicenter integrals over the lattice is carried to convergence. The calculated density of states (DOS) of the crystal is in good agreement with the x-ray photoemission measurements. Studies of Fermi surfaces show an electron pocket near the $\ensuremath{\Gamma}Z$ line and a hole pocket near the $\mathrm{CY}$ line of the Brillouin zone. While the DOS shows a shallow and flat minimum at the Fermi energy for the one-dimensional ${(\mathrm{SN})}_{x}$ chain, in the case of the three-dimensional crystal, a very steep valley is found in the DOS curve near the Fermi energy. This steep valley is a direct consequence of the interchain coupling. The very small theoretical value of DOS at the Fermi energy [0.01 states/(eV spin molecule)] is consistent with the ultraviolet-photoemission-spectra measurements but is much less than the value deduced from the specific-heat experiment. Because the DOS at the Fermi energy [$N(0)$] is very near the minimum of the steep valley, one may speculate that a slight change in the atomic distances may bring about a large increase in $N(0)$. This would provide a simple explanation for the observed increase in the superconducting transition temperature as well as the normal-state conductivity under pressure. Energy bands and DOS for the ${\mathrm{S}}_{2}$${\mathrm{N}}_{2}$ crystal calculated by the same LCAO procedure are also presented.

Structural, electronic and magnetic behaviour of a less-explored material Sr2CrTiO6 has been investigated using ab initio calculations with generalized gradient approximation (GGA) and GGA + U methods, where U is the Hubbard parameter. For each of the three possible Cr/Ti cationic arrangements in the unit cell, considered in this work, the non-magnetic band structure shows distinct traits with significant flat-band regions leading to large t 2g density of states around the Fermi energy. The Cr4+ ion in Sr2CrTiO6, which is a d 2 system, shows a reverse splitting of the t 2g orbitals. The calculated hopping matrix contains non-zero off-diagonal elements between the d xz and d yz orbitals, while the d xy orbitals remain largely unmixed. A minimal tight binding model successfully reproduces the six t 2g bands around the Fermi energy. The Fermi surface shows a two-dimensional nesting feature for the layered arrangement of Cr and Ti atoms. Fixed spin moment studies suggest that the magnetism in this compound cannot be explained by Stoner’s criterion of an itinerant band ferromagnet. In the absence of Hubbard U term, the ground state is a half-metallic ferromagnet. Calculations for the anti-ferromagnetic spin arrangement show re-arrangement of orbital character resulting in (a) narrow d xz /d yz bands and sharp peaks in the density of states at the Fermi energy and (b) highly dispersive d xy bands with a broader density of states around the Fermi energy. The metallicity persists even with increasing U for both the spin arrangements, thus suggesting that Sr2CrTiO6 belongs to the class of weakly correlated metals, while the shifting of O 2p states towards the Fermi energy with U indicates a negative charge-transfer character in Sr2CrTiO6.

The electrical and optical properties of materials which are characterized by narrow bands in the vicinity of the Fermi energy are discussed. For such materials, electronic correlations and the electron-phonon coupling must be considered explicitly. Correlations in $f$ bands and in extremely narrow $d$ bands can be handled in the ionic limit of the Hubbard Hamiltonian. It is shown that free carriers in such bands form small polarons which contribute to conduction only by means of thermally activated hopping. Wider bands may also exist near the Fermi energy. Carriers in these bands may form large polarons and give a bandlike contribution to conductivity. A model is proposed for determining the density of states of pure stoichiometric crystals, beginning with the free-ion energy levels, and taking into account the Madelung potential, screening and covalency effects, crystalline-field stabilizations, and overlap effects. Exciton states are considered explicitly. The Franck-Condon principle necessitates the construction of different densities of states for electrical conductivity and optical absorption. Because of the bulk of experimental data presently available, the model is applied primarily to NiO. The many-particle density of states of pure stoichiometric NiO is calculated and is shown to be in agreement with the available experimental data. When impurities are present or nonstoichiometry exists, additional transitions must be discussed from first principles. The case of Li-doped NiO is discussed in detail. The calculations are consistent with the large mass of experimental information on this material. It is concluded that the predominant mechanism for conduction between 200 and 1000 \ifmmode^\circ\else\textdegree\fi{}K is the transport of hole-like large polarons in the oxygen $2p$ band. A method for representing the many-particle density of states on an effective one-electron diagram is discussed. It is shown that if correlations are important, donor or acceptor levels cannot be drawn as localized levels in the energy gap when multiple conduction or valence bands are present. This result comes about because extrinsic ionization energies of two correlated bands differ by an energy which bears no simple relation to the difference in energies of the intrinsic excitations, which are conventionally used to determine the relative positions of the bands.

The microstructure, lattice parameters, and electrical conductivity mechanisms for Fe doping on B‐site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ were systematically investigated. The oxygen nonstoichiometry was measured by means of thermogravimetry as a function of oxygen partial pressure. In this study, the concept of defect chemistry is used to explain the relationship between the concentration of electron hole with the electrical conductivity. Based on the result of electrical conductivity in air, it is concluded that the concentration of electron hole at high oxygen activity is larger than that at low oxygen activity. This is due to the fact that (La0.8Ca0.2)CrO3−δ‐based ceramics are p‐type conductors, the electrical conductivity is dominated by the concentration of hole. At higher Fe‐doping level, the compensation mechanism at high oxygen activity is significantly dominated by the formation of oxygen vacancy, that is, ionic compensation. The compensation mechanism at low oxygen activity is significantly dominated by the formation of the formation of Cr4+, that is, electrical compensation at lower Fe‐doping level. Based on oxygen nonstoichiometry data, it is found that with increasing Fe‐doping amount on B‐site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ specimens, the initial weight‐stable temperature shifted to lower temperature which might be highly related with the change in compensation mechanism at the temperature.