Computer simulation of the oxygen mobility in CaMnO3-x

The perovskite-type metal oxide CaMnO_3-x is known to accommodate substantial amounts of oxygen vacancies. High-resolution electron microscope investigations give evidence for ordering of the vacancies, i.e. well-defined structures in the compositional range of CaMnO_2.5 < CaMnO_3-x < CaMnO₃. Within this range the metal cation positions do not change, i.e. perovskitic framework is conserved while a remarkably high oxygen anion mobility is recorded. In addition, the electronic and magnetic structure, and thus the physical properties, depend directly on the oxygen stoichiometry.

This contribution focusses on the oxygen mobility in different CaMnO_3-x phases exhibiting oxygen vacancy ordered structures, i.e. CaMnO_3.0, CaMnO_2.80, CaMnO_2.75, CaMnO_2.66, CaMnO_2.55 and CaMnO_2.50. In these compounds the formal oxidation state of manganese changes from Mn⁴⁺ (x=0) to Mn³⁺ (x=0.5).

For the computer simulation of the defect structure and for the mobility of the oxygen anions within these defect structures we applied the method of interatomic potentials in a simple rigid-ion approximation. The parameters of interaction were calibrated on the basis of empirical data. i.e. equilibrium geometry and cohesive energies of the binary oxides CaO, MnO₂ and Mn₂O₃ were taken into account for the present calculations.

Stabilities, oxygen migration barriers and dielectric constants of selected representants of CaMnO_3-x are presented.     

Photoluminescence of hexagonal boron nitride: Effect of surface oxidation under UV-laser irradiation

We report on the UV laser-induced fluorescence of hexagonal boron nitride (h-BN) following nanosecond laser irradiation under vacuum and in different environments of nitrogen gas and ambient air. The observed fluorescence bands are tentatively ascribed to impurity and mono (V_N) or multiple (m-V_N with m=2 or 3) nitrogen vacancies. A structured fluorescence band between 300 and 350 nm is assigned to impurity-band transition and its complex lineshape is attributed to phonon replicas. An additional band at 340 nm, assigned to V_N vacancies on surface, is observed under vacuum and quenched by adsorbed molecular oxygen. UV-irradiation of h-BN under vacuum results in a broad asymmetric fluorescence at ∼400 nm assigned to m-V_N vacancies; further irradiation breaks more B-N bonds enriching the surface with elemental boron. However, no boron deposit appears under irradiation of samples in ambient atmosphere. This effect is explained by oxygen healing of radiation-induced surface defects. Formation of the oxide layer prevents B-N dissociation and preserves the bulk sample stoichiometry.     

Defect formation and water incorporation in Y2O3

The molecular statics method is used to study the formation of defects and water incorporation in Y₂O₃. The crystal structure, the isothermal compressibility, and the formation enthalpy of Y₂O₃ calculated with the chosen interaction potentials are in good agreement with the experimental data. The formation energies of intrinsic and impurity defects are evaluated. The binding energy of protons and oxygen vacancies with an acceptor impurity at different distances is calculated. Various water incorporation reactions in the oxide are examined, including the mechanisms involving oxygen interstitial sites and oxygen vacancies produced by the acceptor doping. It is shown that the water incorporation in pure Y₂O₃ is energetically less favorable than in the acceptor doped oxide.     

The influence of chromium on the defect structure and their mobility in nonstoichiometric cobaltous oxide

Defect structure and the mobility of point defects in pure metal deficient cobalt oxide (Co_1−yO) and in Co_1−yO-Cr₂O₃ solid solutions have been studied as a function of temperature (1223-1573 K) and oxygen pressure (10-10⁵ Pa) using microthermogravimetric techniques. It has been shown that the predominant defects in pure and Cr-doped cobaltous oxide are singly ionized cation vacancies, and 3% at of dopant is high enough to fix the concentration of predominant defects in such solid solutions on a constant level being much higher than in pure Co_1−yO. Re-equilibration rate measurements have demonstrated that the chemical diffusion coefficient and thereby the mobility of point defects in pure Co_1−yO is concentration independent, strongly suggesting that in spite of rather high their concentration no interactions and clustering of defects is to be expected. On the other hand, in Cr-doped cobaltous oxide, re-equilibration rate measurements have shown, that in this case the defect structure is more complicated, although singly ionized cation vacancies seem to be still predominant defects.     

An improved method for preparation of Ce<Subscript>0.8</Subscript>Pr<Subscript>0.2</Subscript>O<Subscript>Y</Subscript> solid solutions with nanoparticles smaller than 10 nm

Ce_0.8Pr_0.2O_Y solid solutions with ultrafine crystalline sizes and high specific surface area were prepared by an improved citrate precursor method, where a nitrogen treatment was added prior to calcinations in air. The samples were characterized by TG-DSC, Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET nitrogen adsorption. XRD and Raman results show that the formation of Ce_0.8Pr_0.2O_Y solid solutions typical of the fluorite-like cubic structure with oxygen vacancies occurs when the Ce–Pr citrate precursors are heated at high temperature in the nitrogen atmosphere. Subsequent calcinations at a low temperature in air to remove carbon species have no apparent effects on the formed solid solutions. Ce_0.8Pr_0.2O_Y solid solution prepared by the improved citrate precursor method at 800°C has ultrafine nanoparticles of less than 10 nm and high specific surface area of 92.1 m²/g, while the sample prepared by the conventional citrate precursor method has mean particle size of 62.1 nm and specific surface area of 18.1 m²/g. Furthermore, Ce–Pr solid solution by the improved method have the mesoporous structure, more lattice defects and oxygen vacancies, which will have a promising application in the catalyst region as well as SOFC field.     

Effect of oxygen vacancies on adhesion at the Nb/Al2O3 and Ni/ZrO2 interfaces

The atomic and electronic structures of the Nb/Al₂O₃(0001) and Ni/ZrO₂(001) interfaces are calculated using density-functional theory. The formation energy of oxygen vacancies is estimated in bulk materials and in surface layers and interfaces for different uppermost atomic layers of oxide surfaces. The work of separation of metal films from oxide surfaces is determined. The effect of oxygen vacancies on the bonding of transition metals to atoms of a substrate determining adhesion at the metal-oxide interfaces is discussed. It is shown that the Nb(Ni)-O interaction at the interfaces weakens in the presence of surface oxygen vacancies.     

Structure and surface properties of praseodymium modified alumina

Mixed PrO₂-Al₂O₃ oxides with different PrO₂ content (1-20 wt.%) were prepared by wetness impregnation of γ-alumina with aqueous solution of praseodymium nitrate. The samples were characterized by different techniques, using surface adsorption-desorption of N₂ (S_BET), thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), temperature-programmed reduction (TPR) and temperature-programmed desorption of CO₂ (TPD-CO₂). TGA and XRD showed the presence of small praseodymium oxide species on the alumina surface. XPS and DRS detected electron deficient interaction between deposited praseodymium oxide and alumina. It was observed a lower reduction temperature for supported Pr oxide species compared to that of the bulk Pr₆O₁₁. TPD-CO₂ studies suggested that the deposition of Pr oxide on alumina leaded to increase of the basicity of mixed oxides.     

Growth of ultra-thin cerium oxide layers on Cu(1 1 1)

The reactive vacuum deposition of CeO₂ on Cu(1 1 1) surface in oxygen atmosphere provides high quality epitaxial ceria overlayers. We report the growth characteristics of Ce oxide, the structures, and the temperature stability of the oxide phases as investigated by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy. We find that Ce oxide on the Cu(1 1 1) grows initially in the form of islands giving sharp hexagonal LEED pattern of the CeO₂(1 1 1) structure corresponding to the (1.5 × 1.5) structure. The CeO₂-Cu(1 1 1) films exhibited mixed valence states and temperature dependent CeO₂-Ce₂O₃ transition above 900 K due to the vacuum annealing. The transition progressed more rapidly at the surface, probably by formation of oxygen vacancies.     

Density functional investigation of fluorite-based Pa2O5 phases: Structure and properties

Protactinium pentoxide (Pa₂O₅) represents the fluorite and the layered structures of the protactinium-oxygen system and the least studied oxide in the group. Our DFT work explores some possible structures of Pa₂O₅ stoichiometry by starting from the fluorite PaO₂ structure and adding oxygen in octahedral position of protactinium. For all simulated structures, the structural, thermodynamic and electronic properties including lattice parameters, bulk moduli, entropy, zero point energy (ZPE) and band gaps were predicted. The fluorite Pa₂O₅ in the b- and g-Pa₂O₅ structures were found to be the most stable overall. Indeed, the doping of oxygen atoms in the octahedral position of Pa is significantly better than other doping positions for the fluorite PaO₂ structure. In particular, we found that the PaO_2.5 structure is the most thermodynamically stable under ambient conditions. Furthermore, while x from 0 increases to 0.5 for protactinium oxides (PaO_2+x, x = 0, 0.25, 0.5, 0.75, 1), the decreasing trend with increasing x of the optimized volume is in reasonable agreement with experimental findings.     

Simulation of the Atomic and Electronic Structure of Oxygen Vacancies and Polyvacancies in ZrO<Subscript>2</Subscript>

Cubic, tetragonal, and monoclinic phases of zirconium oxide with oxygen vacancies and polyvacancies are studied by quantum chemical modeling of the atomic and electronic structure. It is demonstrated that an oxygen vacancy in ZrO₂ may act as both an electron trap and a hole one. An electron added to the ZrO₂ structure with an oxygen vacancy is distributed between two neighboring Zr atoms and is a bonding orbital by nature. It is advantageous for each subsequent O vacancy to form close to the already existing ones; notably, one Zr atom has no more than two removed O atoms related to it. Defect levels from oxygen polyvacancies are distributed in the bandgap with preferential localization in the vicinity of the oxygen monovacancy level.     

Role of an oxide interface in a resistive switch

In the present era of data-driven architectures like 5G, Internet of things (IoT), Artificial Intelligence (AI), etc, the requirement of fast-switchable memory storage is more than ever. Oxide resistive switches are considered to be a primary choice in the non-volatile memory design. In this work, we have engineered the conventional metal-insulator-metal (MIM) structure of an oxide memristor (Ag/ZnO/ITO) by inducing an additional oxide layer La_0.7Sr_0.3MnO₃ (LSMO) at the interface between the active layer (ZnO) and Ag electrode. The presence of LSMO acts as a reservoir for the oxygen vacancies, easing the conducting filament formation process in ZnO, thereby enabling drastic improvement of the switching performance and offering reliable endurance over multiple switching cycles. First-principles-based calculations suggested the role of oxygen vacancies in controlling the electronic state of ZnO and formation of vacancies in the resistive switching process, which is in agreement with the experimental observation. The current results pave ways for improving the switching performance of resistive memory circuits through simple structural engineering incorporation, which lies at the heart of oxide electronics.     

Modified thermal evaporation process using GeO2 for growing ZnO structures

In the current work, zinc oxide (ZnO) nano/microstructures are synthesized using a modified thermal-evaporation process by introducing germanium oxide (GeO₂) powder mixed with metallic Zn powder as the raw material. Without the use of any catalyst and oxygen flow in the furnace system, GeO₂ is utilized to provide an oxygen source for the growth of ZnO structure. The samples are treated by different temperatures ranging from 500 to 900 °C. Morphology, phase structure, and photoluminescence properties are investigated by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and photoluminescence (PL) spectrometer. The structures and morphologies of the samples were found to vary with growth temperature. The XRD diffraction peaks show that the films grown at temperature from 600 to 800 °C consist of hexagonal wurtzite ZnO structures. Room-temperature PL measurement revealed ZnO spectra representing two bands: near-band-edge emission in the ultraviolet (UV) region and broad deep-level emission centered at about 500 nm. The strong UV emission in the PL spectra indicates that the GeO₂ supplies sufficient oxygen for formation of ZnO structures with few oxygen vacancies. The growth mechanism and the roles of GeO₂ for formation of ZnO structures are discussed in detail.     

Transport properties of the Pr2−xSrxNiO4±δ ceramics with x = 0.3 and 0.6

As potential cathode materials Pr_2−xSr_xNiO_4±δ compositions with x = 0.3 and 0.6 were prepared at 1300 °C in air and their electrical conductivity and oxygen non-stoichiometry were investigated in the temperature range 20–1000 °C and oxygen partial pressure (pO₂) 1–21,000 Pa. Sr-doping allows partially to stabilize the Pr₂NiO_4±δ structure, but some phase transitions were observed in spite of that. The electrical conductivity and the oxygen mobility of the Pr- and La-containing ceramic nickelates with K₂NiF₄- type structure are 10–15% higher for Pr_2−xSr_xNiO_4−δ compounds at the same temperature and oxygen partial pressure.     

Large enhancement of photoluminescence in SrTiO3:Pr powders by fluorhydric acid treatment

We have investigated the effect of fluorhydric acid (HF) treatment on the photoluminescence (PL) properties of SrTiO₃:Pr³⁺ powders prepared by sol-gel method. The red emission intensity increased significantly up to 18 times when the powders were subjected into a water-diluted 5% HF solution for 10 min. The origin of the PL enhancement was ascribed to the increase of oxygen vacancies in HF-treated SrTiO₃:Pr³⁺ powders. This study also manifested that HF etching is more effective to improve the red PL intensity than vacuum-annealing for the sol-gel made SrTiO₃:Pr³⁺ powders.     

Low-temperature structural anomalies in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript>

The magnetic and crystal structures of the Pr_0.5Sr_0.5CoO₃ metallic ferromagnet have been studied by the neutron diffraction technique. It is demonstrated that below 150 K, the compound is mesoscopically separated into two crystalline phases with different spatial symmetries and with different directions of the magnetic anisotropy. The phase separation exists down to 1.5 K, and at temperatures below 90 K, the low-symmetry phase occupies about 80% of the sample volume. The main structural difference between the phases is the configuration of oxygen atoms around praseodymium and, to a certain extent, around cobalt. The ferromagnetic structure with the magnetic moment lying in the basal plane of the structure (μ_Co ≈ 1.7 μ _B at 1.5 K) arises at 234 K, whereas the component directed along the long axis of the unit cell appears at 130 K. The formation of the new structural phase and change in the orientation of the magnetic moment give rise to the anomalies of the physical and magnetic characteristics of this compound observed earlier at temperatures about 120 K.     

Volume collapse phase transitions in cerium-praseodymium alloys under high pressure

Cerium-12at%Praseodymium(Ce_0.88Pr_0.12) and Ce-50at%Praseodymium(Ce_0.50Pr_0.50) alloy samples that contain a random solid-solution of Ce (4f¹ (J?=?5/2)) and Pr (4f² (J?=?4)) localized f-states have been studied by angle dispersive x-ray diffraction in a diamond anvil cell to a pressure of 65?GPa and 150?GPa respectively using a synchrotron source. Ce_0.88Pr_0.12 alloy crystallizes in a face-centered cubic (γ-phase) structure at ambient conditions, while Ce_0.50Pr_0.50 alloy crystallizes in the double hexagonal close packed (dhcp) structure at ambient conditions. Two distinct volume collapse transitions are observed in Ce_0.88Pr_0.12 alloy at 1.5?GPa and 18?GPa with volume change of 8.5% and 3% respectively. In contrast, Ce_0.50Pr_0.50 alloy shows only a single volume collapse of 5.6% at 20?GPa on phase transformation to α-Uranium structure under high pressure. Electrical transport measurements under high pressure show anomalies in electrical resistance at phase transitions for both compositions of this alloy.     

Interface-related switching behaviors of amorphous Pr0.67Sr0.33MnO3-based memory cells

The resistive switching properties in amorphous Pr0.67Sr0.33MnO3 films deposited by pulsed laser deposition are investigated.Reproducible and bipolar counter-8-shape and 8-shape switching behaviours of Au/Pr0.67Sr0.33MnO3 /F：SnO2 junctions are obtained at room temperature.Dramatically,the coexistence of two switching polarities could be reversibly adjusted by an applied voltage range.The results allocated those two switching types to areas of different defect densities beneath the same electrode.The migration of oxygen vacancies and the trapping effect of electrons under an applied electric field play an important role.An interface-effect-related resistance switching is proposed in an amorphous Pr0.67Sr0.33MnO3-based memory cell.     

A thermodynamic model of ordering of nonstoichiometric titanium monoxide TiO y with structural vacancies in the oxygen and titanium sublattices

A thermodynamic model of ordering of nonstoichiometric titanium monoxide TiO _y with structural vacancies in the oxygen and titanium sublattices has been developed taking into account the long-range Coulomb interactions. Methods for calculating the internal energy and entropy of this compound have been proposed. The free energy is represented in the form of a function of long-range order parameters, stoichiometry, and temperature. It has been found that the order-disorder phase transition in the titanium monoxide should occur according to the mechanism of a first-order phase transition. The calculated dependences of the critical values of the long-range order parameters and temperatures on the composition y have demonstrated that, when the composition deviates from stoichiometry, the disordering of structural vacancies in different sublattices should proceed at different temperatures.     

Application of transmission electron microscopy for studying phase relations in the bismuth-rich region of the Ba-Bi-O system

The crystal structure and cation composition of oxides of the Ba-Bi-O system in the composition range 80–100 mol % BiO_1.5 have been investigated by the methods of transmission electron microscopy. Ordered phases of the compositions Ba: Bi = 2: 9, 1: 6, and 1: 15 with a rhombohedral structure have been revealed. In the range of the compositions Ba: Bi from 1: 36 to 1: 46, phases with triclinic, monoclinic, and cubic structures have been found. The monoclinic and cubic phases have structures similar to the structures of the α and γ modifications of bismuth oxide Bi₂O₃, respectively. Phase formation was found to be dependent on the following parameters: annealing temperature, partial oxygen pressure, oxygen content in the initial pairs of reacting components (BaO₂-Bi₂O₃, Ba(NO₃)₂-Bi₂O₃, BaCO₃-Bi₂O₃), and crucible material (alundum or platinum).