Sr Vacancies Research Articles

Effects of Gd substitution on microstructures and low temperature dielectric relaxation behaviors of SrTiO3 ceramics

In this paper, Sr1 − 3x/2GdxTiO3 ceramics (0.00 ≤ x ≤ 0.05) were prepared from powders obtained by a sol–gel method. X-ray diffraction results showed that the Gd ion substituted Sr site, and the unit cell volume of doped samples decreased with increasing Gd doping concentrations, which was due to the difference between Sr/Gd ionic radii. Moreover, the structural change also led to an increase in the antiferrodistortive transition temperature. Dielectric properties of all samples were investigated over broad temperature and frequency ranges, and four dielectric relaxation behaviors were identified in different samples. It was found that two intrinsic dielectric relaxation behaviors for the pure SrTiO3 ceramics could be gradually disappeared with increasing Gd doping concentrations, which was ascribed to the change of domain state induced by Sr vacancies. On the other hand, the other two dielectric relaxation behaviors were induced by Gd doping. The relaxation behavior with activation energy in the range of 100–130 meV was originated from oxygen vacancies, while another relaxation behavior with activation energy in the range of 200–230 meV corresponded to the thermal motions of Ti4+ ions.

Crystal structure, local structure, and defect structure of Pr-doped SrTiO3

X-ray diffraction studies showed that the structure of (Sr1−xPrx)TiO3 solid solutions at 300 K changes from the cubic Pm3m to the tetragonal I4/mcm with increasing x. The analysis of XANES and EXAFS spectra of the solid solutions revealed that Pr ions are predominantly in the 3+ oxidation state, they substitute for Sr atoms and are on-center regardless of the preparation conditions. The weak dependence of the lattice parameter in (Sr1−xPrx)TiO3 on the Pr concentration was explained by the competition between the relaxation of the Sr–O bond length, which results from the difference in ionic radii of Sr and Pr ions, and the repulsion of positively charged Pr3+ and Ti4+ ions. It was shown that the most important defects in the crystals are charged Sr vacancies and SrO planar faults; praseodymium does not enter the Sr sites in the planar faults.

Identification of hydrogen defects in SrTiO3by first-principles local vibration mode calculations

For over three decades, the infrared spectroscopy peaks of around 3500 cm${}^{\ensuremath{-}1}$ observed in hydrogen-doped SrTiO${}_{3}$ samples have been assigned to an interstitial hydrogen (H${}_{i}$) attached to a lattice oxygen with two possible configuration models: the octahedral edge (OE) and the cubic face (CF) models. Based on our first-principles calculations of H${}_{i}$ around O, both OE and CF configurations are not energetically stable. Starting from either configuration, the H${}_{i}$ would spontaneously relax into an off axis (OA) site; lowering the energy by 0.25 eV or more. The calculated vibrational frequency of 2745 cm${}^{\ensuremath{-}1}$ for OA invalidates the assignment of H${}_{i}$ to the observed 3500 cm${}^{\ensuremath{-}1}$ peak. In addition, the calculated diffusion barrier is low, suggesting that H${}_{i}$ can be easily annealed out. We propose that the observed peaks around 3500 cm${}^{\ensuremath{-}1}$ are associated with defect complexes. A Sr vacancy (${V}_{\mathrm{Sr}}$) can trap H${}_{i}$ and form a H-${V}_{\mathrm{Sr}}$ complex which is both stable and has the frequency in agreement with the observed main peak. The complex can also trap another H${}_{i}$ and form 2H-${V}_{\mathrm{Sr}}$; consistent with the observed additional peaks at slightly higher frequencies (3510--3530 cm${}^{\ensuremath{-}1}$).

Vacancy induced magnetism in SrTiO3

Vacancy-induced magnetism in perovskite SrTiO3 is investigated by ab initio calculations and magnetic measurements. The calculations of the generalized gradient approximation (GGA), the local density approximation (LDA) and the local density approximation with on-site effect U (LDA+U) methods show that stoichiometric SrTiO3 is nonmagnetic. The GGA calculated results indicate that Ti or O vacancy could induce magnetism rather than Sr vacancy. The LDA and LDA+U calculations show that the Ti vacancy could induce magnetism, while Sr and O vacancies couldn't. The experimental results confirm that SrTiO3 nanocrystalline powders exhibit room-temperature ferromagnetism (FM) and the magnetic moment results from cation vacancies.

Suppression of vacancy defects in epitaxial La-doped SrTiO3 films

Variable energy positron annihilation lifetime spectroscopy of high-mobility La-doped SrTiO3 grown by molecular beam epitaxy found that the films contained sufficiently low concentrations of Sr vacancies and vacancy cluster defects to allow the observation of positron annihilation events from the perfect lattice. This enabled the concentrations of charged cation vacancies to be estimated, and these were found to be at least an order of magnitude below the La-dopant concentrations.

The effect of vacancies on the annular dark field image contrast of grain boundaries: A SrTiO3 case study

The analysis of grain boundary structure in high resolution electron microscopy is often hindered by contrast variation within the grain boundary region which is not explained by simple models of the grain boundary structure. Recent work suggests that structural disorder along the beam direction and the presence of vacancies contribute significantly to this effect. One might expect a significant reduction in contrast in a Z-contrast image of a grain boundary would imply that vacancies present must result from the absence of heavier elements. Using a [001](210) Σ5 grain boundary in SrTiO(3) as a test case and first principles structure relaxation to calculate stable defect structures, we show that the reduction in the intensity from fully occupied Sr columns due to the structural distortion resulting from a nearby O vacancy can be as great as that due to introducing a Sr vacancy in the column itself. The effect on energy dispersive X-ray spectroscopy signals is also considered, but found to be smaller than that on Z-contrast images.

Strontium vacancy clustering in Ti-excess SrTiO3 thin film

The defect structure of a homoepitaxial thin film of SrTiO3 containing excess Ti grown by pulsed laser deposition is investigated. The Sr/Ti ratio of the film is controlled to possess a slight excess of Ti by tuning the laser fluence. High-resolution transmission electron microscopy and electron energy loss spectroscopy studies reveal that the film contains Sr vacancies because of the presence of excess Ti. These vacancies tend to cluster as platelets several nanometers in diameter on (001) planes in the films. The SrO planes that contain the vacancy clusters exhibit lattice expansion due to Coulomb repulsion.

Role of trivalent Sr substituents and Sr vacancies in tetragonal and polar states of SrTiO 3

Inelastic light scattering is used to study lattice dynamics of strontium titanate (STO) ceramics with several heterovalent dopants (La 3+, Gd 3+, Y 3+), which substitute Sr 2+ ions. An extraordinary shift of the antiferrodistortive transition temperature ( T a) is ascertained when just a small percentage of any of the dopants is used. T a is dependent on the tolerance factor ( t). In this work, it is clearly shown that, regardless of the dopant used, a common linear dependence of T a vs. t is obtained if strontium vacancies are taken into account. A vacancy size of ∼1.547 Å was estimated, which is ∼7% larger than the Sr 2+ radius. The vacancy size obtained can directly explain the increase in lattice parameter with increasing Bi 3+ content in Bi-doped STO, as opposed to the dopants referred to above. Furthermore, the introduction of La 3+, Gd 3+ or Y 3+ ions at the Sr site causes a considerable stiffening of the transverse optic TO 1 mode at low temperatures, thereby decreasing the phonon contribution to the dielectric permittivity. Thus, no traces of a ferroelectric phase are found for any of the dopants used in this work.

First-principles study on migration mechanism in SrTiO3

The atomistic behavior of migration in SrTiO3 was investigated by first-principles nudged elastic band calculations. Calculated migration energies for Sr and oxygen are consistent with experimental values. In contrast, the calculated energy for Ti with a simple Ti-vacancy mechanism is far larger than the experimental value. In examining different Ti-migration mechanisms, the Ti-migration energy is found to decrease and become comparable to the Sr-migration energy by introducing a Sr vacancy. This Sr-vacancy-mediated Ti migration, which is consistent with the experimentally proposed mechanism by Gömann et al. [Phys. Chem. Chem. Phys. 6, 3639 (2004)], is confirmed theoretically by the present calculations.

Enhancement of tetragonality and role of strontium vacancies in heterovalent doped SrTiO3

The effect of Sr vacancies on the behavior of strontium titanate with trivalent dopants (La3+, Gd3+, and Y3+) substituting Sr2+ ions is reported. A remarkable shift of the antiferrodistortive transition temperature Ta is revealed by Raman spectroscopy for just a small content of dopant. It is shown that a unique linear dependence of Ta versus tolerance factor is obtained when Sr-vacancies are taken into account. A vacancy size value of ∼1.54 Å is estimated, which is ∼7% larger than Sr2+ radius. This size difference enables explaining the unexpected increase of lattice parameter with increasing Bi3+ content in Sr1−1.5xBixTiO3.

SrAlxOy:Eu2+, Dy3+ (x = 4) nanostructures: Structure and morphology transformations and long-lasting phosphorescence properties

Long-lasting phosphor SrAlxOy:Eu2+, Dy3+ (x = 4) nanostructures were synthesized through a facile but efficient combustion technique followed by a post-annealing reaction at different temperatures. During the synthesis process, triethanolamine (TEA) can not only be used as an organic fuel, but also can control the morphology of nanostructured combustion products. With an increase in post-annealing temperature, their morphology transforms from nanosheets into semi-curled nanobelts due to the crystal growth and the edge curl effect, and moreover, the products with various crystallographic structures can be obtained, whose transition presents the following rule: amorphous → hexagonal β-SrAl2O4 (1000 °C) → monoclinic SrAl4O7 (1100 °C) → orthorhombic Sr4Al14O25 (1300 °C). Excess Al facilitates the stabilization of the β-SrAl2O4 high-temperature phase at room-temperature. Due to the difference of Eu2+ surroundings in various crystallographic structures, their excitation and emission spectra exhibit some discrepancy in peak position and intensity. Surface and boundary defects, originating from small size and poor crystallinity, can both increase trap densities and depth levels; while Sr vacancies (VSr), donated by the deficiency of Sr2+ ions in β-SrAl2O4 and Sr4Al14O25, mainly increase the concentration of relatively shallow traps. Thus, β-SrAl2O4 shows a relatively small afterglow decay constant compared to SrAl4O7. Moreover, the afterglow decay constant of Sr4Al14O25 is initially much smaller than those of β-SrAl2O4 and SrAl4O7, then increases gradually, and finally exceeds them. Additionally, the maximum value of initial brightness and effective afterglow time emerges in the SrAl4O7 and Sr4Al14O25 composite phosphor annealed at 1200 °C.

Thermal Expansion Properties of Sr1 − xLaxTiO3 ( 0 ⩽ x ⩽ 0.3 ) Perovskites in Oxidizing and Reducing Atmospheres

perovskites are strong candidate materials for the interconnects and anodes in solid oxide fuel cells. In this paper, the thermal expansion mechanism of with La content of during reduction-oxidation (redox) cycling is discussed. It was found that for La-doped , air-sintered samples shrank during cycling in the reducing atmosphere. On the other hand, the reduced samples expanded during cycling in air. This thermal expansion behavior is considered to be related to the formation and disappearance of Sr vacancies in the perovskites through the release and absorption of oxygen, respectively.

Identification of vacancy defects in a thin film perovskite oxide

Vacancies are the dominant point defects in perovskite oxides, however, detecting and identifying the nature of vacancy defects in thin films remains challenging. This can be achieved using electron-beam methods but concentrations of several percent are required. Here we use a high-flux positron beam, providing high statistics positron lifetime measurements, to identify vacancies in laser ablated ${\text{SrTiO}}_{3}$ on ${\text{SrTiO}}_{3}$. The method is capable of subparts per million sensitivity and when combined with density-functional theory provides local structure information. The positron lifetime spectrum depth profile detects the presence of large vacancy clusters in a surface layer, a uniform distribution of Sr vacancies through the bulk of the film and resolves the interface with the substrate.

(Sr1–xCax)(11+16y–25z)/2(Si1–yAly)16(N1–zOz)25 (x ≈ 0.24, y ≈ 0.18, z ≈ 0.19) – A Novel Sialon with a Highly Condensed Silicate Framework

Abstract(Sr1–xCax)(11+16y–25z)/2(Si1–yAly)16(N1–zOz)25 (x ≈ 0.24, y ≈ 0.18, z ≈ 0.19) was obtained by high‐temperature synthesis (1650 °C – 1700 °C) using CaSiAlN3, Si3N4, AlN, SiO2, and Sr2N as starting materials. The crystal structure [Imm2 (no. 44), a = 20.6973(6), b = 10.7292(4), c = 4.8881(2) Å, Z = 2, R1 = 0.0326, wR2 = 0.0735] was determined from single‐crystal X‐ray data using dual space methods and confirmed for the bulk material by Rietveld refinement on X‐ray powder diffraction data. The structural results are corroborated by lattice energy calculations (MAPLE). The compound is the first representative of a novel silicate framework. The anionic part of the structure is built up from highly‐condensed dreier ring layers extending parallel (100), which are interconnected by common N and O atoms. In the resulting voids of this framework, there are three different cation sites, which are coordinated by six, eight, and twelve nitrogen and oxygen atoms, respectively. The largest one is fully occupied by Sr atoms, whereas the other two sites host Sr and Ca atoms or vacancies, respectively.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Defect and electrical transport properties of Nb-doped SrTiO 3

This study reports the defect and electrical transport properties of Nb-doped SrTiO 3. Samples with various A/B-ratios were synthesized by a modified glycine-nitrate combustion process and evaluated as a constituent in a SOFC anode. The phase purity and defect structure of the materials have been analyzed with SEM, XRD, TGA, and XANES. The electrical conductivity of Nb-doped strontium titanate (Sr 0.94Ti 0.9Nb 0.1O 3 — sintered in 9% H 2/N 2 at 1400 °C for 12 h) decreased with increasing temperature and showed a phonon scattering conduction mechanism with σ > 120 S/cm at 1000 °C (in 9% H 2/N 2). The results were in agreement with the defect chemistry model of donor-doped SrTiO 3 where the charge compensation changes from Sr vacancy compensation to the electronic type when samples are sintered in reducing atmosphere. XANES in combination with TGA indicated that Ti is the only species that is reduced to a lower oxidation state (from Ti 4+ to Ti 3+). The pre-edge fine structure (PEFS) from the XANES results indicated that Nb improved the overlap of the Ti atomic orbitals and thereby provided one more explanation for the positive effect of Nb on the electronic conductivity of Nb-doped SrTiO 3.

Sintering kinetic studies in nonstoichiometric strontium titanate ceramics

The effect of nonstoichiometry on the densification of SrTiO 3 ceramics with Sr/Ti ratios from 0.997 to 1.02 was systematically addressed. The kinetics of densification was studied by dilatometric analysis. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for crystallographic and microstructure characterization. Ti excess enhanced matter transport during sintering whereas Sr excess decreased it. The shrinkage rate and average grain size increased with the decrease of Sr/Ti ratio. Close values of the activation energy for the initial densification and the near constant onset temperature for densification suggest that identical transport mechanisms control the densification of all the compositions. Small excesses of TiO 2 and SrO were mostly incorporated into the perovskite lattice inducing alterations in the defect chemistry of the material and the mass transport during sintering is controlled by Sr vacancies. Very small stoichiometric variations have a strong influence on the sintering kinetics and resulting microstructure of ST ceramics.

Grain Boundary Atomic Structures in SrTiO<sub>3</sub> and BaTiO<sub>3</sub>

SrTiO3 bicrystals with various types of grain boundaries were prepared by joining two single crystals at high temperature. By using the bicrystals, we examined their current-voltage characteristics across single grain boundaries from a viewpoint of point defect segregation in the vicinity of the grain boundaries. Current-voltage property in SrTiO3 bicrystals was confirmed to show a cooling rate dependency from annealing temperature, indicating that cation vacancies accumulate due to grain boundary oxidation. The theoretical results obtained by ab-initio calculation clearly showed that the formation energy of Sr vacancies is the lowest comparing with Ti and O vacancies in oxidized atomosphere. The formation of a double Schottky barrier (DSB) in n-type SrTiO3 is considered to be closely related to the accumulation of the charged Sr vacancies. Meanwhile, by using three types of low angle boundaries, the excess charges related to one grain boundary dislocation par unit length was estimated. In this study, we summarized our results obtained in our group.

Enhancement of Ionic Conductivity of Apatite‐Type Lanthanum Silicates Doped With Cations

The oxide ion conductivity of apatite‐type lanthanum silicates has been enhanced by cation doping into both La and Si sites. The Sr and Nd fill vacancies on the La site by creating excess oxide ions and increasing the conductivity. Al and Mg, which substitute for Si at a fixed oxygen content, result in an enhancement of conductivity, accompanied by lattice enlargement. The Mg‐doped lanthanum silicate, La9.533(Si5.7Mg0.3)O26, has been shown to have the highest conductivity of 4.4 × 10−2 S/cm at 800°C. The conduction channel consists of La triangles and O triangles, and the channel sizes have been calculated from the Rietveld structure analysis of the powder X‐ray diffraction patterns. Expansions of the O triangles have been observed when introducing excess oxide ions by filling vacancies on the La site or by the substitution of Si using dopants.

First principle prediction of vacancy-induced magnetism in non-magnetic perovskite SrTiO3

Abstract A theoretical study of magnetization in non-magnetic cubic perovskite SrTiO3 induced by vacancies in Sr, Ti and oxygen sublattices is presented using the full-potential linear augmented plane wave (FP-LAPW) method within the generalized gradient approximation (GGA) for the exchange-correlation potential. The results show that the oxygen and titanium vacancies lead to the magnetization of non-magnetic perovskite SrTiO3 due to the spin-polarization of valence states for the Ti and O atoms, nearest to these vacancies. In result the insulating SrTiO3 becomes metallic-like with magnetic moments (per cell) 1.30 μB and 3.54 μB for non-stoichiometric SrTiO 3 − δ and SrTi 1 − δ O , respectively. On the contrary, our calculations reveal that for Sr vacancy the non-magnetic state of perovskite is retained, whereas the electronic spectrum of Sr 1 − δ TiO 3 also adopts metallic-like type.

Magnetocaloric effect on strontium vacancies in polycrystalline La 0.7Sr 0.3−x□ xMnO 3

In this paper, we present the magnetocaloric effect of Sr vacancies in La 0.7Sr 0.3− x □ x MnO 3 compounds with x=0 and 0.05. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered a large magnetic entropy change (Δ S M) at Curie temperature. Δ S M is found to reduce for La 0.7Sr 0.25□ 0.05MnO 3 sample (−1.78 J/kg K for x=0 and 1.60 J/kg K for x=0.05 under magnetic field change μ 0Δ H=2 T). This reduction is accompanied by a decrease of the Curie temperature ( T C =374 and 366 K for x=0 and 0.05, respectively). These results show that these materials are potential candidates for magnetic refrigerants working in above room temperature.