Cation Vacancy Defects Research Articles

Influence of Ga3+ ion dopant on the structure and magnetic properties of YCrO3

In this work, Ga3+ doped YCr1-xGaxO3 (x = 0, 0.1, 0.2 and 0.3) polycrystalline samples were prepared by solid state reaction method and their crystal structure, morphology, internal defect, specific heat and magnetic properties were studied. The results show that the lattice constants, lattice volume and particle size increase, while Y/Cr–O bond constant decreases with replacing Cr3+ by Ga3+. The positron annihilation spectrum (PAS) measurement shows that cation vacancy defect exists in all samples and the internal defect concentration fluctuates slightly with increasing of Ga3+ ions doping concentration. For low doping content, an additional magnetization anomaly emerges in the zero-field-cooling curves measured in H = 1000Oe. With the increase of magnetic field, the anomaly can be well suppressed. Simultaneously, for the specific heat curve peak cannot be observed at the corresponding temperature. These results imply the establishment of short-range magnetic order above the antiferromagnetic transition temperature (TN) due to the dilution effect of nonmagnetic Ga3+ doping.

The structural, dielectric, and magnetic properties of GdMnO3 multiferroic ceramics

GdMnO3 (GMO) ceramics were prepared by solid-state reaction method, the structure, defects, dielectric, and magnetic properties of GMO ceramics sintered at different temperatures have been studied. Characterization by XRD indicates that impurity phase-free GMO can be synthesized at 1350 °C. SEM measurements show that the grain size of GMO increases and the microstructure becomes denser with increasing sintering temperature. Positron annihilation results reveal that the vacancies concentration decreases when sintering temperature increases from 1300 to 1350 °C, and then increases when sintering temperature increases from 1350 to 1400 °C. Dielectric measurements indicate that the GMO ceramic sintered at 1350 °C presents higher dielectric constant and smaller dielectric loss. Magnetic properties measurements show that the magnetic properties of GMO can be modulated by sintering temperature. The relation between structure and properties implies that the cation vacancy defect has remarkable effects on the dielectric and magnetic properties of GMO.

Cation and anion immobilization through chemical bonding enhancement with fluorides for stable halide perovskite solar cells

Defects play an important role in the degradation processes of hybrid halide perovskite absorbers, impeding their application for solar cells. Among all defects, halide anion and organic cation vacancies are ubiquitous, promoting ion diffusion and leading to thin-film decomposition at surfaces and grain boundaries. Here, we employ fluoride to simultaneously passivate both anion and cation vacancies, by taking advantage of the extremely high electronegativity of fluoride. We obtain a power conversion efficiency of 21.46% (and a certified 21.3%-efficient cell) in a device based on the caesium, methylammonium (MA) and formamidinium (FA) triple-cation perovskite (Cs0.05FA0.54MA0.41)Pb(I0.98Br0.02)3 treated with sodium fluoride. The device retains 90% of its original power conversion efficiency after 1,000 h of operation at the maximum power point. With the help of first-principles density functional theory calculations, we argue that the fluoride ions suppress the formation of halide anion and organic cation vacancies, through a unique strengthening of the chemical bonds with the surrounding lead and organic cations. Defects and defect migration are detrimental for perovskite solar cell efficiency and long-term stability. Li et al. show that fluoride is able to suppress the formation of halide anion and organic cation vacancy defects by restraining the relative ions via ionic and hydrogen bonds.

Cation vacancy stabilization of single-atomic-site Pt1/Ni(OH)x catalyst for diboration of alkynes and alkenes

Development of single-atomic-site catalysts with high metal loading is highly desirable but proved to be very challenging. Although utilizing defects on supports to stabilize independent metal atoms has become a powerful method to fabricate single-atomic-site catalysts, little attention has been devoted to cation vacancy defects. Here we report a nickel hydroxide nanoboard with abundant Ni2+ vacancy defects serving as the practical support to achieve a single-atomic-site Pt catalyst (Pt1/Ni(OH)x) containing Pt up to 2.3 wt% just by a simple wet impregnation method. The Ni2+ vacancies are found to have strong stabilizing effect of single-atomic Pt species, which is determined by X-ray absorption spectrometry analyses and density functional theory calculations. This Pt1/Ni(OH)x catalyst shows a high catalytic efficiency in diboration of a variety of alkynes and alkenes, yielding an overall turnover frequency value upon reaction completion for phenylacetylene of ~3000 h−1, which is much higher than other reported heterogeneous catalysts.

The structure, defects, electrical and magnetic properties of BiFe1−x Zr x O3 multiferroic ceramics

BiFe1−x Zr x O3 (x = 0.00–0.30) ceramics were synthesized using solid state reaction method followed by rapid liquid phase sintering, and the microstructure, electrical and magnetic properties of the synthesized ceramics were systematically investigated. The XRD patterns show that no impurity phases exist in Zr doped samples; Zr doping induces the crystal structure distortion when x ≤ 0.10, and a structural phase transition occurs when the content of Zr varies from 0.10 to 0.20. SEM observations indicate that the average grain size is remarkably decreased by Zr doping. Positron annihilation lifetime spectra results indicate that cation vacancy-type defects exist in all samples, the cation vacancy concentration increases with increasing Zr content from 0.00 to 0.20, and then decreases with further increase of Zr content. Electrical and magnetic measurements show that enhanced leakage, ferroelectric and magnetic properties are observed in Zr doped ceramics. The analysis of microstructure and properties show that the cation vacancy defect plays an important role in modulating the electrical and magnetic properties of BiFeO3.

Structure and stability of pyrophyllite edge surfaces: Effect of temperature and water chemical potential

The surfaces of clay minerals, which are abundant in atmospheric mineral dust, serve as an important medium to catalyze ice nucleation. The lateral edge surface of 2:1 clay minerals is postulated to be a potential site for ice nucleation. However, experimental investigations of the edge surface structure itself have been limited compared to the basal planes of clay minerals. Density functional theory (DFT) computational studies have provided insights into the pyrophyllite edge surface. Pyrophyllite is an ideal surrogate mineral for the edge surfaces of 2:1 clay minerals as it possesses no or little structural charge. Of the two most-common hydrated edge surfaces, the AC edge, (110) surface in the monoclinic polytype notation, is predicted to be more stable than the B edge, (010) surface. These stabilities, however, were determined based on the total energies calculated at 0K and did not consider environmental effects such as temperature and humidity. In this study, atomistic thermodynamics based on periodic DFT electronic calculations was applied to examine the effects of environmental variables on the structure and thermodynamic stability of the common edge surfaces in equilibrium with bulk pyrophyllite and water vapor. We demonstrate that the temperature-dependent vibrational energy of sorbed water molecules at the edge surface is a significant component of the surface free energy and cannot be neglected when determining the surface stability of pyrophyllite. The surface free energies were calculated as a function of temperature from 240 to 600K and water chemical potential corresponding to conditions from ultrahigh vacuum to the saturation vapor pressure of water. We show that at lower water chemical potentials (dry conditions), the AC and B edge surfaces possessed similar stabilities; at higher chemical potentials (humid conditions) the AC edge surface was more stable than the B edge surface. At high temperatures, both surfaces showed similar stabilities regardless of the water chemical potential. The equilibrium morphology of pyrophyllite crystals is also expected to be dependent on these two environmental variables. Surface defects may impact the surface reactivity. We discuss the thermodynamic stability of a possible Si cation vacancy defect which provides additional hydroxyl group on the surface.

Electronic and magnetic properties of the cation vacancy defect inm−HfO2

The electronic and magnetic properties of cation vacancies in $m\text{\ensuremath{-}}{\mathrm{HfO}}_{2}$ are predicted using density functional theory. The hafnium vacancy is found to introduce a series of charge transition levels in the range 0.76--1.67 eV above the valence band maximum associated with holes localized on neighboring oxygen sites. The neutral defect adopts a $S=2$ spin state, and we compute corresponding $g$ tensors to aid electron experimental identification of the defect by electron spin resonance spectroscopy. We find that separated vacancies exhibit weak ferromagnetic coupling and the interaction is highly anisotropic---being much stronger when mediated by planes of three-coordinated oxygen ions. Further, we characterize the process of thermal detachment of a hole from a neutral vacancy providing an atomistic model for the $p$-type conductivity observed experimentally at high temperature. These results provide invaluable information on the electronic and magnetic properties of cation vacancies in ${\mathrm{HfO}}_{2}$ and can aid future experimental identification of these complex defects.

At last! The single-crystal X-ray structure of a naturally occurring sample of the ilmenite-type oxide FeCrO3.

A natural single crystal of the ferrimagnetic oxide FeCrO3, which was found in an opencast mine situated in the San Luis Potosí State in Mexico, has been characterized in order to elucidate some outstanding issues about the actual structure of this material. The single-crystal X-ray analysis unambiguously shows that transition metal cations are segregated in alternating layers normal to the threefold crystallographic axis, affording a structure isomorphous to that of ilmenite (FeTiO3), in the space group R3̅. The possible occurrence of cation antisite and vacancy defects is below the limit of detection available from X-ray data. Structural and magnetic results are in agreement with the coherent slow intergrowth of magnetic phases provided by the two antiferromagnetic corundum-type parent oxides Fe2O3 (hematite) and Cr2O3 (eskolaite). Our results are consistent with the most recent density functional theory (DFT) studies carried out on digital FeCrO3 [Sadat Nabi & Pentcheva (2011). Phys. Rev. B, 83, 214424], and suggest that synthetic samples of FeCrO3 might present a cation distribution different to that of the ilmenite structural type.

Cation vacancies and electrical compensation in Sb-doped thin-film SnO2 and ZnO

We present positron annihilation results on Sb-doped SnO2 and ZnO thin films. The vacancy types and the effect of vacancies on the electrical properties of these intrinsically n-type transparent semiconducting oxides are studied. We find that in both materials low and moderate Sb-doping leads to formation of vacancy clusters of variable sizes. However, at high doping levels cation vacancy defects dominate the positron annihilation signal. These defects, when at sufficient concentrations, can efficiently compensate the n-type doping produced by Sb. This is the case in ZnO, but in SnO2 the concentrations appear too low to cause significant compensation.

Probing optical band gaps at nanoscale from tetrahedral cation vacancy defects and variation of cation ordering in NiCo2O4 epitaxial thin films

High resolution electron energy loss spectroscopy (HREELS) is utilized to probe the optical band gaps at the nanoscale in epitaxial NiCo2O4 (NCO) thin films with different structural order (cation/charge). The structure of NCO deviates from the ideal inverse spinel (non-magnetic and insulating) for films grown at higher temperatures (>500 °C) towards a mixed cation structure (magnetic with metallic conductivity) at lower deposition temperatures (<450 °C). This significantly modifies the electronic structure as well as the nature of the band gap of the material. Nanoscale regions with unoccupied tetrahedral A site cations are additionally observed in all the samples and direct measurement from such areas reveals considerably lower band gap values as compared to the ideal inverse spinel and mixed cation configurations. Experimental values of band gaps have been found to be in good agreement with the theoretical mBJLDA exchange potential based calculated band gaps for various cation ordering and consideration of A site cation vacancy defects. The origin of rich variation in cation ordering observed in this system is discussed.

The origin of p-type conductivity in ZnM2O4 (M = Co, Rh, Ir) spinels

ZnM2O4 (M = Co, Rh, Ir) spinels are considered as a class of potential p-type transparent conducting oxides (TCOs). We report the formation energy of acceptor-like defects using first principles calculations with an advanced hybrid exchange-correlation functional (HSE06) within density functional theory (DFT). Due to the discrepancies between the theoretically obtained band gaps with this hybrid functional and the - scattered - experimental results, we also perform GW calculations to support the validity of the description of these spinels with the HSE06 functional. The considered defects are the cation vacancy and antisite defects, which are supposed to be the leading source of disorder in the spinel structures. We also discuss the band alignments in these spinels. The calculated formation energies indicate that the antisite defects ZnM (Zn replacing M, M = Co, Rh, Ir) and VZn act as shallow acceptors in ZnCo2O4, ZnRh2O4 and ZnIr2O4, which explains the experimentally observed p-type conductivity in those systems. Moreover, our systematic study indicates that the ZnIr antisite defect has the lowest formation energy in the group and it corroborates the highest p-type conductivity reported for ZnIr2O4 among the group of ZnM2O4 spinels. To gain further insight into factors affecting the p-type conductivity, we have also investigated the formation of localized small polarons by calculating the self-trapping energy of the holes.

Positron lifetime measurements of hydrogen passivation of cation vacancies in yttrium aluminum oxide garnets

A charge compensation mechanism is proposed for cation vacancy defects in complex oxides based on positron lifetime measurements, infrared spectroscopy, and composition analysis. Defects were characterized in samples of yttrium aluminum garnet grown in O${}_{2}$ or Ar. However, no positron trapping was detected in samples grown in H${}_{2}$. This is attributed to decoration of cation vacancies with hydrogen, thereby passivating charges of vacancies that otherwise function as positron traps. Infrared spectroscopy gave direct evidence of the presence of hydrogen. Passivation of cation vacancies with hydrogen is proposed as an important mechanism for charge compensation in the defect physics of oxides.

Bulk and Surface Tunneling Hydrogen Defects in Alumina

We perform abinitio calculations of hydrogen-based tunneling defects in alumina to identify deleterious two-level systems (TLS) in superconducting qubits. The defects analyzed include bulk hydrogenated Al vacancies, bulk hydrogen interstitial defects, and a surface OH rotor. The formation energies of the defects are first computed for an Al- and O-rich environment to give the likelihood of defect occurrence during growth. The potential energy surfaces are then computed and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. Finally, the tunneling energy is computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz frequency TLSs in alumina.

Nonstoichiometry accommodation in SrTiO3thin films studied by positron annihilation and electron microscopy

Accommodation of nonstoichiometry in SrTiO${}_{3}$ pulsed laser deposited (PLD) films was investigated using positron annihilation lifetime spectroscopy and (scanning) transmission electron microscopy. Increasing PLD laser fluence changed the stoichiometry from Ti to Sr deficient. Cation vacancy defects were detected, and the concentration ratio of Sr to Ti vacancies, [${V}_{\mathrm{Sr}}$]/[${V}_{\mathrm{Ti}}$], was observed to increase systematically in the Sr-deficient region, although no change in the electron microscopy lattice images was detected. Increasing Ti deficiency resulted in the accommodation of SrO layers in planar defects, and in the formation of vacancy cluster defects. A change from ${V}_{\mathrm{Ti}}$ to ${V}_{\mathrm{Sr}}$ defect positron trapping was also detected.

Ab initio study of structurally bound water at cation vacancy sites in Fe- and Al-oxyhydroxide materials

The structure and energetics of proton-compensated cation vacancies in crystalline Fe- and Al-oxide and oxyhydroxide materials are investigated using ab initio methods. In this defect model, a vacant Me3+ cation site is charge compensated by the presence of three protons, forming hydroxyls with the O atoms surrounding the vacant cation site. Proton-compensated cation vacancies are chemically equivalent to excess hydroxyl content, and are also known as hydrogarnet defects, or Ruetschi defects. These defects can be considered a particular form of structurally bound water, as the formation of the defect can be written as the product of the ideal crystalline material and water. Proton-compensated cation vacancy defects are shown to cause lattice expansion in all calculated Fe and Al materials, and are shown to destabilize all materials relative to the ideal crystalline phases. The magnitude of the destabilization due to the vacancy defects is structure dependent, thus defect content can induce shifts in the relative stability between crystalline phases.In all of the Fe-oxyhydroxide materials, proton-compensated cation vacancy defects are shown to be slightly co-stabilized in the presence of nearby Al dopant atoms, likely due to the stronger nature of Al–OH bonding (relative to Fe–OH), or from the cancellation of the opposing lattice strains introduced by the two defect types when considered in isolation. FTIR data in the literature confirms that these two defect types (proton-compensated cation vacancies and Al substitutions) have been observed to occur in tandem.Infrared vibrational frequencies are calculated for the non-stoichiometric hydroxyl groups found at the vacancy defect sites and compared with those of stoichiometric OH groups found in ideal crystalline (oxy)hydroxides. The calculated O–H stretching modes of the defect hydroxyls have higher frequencies than the modes of stoichiometric hydroxyl groups found in Fe-oxyhydroxide materials, consistent with experimental FTIR observations.

Can cation vacancy defects induce room temperature ferromagnetism in GaN?

The unique properties of gallium nitride (GaN) crystal, such as a wide band-gap and high thermal conductivity, make it ideal material for electronic and optoelectronic devices. Achieving room temperature (RT) ferromagnetism in GaN becomes crucial. In previous works, gallium vacancy (VGa) was expected to be promising for reaching this goal. However, using an accurate hybrid exchange-correlation functional, we show that the largest value of J0 is only 3.3 meV at the VGa density of 1.28 × 1021 cm−3, corresponding to a Curie temperature of 150 K. This suggests that VGa cannot induce RT ferromagnetism at the density lower than that value.

Variable Energy Positron Annihilation Spectroscopy of Perovskite Oxides

The application of variable energy positron annihilation spectroscopy (VE-PAS) methods to the study of perovskite oxide, ABO3, material thin films and near-surface regions is reviewed. The primary focus is on ferroic perovskite titanate oxide materials SrTiO3and Pb (ZrxTi1-x)O3, but studies of BaTiO3, LaxSr1-xCoO3, La0.67Sr0.33MnO3and YBa2CuO7-δare also included. Characterization of single layer and multilayer structures is discussed. The methods, in particular positron annihilation lifetime spectroscopy, allow the identification of cation vacancy defects at both the A-and B-sites with parts per million sensitivity. Varying oxygen deficiency is often observed to result in marked changes in PAS spectra; these effects are reviewed and discussed.

Doping Effect of Nb5+ on the Microstructure and Defects of LiFePO4

An Nb5+-doped cathodic material, Li1−xNbx+yFe1−yPO4, was synthesized via the high-temperature solid-state method. The crystal structure, micro-defects and doping ion occupancy of doped LiFePO4 have been studied via the Rietveld crystal structure refinement, HRTEM, EPR and positron annihilation lifetime spectroscopy (PALS) methods. The effect of ion doping on the electrochemical properties was studied by measuring the changes in the electronic localization and structural defects. The results of the Rietveld crystal structure refinement showed that Nb5+ occupied positions on both Li and Fe in [Li0.99−xNbx][Fe1−yNby]PO4, with an occupancy rate on Fe approximately six times greater than that in the Li position. The HRTEM results indicated that, when Nb5+ was doped into the lattice well, there were no other phases on the grain surface. Furthermore, the EPR results indicated that Nb5+ occupying the Fe position broke the Fe2+-O-Fe2+ electron spin correlation, which distorted the crystal ligand field around Fe2+. The PALS investigation showed that Nb5+ could be doped into the internal LiFePO4 lattice to produce cation vacancy defects. The internal defects and outer electron energy of the cations in the lattice were changed because of the Nb5+ doping, which could improve the electrochemical performance of the material.

Theoretical Study on Cation Vacancy Induced Magnetism in Bulk ZnO

Based on first principle FP-LAPW calculations, the electronic structure and magnetic properties of ZnO with Zn cation vacancy has been investigated. We find that the cation vacancy defect can induce a magnetic moment of about 2μB/supercell. The magnetic moment mainly comes from 2p-orbitals of O atoms which surround the Zn vacancy. We also find that the two Zn vacancies in ZnO always coupled Ferro-magnetically. Furthermore, the system shows half metallic Ferro-magnetic properties with high Curie temperature.

Non-stoichiometry in MgAl2O4 spinel

Stoichiometric magnesium aluminate spinel, MgAl2O4, contains equimolar proportions of Al2O3 and MgO. Spinel can, however, exhibit significant deviations from this stoichiometric composition. There is considerable disagreement concerning which species compensate for either excess Al2O3 or MgO non-stoichiometry. Here, we use empirical and quantum mechanical (density functional theory) atomistic simulation techniques to investigate the defect chemistry accommodating non-stoichiometry. The incorporation of excess Al2O3 was found to be a lower energy process than the solution of excess MgO. Elevated magnesium and aluminium cation vacancy defect concentrations are predicted in Al2O3 rich spinels, whilst MgO excess is facilitated by a combination of oxygen vacancy and magnesium interstitial defects.