Defect Chemical Model Research Articles

Reversible conductivity control and quantitative identification of compensating defects in ZnSe bulk crystals

Donor compensation by associates of donor impurities and cation vacancies is investigated in vapor grown ZnSe : I and Al diffused melt grown bulk crystals. Both the (I SeV Zn) and the (Al ZnV Zn) A-centers could be identified by electron paramagnetic resonance (EPR) spectroscopy. The concentrations of the compensating defects can be reversibly changed by vapor-phase equilibration and a reproducible adjustment of room temperature (RT) carrier concentrations n e up to 1.4×10 18 cm −3 (ZnSe : I) and 2.7×10 18 cm −3 (ZnSe : Al) is possible. The results are explained in terms of a defect chemical model. The supposed high concentrations of one type of vacancy associated defects are confirmed using positron annihilation (PA). Lowered compensation leads to an increase of carrier mobility μ e with increasing n e. In low doped systems mobilities of 500 cm 2/Vs can be obtained. A similarly drastic shift of the absorption edge to lower photon energies is observed for both I- and Al-doped crystals.

Electrical conductivity and nonstoichiometry in doped Sr 3Ti 2O 7

A series of doped Ruddlesden–Popper phases, of general formula Sr 3Ti 2- x M x O 7- δ ( M= Al, Ga, Co) , were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O 2)> 10 −5 atm, followed by a p(O 2)-independent electrolytic regime, and n-type electronic conductivity at very low p(O 2). The electrolytic regime exhibits activation energies in the range 1·7–1·8 eV. Doping with transition metals such as Co results in a very significant increase in total conductivity with a p-type conductivity at high p(O 2). Furthermore, an apparent ionic regime at intermediate p(O 2) is observed, characterized by high conductivity (> 10 −2 S/cm at 700°C) and low activation energy (0·6 eV). This interpretation is consistent with iodometric measurements as interpreted by a defect chemical model. Other measurements are in progress to confirm this conclusion.

Electrical and defect thermodynamic properties of nanocrystalline titanium dioxide

The electrical properties of dense nanocrystalline TiO2 ceramics with an average grain size of 35 nm were investigated by impedance spectroscopy and compared with a coarsened material with micron size grains. The nanocrystalline ceramics show an uncommon domain of ionic conductivity at high oxygen pressures and a steep increase of electronic conductivity at low oxygen pressures. These results are discussed in terms of defect chemical models. The enthalpy of reduction of the nanocrystalline TiO2 was deduced to be significantly lower than that of the coarsened material. This may be related to lower defect formation energies at interface sites.

New Mixed Conductors Based on Doped Layered Perovskites

ABSTRACTA series of doped Ruddlesden-Popper phases, of general formula Sr3Ti2−xMxO7−δ (M=Al, Ga, Co), were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O2)>10−5 atm, followed by a p(O2)-independent electrolytic regime, and n-type electronic conductivity at very low p(O2). The electrolytic regime exhibits activation energies in the range 1.7-1.8 eV. Doping with transition metals such as Co results in a very significant increase in total conductivity with a p-type conductivity at high p(O2). Furthermore, an apparent ionic regime at intermediate p(O2) is observed, characterized by high conductivity (>10−2 S/cm at 700 °C) and low activation energy (0.7 eV). This interpretation is consistent with iodometric measurements as interpreted by a defect chemical model. Other measurements are in progress to confirm this conclusion.

Defect Chemistry of Donor‐Doped and Undoped Strontium Titanate Ceramics between 1000° and 1400°C

The electrical conductivity, sigma, of donor‐doped and undoped strontium titanate (SrTiO3) ceramics and, in some cases, single crystals, Sr1‐xLaxTiO3 (0 ≤ to x ≤ 0.1), was investigated in the temperature range of 1000°‐1400°C under oxygen partial pressures, PO2, of 10‐20‐1 bar. In conjunction with Hall data and thermopower data from related papers, a set of constants for a defect‐chemical model was determined, precisely describing point‐defect concentrations and transport properties of these materials. In contrast to former works, temperature‐dependent transport parameters and their non‐negligible influence on the determination of the constants was considered, as well as the equilibrium restoration phenomena of the cation sublattice, which can be studied only at such high temperatures. It was shown that defects in the cation sublattice completely govern the electrical behavior of donor‐doped and undoped SrTiO3. In the latter case, frozen‐in strontium vacancies act as intrinsic acceptors, determining the sigma(PO2) curves at lower temperatures. This intrinsic acceptor concentration also can be calculated with this model. The very good agreement between calculation and measurement is shown in many examples.

Electronic transport properties of Sr1−xLaxTiO3 ceramics

The electrical conductivity of Sr1−xLaxTiO3 ceramics (x≤0.4) and single crystals (x≤0.1) was investigated in the temperature range between 19 and 1673 K. The mobility was calculated from the carrier concentration, n, which was determined by Hall measurements as well as by a chemical Ti3+ titration. The mobility of the single crystals agrees well with a model originally developed for undoped strontium titanate. At low T and high n scattering by ionized donor centers predominates. Above room-temperature phonon scattering becomes predominant. Sr1−xLaxTiO3 ceramics follow the same model, but only if they had been previously reduced in water-free and oxygen-free hydrogen atmospheres. The behavior during the reducing and cooling process can be explained by a defect chemical model, using a set of constants developed in a former work. However, reduced in water saturated hydrogen the conductivity at low T decreases by decades compared to single-crystal data and cannot be explained anymore by the above-mentioned models. In conjunction with impedance spectroscopy experiments it was found, that in this case the electrical behavior of the ceramics is completely governed by grain-boundary phenomena. These high-ohmic grain boundaries may be depletion layers, which are formed or annihilated depending on the conditions of the preceding high-temperature process.

Comments on the Defect Chemistry of Magnesium-Doped Lithium Niobate (LiNbO3)

Congruently grown LiNbO3is known to be highly defective due to its significant Li2O deficiency. At present two stoichiometry-related defect-chemical models are discussed which can be distinguished with respect to the occurrence of Li or Nb vacancies. The Nb-vacancy model takes advantage of ilmenite-structured LiNbO3and predicts a niobium antisite concentration being at least five times as large as in the Li-vacancy model. In the present contribution it is argued that the magnesium-doping mechanisms are essentially independent of these particular differences. It is argued that extended stoichiometry-related defect clusters represent a key for a proper understanding of the defect chemistry of impurity cations. The discussion is mainly based on our recent shell-model simulations of perfect and defective LiNbO3.

Conduction in titanate pyrochlores: role of dopants

Electrical conductivity measurements have been performed on the pyrochlore compounds Y2Ti2O7, Gd2Ti2O7 and Sm2Ti2O7 as a function of temperature, oxygen fugacity and aliovalent dopant concentration. Both the magnitude and type of conduction (ionic and/or electronic) were found to be influenced by dopant size, site location (“A” or “B” cation sublattice) and net dopant density. While Ca doping on the “A” cation sublattice (Ca'Gd,Sm,Y) was found to increase the ionic conductivity by as much as two and half orders of magnitude, Sr and Mg dopants with larger dopant-host size mismatch induced a drop in ionic and an enhancement in electronic conductivity. Acceptor doping on the “B” cation sublattice, e.g. Al'Ti, increased the ionic conduction only at low dopant levels, while RuTi enhanced electronic conduction at high PO2. Trends in ionic and electronic conduction as a function of T, PO2, and dopant density are analyzed with the aid of a defect chemical model.

Oxygen Tracer Diffusion in La2‐xSrxCuO4‐y Single Crystals

The tracer diffusion of 18O in La2‐xSrxCuO4‐y single crystals (x = 0 to 0.12) has been measured from 400° to 700°C in 1 atm of oxygen using SIMS analysis. Evidence for diffusion by a vacancy mechanism was found at low strontium contents. Oxygen diffusivities for x 2≥ 0.07 were depressed by several orders of magnitude below the diffusivity for undoped La2CuO4±y. The observed effects of strontium doping on oxygen diffusivity are discussed in terms of defect chemical models. The decreasing oxygen diffusivity with increasing strontium was attributed to the ordering of oxygen vacancies at large defect concentrations. A diffusion anisotropy, Dab/Dc, of ∼600) was also found at 500°C.

Contributions to the high-temperature behaviour of YBa 2Cu 3O 7− y

The HTSC material YBa 2Cu 3O 7− y is examined in a temperature range between 600 and 900°C under variable oxygen partial pressures between 1 and 10 t4̄ bar. The oxygen deficit y is determined as a function of temperature and p O 2 by a solid-state titration technique. The results are compared with literature data obtained from TGA measurements. Based on these data a defect chemical model is proposed, which describes the electronic defects as a function of temperature and p o 2 . On thermopower measurements the influence of the preparation history of our samples is shown.

Characterization of hydrothermal barium titanate

Abstract Hydrothermally prepared powders of BaTiO 3 show a number of structural peculiarities which are closely connected with a considerable content of water, incorporated in the perovskite lattice. On annealing the powders to 500°C the water is gradually released and the normal structural properties of BaTiO 3 recur. A defect chemical model has been derived for hydrothermal BaTiO 3 showing a loosely packed perovskite structure with large numbers of protons in the oxygen sublattice and vacancies in the sublattices of Ba and Ti.

Ruthenium Doped Gadolinium Titanate: Effects of a Variable Valent Acceptor on Ionic and Electronic Conduction

AbstractWe have investigated ruthenium doped gadolinium titanate, Gd2(RuxTi1-x)2O7-δ with 0 ≤ x ≤ 0.2, to determine the role of variable valent Ru in influencing the defect structure and transport properties of the pyrochlore host. We have developed a defect chemical model to interpret electrical conductivity, thermoelectric power, and thermogravimetry data. We have found that Ru acts as an acceptor compensated in large part by oxygen vacancies, resulting in enhanced ionic conduction at low values of x. For larger values (x ≈ 0.05), electronic conductivity predominates which we attribute to electron migration by hopping through a Ru-derived defect band.

Mixed Ionic-Electronic Conduction in Pyrochlore Oxides

AbstractThe transport properties of the pyrochlore solid solution Gd2(ZrxTil-x)2O7 are investigated to clarify the relationships between composition, structural disorder and ionic and electronic transport. The oxygen ion conductivity has been found to increase sharply with increasing Zr content, x, due to enhanced structural disorder, leading to intrinsic ionic conduction at large values ofx. In contrast, the n—type conductivity predominant at low x, decreases sharply above x=0.2. Defect chemical models are presented to account for the simultaneous contributions of both intrinsic and dopant induced disorder. These models are applied to the σ(T, PO2, dopant) data to extract key thermodynamic and transport parameters. The significance of these parameters and the potential application of these materials in electrochemical devices are discussed.

Defect chemical explanation for the effect of air anneal on CdS/CuInSe2 solar cell performance

We formulate a consistent defect chemical model of the effect of air/O2 anneals on CdS/CuInSe2 devices. The model centers on O-induced neutralization of (near) surface donor states in CuInSe2 grains. The simplest identification of these states is with ionized Se vacancies, due to coordinatively unsaturated In on grain surfaces and boundaries.

Interaction of Oxygen with Native Chemical Defects in CuInSe2 Thin Films

ABSTRACTWe formulate a consistent defect chemical model of the effect of air/02 anneals on CuInSe2 polycrystalline thin films. The model centers on O-induced neutralization of (near) surface donor states in CuInSe2 grains. The simplest identification of these states is with ionized Se vacancies, due to coordinatively unsaturated In on grain surfaces and boundaries.

Role of iron centers in the photorefractive effect in barium titanate

Variably valent iron impurities have been suggested as the source of photorefractive charge carriers in BaTiO3. High-purity BaTiO3 crystals were grown with transition-metal impurity levels below the 1016 cm−3 baseline estimated for photorefractivity. Iron-doped crystals were grown with concentrations of 5, 50, 250, 500, 750, and 1000 parts in 106 of iron. Changes in iron valence monitored by optical absorption in the PO2 range 1–10−4 atm were found to be consistent with a defect-chemical model indicating Fe3+ and Fe4+ to be the stable valence states in this range. Photorefractive characterization of high-purity BaTiO3 suggests that variably valent iron ions are not the dominant photorefractive species in these crystals, whereas the role of iron centers in doped crystals is complicated by large absorption losses.

Defect chemistry of YBa 2Cu 3O 7−δ

Thermogravimetric, electrical resistance and thermopower measurements on YBa 2Cu 3O x are reported as function of the ambient oxygen pressure. Th occurrence of monovalent copper in the complete composition range 6.0 < x < 7.0 seems to be essential to explain the defect chemistry of this compound, although no simple defect chemical model can be constructed from the data reported so far.

Luminescence of CuInS 2: II. Exciton and near edge emission

The near-edge (exciton) emission of CuInS 2 is investigated for various material-compositions as a function of temperature. From these investigations the exciton ionization energy (20 meV) and the temperature dependence of the energy gap were determined. For the first time, recombination of the free exciton belonging to the deeper lying Γ 7 valence bands has been observed. Moreover, six different bound exciton emission lines and a donor to valence band transition were detected. These emissions could be assigned in terms of the defect-chemical model presented in Part I.