Lanthanum Scandate Research Articles

The Influence of Doping with Substitutional and Interstitial Atoms and Oxygen Vacancies on the Electronic Spectrum of Lanthanum Scandate

The electronic spectra of lanthanum scandate LaScO3 doped only in the scandium sublattice La[Sc0.95Co0.05]O3 or in several sublattices simultaneously (such as [La1–xSrx][Sc1–yCoy]O3–δ) are calculated using the coherent potential approximation. The reasons for the change in the electronic spectrum of LaScO3 within its bandgap are identified depending on the concentration of additive (including interstitial) components, at 0 ≤ x ≤ 0.3, y = 0.05, and δ ≤ 0.1, 0 ≤ z in La1–xSrxSc1–yCoyO3–δHz. It is found that the presence of Co substitutional atoms in the Sc sublattice leads to the appearance of additional peaks in the center of the bandgap of LaScO3. Sr and H atoms and oxygen vacancy regulate the filling of the Co3d bands. The movement of the Fermi level within the bandgap due to additive components with different concentrations allows one to predict how the electronic properties of complex‐doped compounds will change.

Insights into Barriers to Increasing Proton Conductivity of Lanthanum Scandate Ceramics via High Sr-Doping

Insights into Barriers to Increasing Proton Conductivity of Lanthanum Scandate Ceramics via High Sr-Doping

Dopant-induced changes of local structures for adjusting the hydration ability of proton-conducting lanthanum scandates

Dopant-induced local distortions in the crystal structure and the electronic structure influence the hydration of proton-conducting LaScO3-based perovskites.

Sr Doping and Oxygen Vacancy Formation in La1−xSrxScO3−δ Solid Solutions: Computational Modelling

Sr-doped lanthanum scandate La1−xSrxScO3−δ (LSS) is a promising perovskite-type material for electrochemical applications such as proton conductors. Oxygen vacancy is a common defect in ABO3-type perovskites. It controls ion transport as well as proton uptake. The energetic, structural, and electronic properties of oxygen vacancy in LSS are studied deploying the DFT method with meta-GGA functional. The vacancy formation energies in LSS were calculated for various Sr concentrations. Unlike other perovskites, in this material, the electrons are trapped at the oxygen vacancy site (the F-type centres, common in ionic oxides like MgO and Al2O3) rather than localised on the nearest to the vacancy B-cations. The process of oxygen vacancy formation is considered relative to Sr concentration x and oxygen nonstoichiometry factor δ. Three primary regimes are discussed: (I) localized at the vacancy electrons, x/δ < 2, (II) electron charge balanced system, x/δ = 2, and (III) delocalized electron holes, x/δ > 2. For x/δ ≥ 2 oxygen vacancy formation energy reaches the saturation level of ~3.5 eV, which is potentially beneficial for the proton uptake.

Oxygen Vacancy Formation and Migration within the Antiphase Boundaries in Lanthanum Scandate-Based Oxides: Computational Study.

The atomic structure of antiphase boundaries in Sr-doped lanthanum scandate (La1−xSrxScO3−δ) perovskite, promising as the proton conductor, was modelled by means of DFT method. Two structural types of interfaces formed by structural octahedral coupling were constructed: edge- and face-shared. The energetic stability of these two interfaces was investigated. The mechanisms of oxygen vacancy formation and migration in both types of interfaces were modelled. It was shown that both interfaces are structurally stable and facilitate oxygen ionic migration. Oxygen vacancy formation energy in interfaces is lower than that in the regular structure, which favours the oxygen vacancy segregation within such interfaces. The calculated energy profile suggests that both types of interfaces are advantageous for oxygen ion migration in the material.

Effect of irradiation with Ar ions on properties of proton conductors on the base of LaScO3

The effect of irradiation with low-energy Ar ions on the structure and conducting properties of lanthanum scandate synthesized by acceptor doping and creation of a lanthanum deficit has been studied.

Antiphase Boundary Defects in Strontium‐Doped Lanthanum Scandate

The microstructure and local structural analysis of undoped and strontium‐doped lanthanum scandates are performed by means of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) on dense ceramic specimens. LaScO3 is found to be homogeneous: there are no impurities and all the grains in the polycrystalline sample are structurally uniform and described in the framework of the same structural model of orthorhombic perovskite (space group Pnma). However, the existence of structural inhomogeneity in a polycrystalline sample of the La0.91Sr0.09ScO3–δ oxide is revealed and described. Possible reasons for the structural inhomogeneities are considered.

INFLUENCE OF DEFECTS FORMATION WAY ON PROPERTIES OF PROTON CONDUCTORS BASED ON LANTHANUM SCANDATE

The structure and properties of proton conductors based on LaScO3 synthesized by acceptor doping and the creation of a deficiency in La cation were studied. It has been shown that the number of intercalated protons in the oxide lattice increases with an increase in the degree of cationic doping and is higher when doped with strontium than calcium. It has been noted that in lanthanum scandate samples with cationic non-stoichiometry, the number of intercalated protons decreases with increasing lanthanum deficiency, which is probably due to the possibility of binding oxygen vacancies VO•• to (LaLa''' - VO••)' complexes and blocking their participation in the process of water dissolution. The activation energies of the conductivity of the volume and grain boundaries of lanthanum scandate synthesized by various methods are determined. It was shown that the conductivity and activation energy of conductivity of the volume of grains of samples doped with strontium are higher than when doped with calcium. It is noted that the activation energy of conductivity of samples with cationic non-stoichiometry is practically independent of the lanthanum deficiency.

Oxygen isotope exchange in proton-conducting oxides based on lanthanum scandates

A method of the oxygen isotope exchange with equilibration of the gas phase was used in order to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients of the proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600–900 °C and oxygen pressure of 1.01 kPa. The diffusion and oxygen surface exchange coefficients increase with the strontium content in oxides. The rates of the individual stages of the oxygen exchange process on the surface of the oxides were determined. The oxygen incorporation was revealed to be rate-determining for the undoped LaScO3 oxide. However, for the strontium-doped oxides La1–xSrxScO3–δ (x = 0.04; 0.09), the difference between the rates of oxygen dissociative adsorption and oxygen incorporation decreases with increasing the strontium concentration, in a way that for the La0.91Sr0.09ScO3−δ oxide these stages become competing. The possible reasons for these differences in oxygen surface exchange kinetics were analyzed in this paper. Using the obtained oxygen diffusion coefficients, the oxygen-ionic conductivities in accordance with the Nernst-Einstein equation, and the contributions of the oxygen-ion and proton components of the total conductivity of the La1–xSrxScO3–δ (x = 0.04; 0.09) oxides in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa) were calculated. The proton transference numbers were found to be close to unit at the wet reducing atmospheres in the temperature range of 500–600°С.

Interaction of O2, H2O and H2 with proton-conducting oxides based on lanthanum scandates

An interaction of components from the gas phase, containing gaseous oxygen, hydrogen and water, with the La1–xSrxScO3–α oxides in the temperature range of 300–950 °С and partial pressure of 6.1–24.3 kPa 8.1–50.7 kPa has been studied using a high temperature thermogravimetric analysis. The effects of partial pressure of the gaseous oxygen, water and hydrogen on the apparent uptake level of protons with oxides has been found. The studies of incorporation processes of hydrogen from molecular hydrogen atmosphere into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates were performed in the temperature range of 300–800 °C and hydrogen pressure of 0.2 kPa by means of the isotope exchange method with the equilibration of isotope composition in the gas phase. The protons and deuterons concentrations were determined for the La0.91Sr0.09ScO3–α oxide. The existence of proton defects in the structure of studied oxides after the exposure in H2O and H2-containing atmospheres was revealed using the 1Н NMR. The role of oxygen vacancies in the proton incorporation processes is considered in the present work.

Interaction of H2O, O2 and H2 with Proton Conducting Oxides Based on Lanthanum Scandates

The research uses the method of high-temperature thermogravimetric analysis to study the processes of interaction of the gas phase in the temperature range 300–950 °C in the partial pressure ranges of oxygen 8.1–50.7 kPa, water 6.1–24.3 kPa and hydrogen 4.1 kPa with La1–xSrxScO3–α oxides (x = 0; 0.04; 0.09). In the case of an increase in the partial pressure of water vapor at a constant partial pressure of oxygen (or hydrogen) in the gas phase, the apparent level of saturation of protons is shown to increase. An increase in the apparent level of saturation of protons of the sample also occurs with an increase in the partial pressure of oxygen at a constant partial pressure of water vapor in the gas phase. The paper discusses the causes of the observed processes. The research uses the hydrogen isotope exchange method with the equilibration of the isotope composition of the gas phase to study the incorporation of hydrogen into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates. The concentrations of protons and deuterons were determined in the temperature range of 300–800 °C and a hydrogen pressure of 0.2 kPa for La0.91Sr0.09ScO3–α oxide. The paper discusses the role of oxygen vacancies in the process of incorporation of protons and deuterons from the atmosphere of molecular hydrogen into the structure of the proton conducting oxides La1–xSrxScO3–α (x = 0; 0.04; 0.09). The proton magnetic resonance method was used to study the local structure in the temperature range 23–110 °C at a rotation speed of 10 kHz (MAS) for La0.96Sr0.04ScO3–α oxide after thermogravimetric measurements in an atmosphere containing water vapor, and after exposures in molecular hydrogen atmosphere. The existence of proton defects incorporated into the volume of the investigated proton oxide from both the atmosphere containing water and the atmosphere containing molecular hydrogen is unambiguously shown. The paper considers the effect of the contributions of the volume and surface of La0.96Sr0.04ScO3–α oxide on the shape of the proton magnetic resonance spectra.

Oxygen Isotope Exchange in Proton-Conducting Oxides Based on Lanthanum Scandates

The method of oxygen isotope exchange with the gas phase equilibration have been used to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients with proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600−900°C at oxygen pressure 1.01 kPa. The paper determines that the diffusion and oxygen surface exchange coefficients increase with the increasing of the strontium content in the oxides. We have found out the rates of the individual stages of the oxygen exchange process on the surface of the oxides. It is shown that oxygen incorporation is rate-determining stage of the oxygen exchange on the surface of the undoped oxide, whereas for the strontium-doped oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) with increasing of strontium concentration, the difference between the rates of dissociative adsorption and oxygen incorporation decreases so that for the oxide La0,91Sr0,09ScO3–δ the stage of dissociative adsorption of oxygen becomes rate-determining stage. The paper analyzes the possible reasons of these differences in oxygen surface exchange kinetics. Moreover, the paper using the obtained oxygen diffusion coefficients that have been recalculated in the oxygen-ionic conductivities according to the Nernst-Einstein equation performs the contributions of the oxygen-ion and proton components of the total conductivity of oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa). Proton transference numbers are shown to be close to unit in the temperature range of 500–600 °С at the wet hydrogen-containing reducing atmosphere.

Local levels in La1-xSrxScO3-x/2 band-gap under interaction with components of O2, H2, H2O atmospheres

The paper studies the electronic structure of proton-conducting oxides based on the lanthanum scandate La1-xSrxScO3-x/2 for advancing in understanding the mechanisms of hydrogen uptake from dry and humid atmospheres into the lattice of oxides with a perovskite structure. The process of protons incorporation from H2O containing atmospheres is considered to describe by the reaction H2O+OO×+VO••=2OHO•. However, there is no established concept of a mechanism for proton uptake from a dry H2 atmosphere. At such an uptake, a positively charged proton defect will be formed in the oxide lattice, and a negative charge must appear for compensation of the excess positive charge. Formally, the reaction of this process can be represented as 12H2+OO×=OHO•+e′. In this case an uncompensated electron appears, and the question arises as to where it is localized. In order to answer this question, it is necessary to study the electronic structure of perovskites.With increasing in dopant concentration x the absorption band at 5.6 eV overlapping with the edge of fundamental absorption increases. Most probably it can be related with oxygen vacancies. When protons are incorporated from the H2 atmosphere into the La1-xSrxScO3-x/2 lattice, the absorption intensity in this band decreases, which can be due to the transition of the defects causing this band to another charge state. In addition, specific defects that absorb in the red and IR region at hν < 2.2 eV are formed. They are found to be located deep enough in the bang-gap and not to be an electronic traps. It is also shown that in La1-xSrxScO3-x/2 there are electron traps located at a depth of 2–4.5 eV in the band-gap relative to the bottom of the conduction band. On the basis of the obtained data, it can be assumed that these defects are somehow associated with oxygen vacancies, but their charge state is not obvious. It is important that these traps participate in the capture of uncompensated electrons during the proton uptake from the H2 atmosphere.

Local disorder and water uptake in La1–xSrxScO3–δ

Structure of perovskite oxides La1–xSrxScO3–δ (x=0, 0.04, 0.09) have been studied by X-Ray powder diffraction method. Orthorhombic distortions are found to decrease with increasing of strontium concentration. Water uptake of Sr-doped lanthanum scandates was studied by means of thermogravimetric analysis over the temperature range of 300–950°C in gas mixture with water and oxygen partial pressure of pH2O=0.24 atm and pO2=0.18 atm, respectively. Based on the thermogravimetry data the hydration enthalpy and entropy have been calculated for oxides La1–xSrxScO3–δ (x=0.04, 0.09). 45Sc nuclear magnetic resonance (NMR) spectra were taken at T=300 K and external magnetic field H0=11.7 T. 45Sc NMR study revealed that local symmetry around scandium ions in lanthanum scandate was affected by the presence of both strontium and protonic defects. Possible reasons of 45Sc NMR spectra changes have been considered.

Low-temperature fabrication of an anode-supported SOFC with a proton-conducting electrolyte based on lanthanum scandate using a PLD method

An anode-supported-type solid oxide fuel cell with a proton-conducting electrolyte composed of LaScO3 co-doped with 32.5mol% Sr and 2mol% Co was fabricated using a modified pulsed laser deposition (PLD) method. The modified PLD method, which comprised a conventional PLD method combined with an ultrasonically cleaning procedure, enabled deposition at a temperature lower than the cell fabrication temperatures. Thus, this procedure resulted in the deposition of a dense, gas-tight electrolyte of a poorly sinterable material without interaction between the electrode and electrolyte. The fabricated cell functioned in an intermediate temperature range; open circuit voltages and maximum power densities of the cell were 0.89V and 8.8mWcm−2 at 700°C and 0.91V and 1.9mWcm−2 at 600°C.

Rare-Earth Scandate/TiN Gate Stacks in SOI MOSFETs Fabricated With a Full Replacement Gate Process

Terbium scandate (TbScO3) and lanthanum scan date (LaScO3) have been investigated as gate dielectrics for metal-oxide-semiconductor field-effect transistors on both silicon-on-insulator (SOI) and strained SOI (sSOI) substrates. X-ray photoelectron spectroscopy analysis revealed the presence of a silicate at the interface for TbScO3 on Si, whereas a silicate/SiO2-like interface was found for the LaScO3 on Si. A full replacement gate process was developed to fabricate high-κ/metal gate fully depleted transistors on SOI and sSOI. LaScO3 transistors with a gate length of 2 μm show excellent characteristics with steep subthreshold slopes of 72 mV/dec, high Ion/Ioff ratios, and electron mobility of 180 cm2/V · s for SOI and 375 cm2/V · s for sSOI at low field, which is superior to corresponding TbScO3 de vices. All sSOI transistors showed 2 times higher electron mobility than SOI reference devices.

156 ランタンスカンデート系プロトン導電体を用いた中低温作動μSOFC(エンジンシステム/動力エネルギシステム,一般講演)

156 ランタンスカンデート系プロトン導電体を用いた中低温作動μSOFC(エンジンシステム/動力エネルギシステム,一般講演)

Low-Temperature Operating Micro Solid Oxide Fuel Cells with Perovskite-type Proton Conductors

ABSTRACTMicro-solid oxide fuel cells (Micro-SOFCs) with yttrium-doped barium zirconate (BZY) and strontium and cobalt-doped lanthanum scandate (LSScCo) electrolytes were fabricated for low-temperature operation at 300 °C. The micro-SOFC with a BZY electrolyte could operate at 300 °C with an open circuit voltage (OCV) of 1.08 V and a maximum power density of 2.8 mW/cm2. The micro-SOFC with a LSScCo electrolyte could operate at 370 °C; its OCV was about 0.8 V, and its maximum power density was 0.6 mW/cm2. Electrochemical impedance spectroscopy revealed that the electrolyte resistance in both the micro-SOFCs was lower than 0.1 Ωcm2, and almost all of the resistance was due to anode and cathode reactions. Although the obtained maximum power density was not sufficient for practical applications, improvement of electrodes will make these micro-SOFCs promising candidates for power sources of mobile electronic devices.

Effects of annealing on the electrical and interfacial properties of amorphous lanthanum scandate high-κ films prepared by molecular beam deposition

Amorphous LaScO3 thin films were grown on (100) Si by molecular beam deposition and the effects of postdeposition thermal treatments on the film properties were studied after anneals in O2 or inert Ar atmosphere at 400 or 650 °C. Rutherford backscattering spectrometry, transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy (XPS) were employed to investigate the samples. Capacitance-voltage and current-voltage measurements allowed their electrical characterization. Postdeposition annealing in O2 reduces hysteresis, flatband voltage, and also leakage current density. In contrast, films treated in Ar ambient revealed a different behavior. The observations were associated with the interface evolution as studied by XPS, which verify that an O2 atmosphere favors the formation of a SiO2-rich interface between the film and the Si substrate, while a La-Sc-silicate-like compound predominates in this region after treating the samples in Ar. Additionally, postdeposition annealing results in an improvement of the dielectric constant up to 33, which is higher than that previously determined for LaScO3 or other amorphous alternative high-κ oxide films deposited by various techniques.

Thermal stability of lanthanum scandate dielectrics on Si(100)

The authors have examined the thermal stability of amorphous, molecular beam deposited lanthanum scandate dielectric thin films on top of Si (100) after a 1000°C, 10s rapid thermal anneal. After the anneal, crystallization of LaScO3 is observed. Excellent suppression of lanthanum and scandium diffusion into the substrate silicon is indicated by the back-side secondary ion mass spectrometry (SIMS) analyses. In contrast, front-side SIMS and high-resolution electron energy loss analyses of the amorphous Si∕LaScO3∕Si (100) stack indicated the outdiffusion of lanthanum and scandium into the silicon capping layer during the anneal.