Charge transport in CaTiO3: I. Electrical conductivity

The present paper reports the electrical conductivity of polycrystalline undoped CaTiO₃ in the temperature range 973–1323 K under controlled oxygen partial pressure (10–10⁵ Pa). The electrical conductivity data are considered in terms of defect disorder and related semiconducting properties of CaTiO₃. The values of the p(O₂) exponent of electrical conductivity at high p(O₂), that vary between 1/4.3 and 1/6.2 at 973 and 1323 K, respectively, are considered in terms of theoretical defect disorder model of p-type CaTiO₃ and increasing effect of minority charge carriers (electrons) with temperature on p-type conduction. The activation energy of the electrical conductivity, assuming 125.3 kJ/mol at 10 Pa and 94.4 kJ/mol at 72 kPa, has been considered in terms of the formation of defect and their mobility. The band gap, determined from the minimum of electrical conductivity corresponding to the n–p transition is equal to 2.77 eV.     

Solid/melt ZnO-Bi2O3 composites as ion transport membranes for oxygen separation from air

We present the oxygen-permeable plastic solid/melt ZnO — 15-30 wt.% Bi₂O₃ composites, which could be used as ion transport membranes for oxygen separation from air. The composites are made from electron-conducting solid grains of ZnO and ion-conducting liquid channels at the grain boundaries. Ambipolar conductivity of ionic and electronic charge carriers in an oxygen electrochemical potential gradient assures a high oxygen permeation flux through the composites.     

Defect structure of acceptor-doped calcium titanate at elevated temperatures

The defect structure of acceptor (Al or Cr)-doped polycrystalline calcium titanate was investigated by measuring the oxygen partial pressure dependence (at 10° to 10–18 atm) of the electrical conductivity at 1000 and 1050° C. The observed electrical conductivity data were proportional to for the oxygen pressure range < 10^–10 atm and proportional to for the oxygen pressure range ( 10^–7 atm. The conductivity values were observed to increase with the acceptor concentration in the p-type region with the shift in the conductivity minima towards lower oxygen partial pressure. The absolute value of the electrical conductivity in the acceptor-doped samples were lower in the n-type region compared to the values in the undoped CaTiO₃. Aluminium and chromium were found to be equally effective in acting as acceptor impurities in CaTiO₃. The defect chemistry of CaTiO₃ is dominated by the added acceptor impurities for the entire oxygen partial pressure range used in this investigation.     

Defect formation in Gd3Fe5O12-based garnets: a Mössbauer spectroscopy study

The Mössbauer spectroscopy of Gd_2.2Pr_0.8Fe₅O₁₂ garnet, which exhibits higher electronic conduction with respect to Gd₃Fe₅O₁₂ due to the presence of Pr⁴⁺ cations, showed that praseodymium doping decreases the coordination of Fe³⁺ in octahedral sites. Penta-coordinated Fe³⁺ ions, in combination with small quantities of Fe⁴⁺, are also formed in the lattice of Gd_2.5Ca_0.5Fe₅O₁₂ where the variations of ionic and electronic transport properties indicate charge compensation via generation of oxygen vacancies and electron holes. The mechanisms of garnet lattice disorder, induced by acceptor- and donor-type doping, appear thus quite similar; in all cases, the ionic defect formation requires substantial structural reconstruction, probably associated with direct linking of iron–oxygen tetrahedra. Due to the low concentration of charge carriers and the important role of lattice relaxation in the oxygen ion migration processes, this behavior results in similar activation energies for the ionic conductivity in all Gd₃Fe₅O₁₂-based garnets.     

Transport properties and Mössbauer spectra of Fe-substituted La10−x(Si,Al)6O26 apatites

Increasing iron content in apatite-type La_9.83Si_4.5Al_1.5−yFe_yO_26+δ (y=0.5-1.5) leads to increasing unit cell volume, fraction of Fe⁴⁺, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973-1223 K in air, are in the range 0.986-0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10⁻² Pa to 70 kPa, confirm that under oxidizing conditions the ionic conduction in Fe-substituted La_9.83(Si,Al)₆O_26+δ apatites is dominant. Due to stabilization of Fe³⁺, substantially worse transport properties are observed for A-site stoichiometric La₁₀Si₄Fe₂O₂₆, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe⁴⁺ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La_10−x(Si,Al)₆O_26+δ ceramics, calculated from dilatometric data in air, are 8.9×10⁻⁶ to 9.9×10⁻⁶ K⁻¹ at 300-1250 K.     

Effects of mixed and ionic conduction in ZrO2 and ThO2 as CO oxidation catalysts

A comparison is made of the heterogeneous catalytic activities of ZrO₂, Zr_0.91Ca_0.09O_1.91 and Th_0.85La_0.15O_1.925 for the oxidation of CO by O₂. A correlation is evident with the magnitudes of the simultaneous bulk ionic and electronic conductivities. This is in accord with a postulated model for reaction at separate reactant absorption sites in which oxygen ions and electronic charge are transported in and on the surface of the oxide catalyst.     

Morphology and oxygen vacancy investigation of strontium titanate-based photo electrochemical cells

In this paper single band gap photo electrochemical cells (PECs) are presented, which consist of strontium titanate (SrTiO₃) photo anodes on nickel cathodes in potassium hydroxide electrolyte. SrTiO₃ powders are deposited on nickel substrates by electrophoresis before sintering with varying temperatures, times, cooling rates, gas types, and gas flow rates. The external quantum efficiency (EQE) of such PECs mainly depends on the morphology and the amount of oxygen vacancies in SrTiO₃ lattice. At first, the morphology is investigated, which can be adjusted by the particle size as well as the sinter temperature and time. Nanopowder-based PECs sintered above the starting sinter temperature indicate the best charge carrier transport and hence allow high EQEs. The sinter time influences the specific surface area, but not the EQE in this investigation. Secondly, the generation of oxygen vacancies is investigated, which depends on the oxygen partial pressure and the equilibration temperature. Low oxygen partial pressures and high equilibration temperatures increase the amount of oxygen vacancies, which can be set by the gas type and its flow rate or the cooling rate and an additional heating step, respectively. It can be shown that PECs have to possess a low amount of oxygen vacancies to reach high EQE values, but not too low to allow for sufficient conductivity. This point is shown through our finding that the samples with lower and higher concentrations exhibit very low photo activity. Oxygen vacancies can be considered as intrinsic donors and hence increase electrical conductivity which is necessary but also act as recombination centers. For SrTiO₃ nanopowder-based samples, which have been sintered at 1200 °C for 20 min with a cooling rate of 10 K/s in reducing gas (with 5 vol% H₂) and a low flow rate of 1.7 l/h, very high external quantum efficiencies of 64.2 % under 365 nm illumination can be achieved.     

Conductivity and creep in acceptor-dominated polycrystalline Al2O3

Ionic and electronic conductivity and compressive creep of hot-pressed polycrystalline acceptor-dominated Al₂O₃ were measured as a function of oxygen partial pressure and grain size varying from 3 to 200 m. Hole conduction shows a slight preference for grainboundaries; ionic conduction is slightly hindered by grain boundaries, indicating that fast oxygen grain-boundary diffusion involving charged species does not occur. Conductivity and creep are accounted for on the basis of a model in which there is fast grain-boundary migration by a neutral oxygen species.     

Electrical conductivity and defect structure of CeO2-ZrO2 mixed oxide

The electrical conductivity of CeO₂-ZrO₂ system was measured as functions of the temperature and oxygen partial pressure and of the composition. The ionic conduction was prevailing in the ZrO₂ rich phase due to the increase of ionic defect concentration via homovalent doping effect. The enhancement of n-type electronic conductivity was observed in intermediate and CeO₂ rich phase compared with pure CeO₂, which originated either from homovalent doping effect or increase of electronic mobility due to the change of transport mechanism.     

Efficiency enhancement by aluminum addition to CaTiO3:Pr phosphor thin films

The mechanism of enhancement of the red emission efficiency from CaTiO₃:Pr³⁺ thin film by Al addition has been investigated. Al-ions have been attracting interest as a sensitizer to improve the luminescent efficiency of phosphors. Also, influence of Al-doping on the crystallization, surface morphology and luminescent properties of CaTiO₃:Pr³⁺ thin films have been discussed. CaTiO₃:Pr³⁺ and Al-doped CaTiO₃:Pr³⁺ films were grown using pulsed laser deposition technique on Al₂O₃ (0001) substrates under different substrate temperatures and oxygen pressures. The crystalline phase and surface morphology of the films were very dependent on the oxygen pressure and substrate temperature and they affected the luminescent brightness of the films. The crystalline structure and microstructure of these films have been characterized by X-ray diffraction and electron microscopy and their luminescent properties have been evaluated at room temperature using a luminescence spectrometer and excitation by a broadband incoherent ultraviolet light source with a dominant excitation wavelength of 325 nm. In particular, the incorporation of Al³⁺ ions into CaTiO₃ lattice could induce a remarkable increase of photoluminescence. The enhancement of luminescence for Al-doped films may result not only from the improved crystallinity but also from the reduced internal reflections caused by rougher surfaces. Also, the luminescent intensity and surface roughness of the films exhibited similar behavior as a function of oxygen pressure.     

Calcium phosphates formation on CaTiO<Subscript>3</Subscript> coated titanium

In this study, performance of calcium phosphate formation of CaTiO₃ coating film on Ti in Hanks’ balanced saline solution (HBSS) was investigated. CaTiO₃ thin films with a thickness of 50 nm were deposited on Ti using radiofrequency (RF) magnetron sputtering. The temperature of Ti substrate was adjusted to room temperature (RT) and 873 K. Thereafter, the specimens deposited at RT were annealed at 873 K in air for 7.2 ks. The films were characterized by grazing incident angle X-ray diffractometry (GI-XRD) and X-ray photoelectron spectroscopy (XPS). After immersion in HBSS for 60 d, on CaTiO₃ coated Ti, the formation of hydroxyapatite (HAP) was observed. Furthermore, HAP layer formed was thicker on the specimen on which CaTiO₃ film was deposited at RT and annealed than that prepared at 873 K. The major difference between both specimens was the chemical properties of the outermost surface. In summary, CaTiO₃ thin film deposited at RT and followed by annealing at 873 K for 7.2 ks in air enhances calcium phosphate formation ability on Ti.     

Oxygen permeability of LaGa0.65Ni0.20Mg0.15O3−δ ceramics: effect of synthesis method

Oxygen ionic transport in dense LaGa_0.65Ni_0.20Mg_0.15O_3−δ membranes, prepared by the standard ceramic synthesis technique and via glycine-nitrate process (GNP), was studied using measurements of the total conductivity, oxygen permeation and faradaic efficiency (FE). At 1223 K oxygen transfer through LaGa_0.65Ni_0.20Mg_0.15O_3−δ ceramics is mainly determined by the bulk ambipolar conductivity, while decreasing temperature leads to a greater role of the surface exchange rate. In spite of moderate difference in the ceramic microstructures, the surface exchange limitations are considerably higher for the membranes prepared by the standard ceramic route compared to GNP-synthesized material. Thermal expansion and partial ionic and electronic conductivities were found essentially independent of the synthesis method. The level of oxygen ionic conduction in LaGa_0.65Ni_0.20Mg_0.15O_3−δ, characterized by the activation energy of about 150 kJ/mol and ion transference numbers in the range 1×10⁻³-5×10⁻² at 973-1223 K, is higher than that in La(Ga,Ni)O_3−δ perovskites and comparable to La₂NiO₄-based phases.     

First-principles study of electronic and optical properties of cubic perovskite CsSrF3

We present first principles calculations of the electronic, structural and optical properties of the cubic perovskite CsSrF₃ by using the full potential linearized augmented plane wave (FP-LAPW) plus local orbitals method with generalized gradient approximation (GGA) in the frame work of density functional theory. The calculated lattice constant is in a good agreement with the experimental result. The electronic band structure shows that the fundamental band gap is wide and direct at ?? point. The contribution of the different bands was analyzed from the total and partial density of states curves. The charge density plots show strong ionic bonding in Cs-F, ionic and weak covalent bonding between Sr and F. The calculated optical spectra viz., the dielectric function, optical reflectivity, absorption coefficient, real part of optical conductivity, refractive index, extinction coefficient and electron energy loss, are presented for the energy range of 0?C30 eV.     

High-temperature electrical conductivity of aluminium nitride

The electrical conductivity of hot-pressed polycrystalline aluminium nitride doped with oxygen and beryllium was measured as a function of temperature from 800 to 1200° C and over a range of nitrogen partial pressure from 10² to 10⁵ Pa. The effect of beryllium dopant, the independence of conductivity from nitrogen partial pressure, and the observed activation energy suggested extrinsic electronic species or aluminium vacancies as charge carriers. Polarization measurements made with one electrode blocking to ionic species indicated that the aluminium nitride with oxygen impurity was an extrinsic electronic conductor.     

Unipolar resistive switching behavior of BiFeO3 thin films prepared by chemical solution deposition

Bismuth ferrite (BiFeO₃, BFO) thin films were spin-coated on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. The ferroelectric BFO films annealed at 500 °C and 550 °C were found to possess unipolar resistive switching behaviors. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of the unipolar resistance switching is about 10³ for the ferroelectric BFO films. The conduction mechanisms are concluded to be space charge-limited conduction for the initial state and Ohmic conduction for the LRS. As for the HRS, the Poole-Frenkel emission fits well in the whole voltage region. Traps composed of oxygen vacancies are considered to play a key role in forming conducting paths. The relaxation time of electronic carriers is much shorter than that of ionic oxygen vacancies; therefore, the resistance switching is considered more probably due to carrier injection and emission through the Poole-Frenkel model after forming.     

Charge transport in CaTiO3: II. Thermoelectric power

The present work describes the method of the measurement of thermoelectric power of oxide materials under the gas phase of controlled oxygen activity with high accuracy. The present study reports the thermopower data for undoped CaTiO₃ at elevated temperatures (973–1323 K) and under the oxygen partial pressure in the range 10–10⁵ Pa. The obtained experimental data indicate lack of consistency between the thermopower data determined in this work and the electrical conductivity data reported before with respect to the n–p transition point, that is, the p(O₂) corresponding to the minimum of electrical conductivity differs from that at which S=0. The thermopower data were used to establish the temperature dependence of the electrical conductivity at constant thermopower. The obtained activation energy is in the range 119 kJ/mol in the n- and n–p transition regimes and assumes 146 kJ/mol in the p-type regime.     

The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications

Transparent conducting oxides (TCO) have integral and emerging roles in photovoltaic, thermoelectric energy conversion, and more recently, photocatalytic systems. The functional properties of TCOs, and thus their role in these applications, are often mediated by the bulk electronic band structure but are also strongly influenced by the electronic structure of the native surface 2D electron gas (2DEG), particularly under operating conditions. This study investigates the 2DEG, and its response to changes in chemistry, at the (111) surface of the model TCO In₂O₃, through angle resolved and core level X‐ray photoemission spectroscopy. It is found that the itinerant charge carriers of the 2DEG reside in two quantum well subbands penetrating up to 65 Å below the surface. The charge carrier concentration of this 2DEG, and thus the high surface n‐type conductivity, emerges from donor‐type oxygen vacancies of surface character and proves to be remarkably robust against surface absorbents and contamination. The optical transparency, however, may rely on the presence of ubiquitous surface adsorbed oxygen groups and hydrogen defect states that passivate localized oxygen vacancy states in the bandgap of In₂O₃.     

Charge transport in CaTiO3: III. Jonker analysis

The present work considers semiconducting properties of undoped CaTiO₃ in terms of electrical conductivity and thermopower as a function of oxygen partial pressure. Comparison of the two plots indicates that there is a substantial discrepancy between the p(O₂) corresponding to minimum of electrical conductivity and the p(O₂) corresponding to zero value of the thermopower. According to the applied Jonker formalism, the mobility of electrons is substantially larger than that of electron holes, however, the discrepancy between the two is beyond the range which may be explained assuming different mechanisms.     

SrSn1−xFexO3−δ (0≤x≤1) perovskites: a novel mixed oxide ion and electronic conductor

The electrical conductivity of SrSn_1−xFe_xO_3−δ increases with the Fe content and reaches a value of ∼10⁻¹ S/cm at 25°C at x=1. Compounds with low Fe content exhibit both ionic and electronic conductivity, while the higher Fe content perovskites are mainly electronic conductors with a conductivity independent of the oxygen partial pressure over a wide range from 0.21 to 10⁻²² atm.     

Dielectric properties of polycrystalline CaTiO3 doped with yttrium oxide

Y³⁺ was substituted for Ca²⁺ in polycrystalline CaTiO₃ in amounts up to 15 at %. Sintering conditions (1450° C, 15 h) were such that grain sizes were > 25 m. Stoichiometry was adjusted on the assumption that the excess charge of the dopant was compensated by the creation of calcium vacancies. This assumption was supported by measurements of the Ca/Ti ratio in the grains by electron-probe microanalysis. Unlike yttrium-doped SrTiO₃, material sintered in air was light-coloured with no evidence of semiconductivity. On the other hand, when sintering was done in nitrogen, dielectric relaxation characteristic of boundary layers was observed for dopant levels > 1 mol %. The experimental data support the view that dielectric relaxation in SrTiO₃ and CaTiO₃ results from semiconducting grains with resistive surface layers and that the semiconductivity arises because oxygen loss from the grains during sintering is increased by donor doping.