Partial Schottky Defects Research Articles

Novel non-stoichiometric (Ba0.91Ca0.09)x(Zr0.18Ti0.82)O3 ferroelectric ceramics with improved diffuse phase transition and dielectric tunable performance

Dielectric tunable properties in ferroelectric ceramics have been optimized by a variety of approaches, e.g., isovalent/aliovalent substitution, system composite et al., while the study of non-stoichiometric regulation on the performance of BCZT ceramics was rarely concerned. Herein, a series of novel non-stoichiometric (Ba0·91Ca0.09)x (Zr0·18Ti0.82)O3 (BCZTx) ceramic specimens were successfully prepared via solid-state reaction. The microstructures of BCZTx ceramics are simultaneously investigated through XRD and SEM. The diffuse phase transition (DPT) behaviour of BCZTx ceramics are studied by the Lorentz-type empirical formula. The significant enhancement of DPT behaviour is observed at x = 1.03, which probably due to the coaction of the generation of partial Schottky defects and prominent reduction of grain size. Furthermore, a high tunability (k) 87.80%, low dielectric loss (tan δ) 0.141%, and a remarkably enhanced FOM of 623 are achieved in x = 0.99 at a low DC bias electric fields (BEFs) of 7.28 kV/cm and room temperature (RT), which is superior to that of the stoichiometric BCZT ceramics and other available reported BT-based ceramics systems in term of the dielectric tunable properties. Meanwhile, it shows that the temperature dependent CQF value of x = 0.99 remained advantageous around the RT. These findings suggested that non-stoichiometric BCZTx ceramics with x = 0.99 are significantly competitive in the applications of dielectric tunable devices at RT. The non-stoichiometric regulation is an effective approach in improving the dielectric tunability properties of BCZT ceramics.

Effects of non-stoichiometric defects on the dielectric properties of (Ba0.5Sr0.5)xTiO3-ZnGa2O4 composite ceramics

In order to explore the effects of non-stoichiometric defects on the dielectric properties of composite ceramics, 70 wt% (Ba0.5Sr0.5)xTiO3-30 wt%ZnGa2O4 ((BS)xT50-ZG, x = (Ba + Sr)/Ti = 0.99, 1.00, 1.01 and 1.05) composite ceramics were fabricated by the traditional sintering technique. The association between structure and dielectric properties has been studied. The results show that the distortion of the crystal lattice brought by the partial Schottky defects, namely [V″Ba,Sr–V˙˙O]× and [V′′′′Ti–2V˙˙O]×, induces a decrease in the Curie temperature of (BS)xT50-ZG composite ceramics. The orientation of the elastic dipoles brought by oxygen vacancies causes the pinning of the domain walls, which reduces the dielectric loss at low frequencies. Tunability is related to the dipole polarization caused by [V″Ba,Sr–V˙˙O]× and [V′′′′Ti–2V˙˙O]× defect complexes. In addition, compared with the composite ceramic with x = 1, the Q values of the composite ceramics with x < 1 and x > 1 decreases due to the deterioration of the microstructure homogeneity and the enhancement of the disorder of the B-site cations in (BS)xT50.

Dielectric properties and vacancy‐like defects in plasma‐sprayed barium titanate

AbstractPositron annihilation spectroscopy was employed for investigation of vacancy‐like defects in plasma‐sprayed barium titanate. Defect studies were combined with measurement of dielectric properties of barium titanate coatings. Samples prepared by gas‐stabilized plasma spray (GSP) torch and by plasma torch with the hybrid water‐argon stabilization (WSP‐H) were studied. Processing parameters were selected so that GSP coatings were sprayed in reductive conditions, whereas WSP‐H coatings were prepared in oxidizing environment. As‐sprayed GSP coating is dark, whereas WSP‐H one is light. The dielectric properties of WSP‐H coating are superior to those for GSP one. Defect studies revealed that both GSP and WSP‐H coatings contain titanium vacancies. However, GSP coating contains in addition a considerable concentration of oxygen vacancies. Some fraction of oxygen vacancies in GSP coating is coupled with titanium vacancies forming partial Schottky defect. The structure of WSP‐H coating is less disordered and contains only a low concentration of oxygen vacancies. This is consistent with reductive and oxidizing conditions in GSP and WSP‐H spraying, respectively. Annealing at elevated temperatures in air leads to removal of oxygen vacancies which are filled by oxygen diffusing into the samples.

Effect of the (Ba + Sr)/Ti ratio on the microwave‐tunable properties of Ba0.6Sr0.4TiO3 ceramics

AbstractThe impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave‐tunable characteristics of diffuse phase transition (DPT) ferroelectric Ba0.6Sr0.4TiO3 (0.6‐BST) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., and . The magnitude of the dielectric constant, ε′, at room temperature in the absence of an applied electric field was governed by the shift in the dielectric maximum temperature, Tm, because Tm was close to room temperature for the 0.6‐BST. The dielectric loss, tanδ, diminished as the ε′ decreased for 0.98≤A/B≤1.05, while the tanδ was much higher for A/B=0.95 having the greatest A‐site vacancy loading. The negatively charged and were mainly compensated by oxygen vacancies and likely partly compensated by holes, h•, which contributed to the electrical conduction. The tunability, T, at 100 MHz was almost constant at 20%–25% for A/B≥1.00 despite the reduction of the ε′, whereas T decreased for A/B&lt;1.00 to ca. 10% for A/B=0.95 having the greatest A‐site vacancy loading. The results implied that the for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions (PNRs) than the for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, FOM, reached its maximum of 250 at A/B=0.9875, which was ca. 155% higher than that of the stoichiometric BST.

First-principles study of defect properties in tetragonal BaTiO3

Based on density functional first principles method, the defect properties in tetragonal BaTiO 3 have been studied. The results showed that the formation energies of neutral Ti vacancy and partial Schottky defect 2 V 3- Ti +3V 2+ O are the lowest under oxygen-rich condition; while under reducing condition oxygen vacancy becomes the primary defect. The calculated full Schottky formation energy is higher than that obtained in the cubic phase, which may be the result of the strong hybridization between the Ti-O bonds. The hybridization is also responsible for the Frenkel formation energy of Ti. The defect-interactions are important when dealing with Schottky defects.

Schottky defects in cubic lattice of SrTiO 3

Structural and electronic properties produced by formation of Schottky defects in cubic structure of SrTiO 3 crystal are investigated by means of a quantum-chemical simulation based on the Hartree–Fock methodology. The occurrence of Sr partial Schottky defect (V Sr+V O) and two types of Ti partial Schottky defects (V Ti+2V O) is modeled using a supercell containing 135 atoms. Vacancy-induced changes in the positions of their neighboring atoms are analyzed in light of the computed electron density redistribution in the defective region of supercell. The observed local one-electron energy levels in the gap between the upper valence band and the conduction band can be attributed to the presence of anion and cation vacancies.

Modified Phase Diagram for the Barium Oxide–Titanium Dioxide System for the Ferroelectric Barium Titanate

The ferroelectric phase transition behavior in BaTiO3 was investigated for various annealing times, temperatures, and Ba/Ti ratios by means of a differential scanning calorimeter. Coupling these observations with powder X‐ray diffraction and transmission electron microscopy allowed new insights into the barium oxide (BaO)–titanium dioxide (TiO2) phase diagram. The transition temperature was varied systematically with the Ba/Ti ratio at annealing temperatures from 1200° to 1400°C in air. The transition temperature decreased with increasing concentrations of BaO and TiO2 partial Schottky defects, and showed a discontinuous change at the phase boundaries. Beyond the solubility region, two peritectoid reactions were confirmed and revised; first around 1150°C for Ba1.054Ti0.946O2.946→Ba2TiO4+BaTiO3 and second 1250°C for BaTi2O5→Ba6Ti17O40+BaTiO3, respectively. All other regimes of the BaO–TiO2 were found to be consistent with the reported diagrams in the literature.

Influence of nonstoichiometry on ferroelectric phase transition in BaTiO3

The ferroelectric phase transition behavior in equilibrated nonstoichiometric BaTiO3 powder samples was characterized by a differential scanning calorimeter. The transition temperature (TC) and enthalpy of transition (ΔHTC) between the paraelectric and ferroelectric phases were varied systematically with defect concentrations through the Ba∕Ti ratio, temperature, and oxygen partial pressure [p(O2)]. It was observed that the different defects such as titanium, barium, and oxygen vacancies all contribute to the changes of TC and ΔHTC. The TC decreased with increasing concentration of TiO2 partial Schottky defects and BaO partial Schottky defects. The annealing temperature increased the TC due to a complex mechanism with the increase of full Schottky defect reactions. In terms of p(O2), TC is constant in the ionic compensation region, and increases with the nonstoichiometry in the electronically compensated p(O2) region.

Atomistic simulation of Mg2SiO4 and Mg2GeO4 spinels: a new model

We have developed a new interatomic potential model for the simulation of ringwoodite, the high-pressure phase of Mg2SiO4, and its low-pressure analogue, Mg2GeO4 spinel. The main novelty is the addition of a breathing shell model that enables us to accurately describe the structural and elastic parameters of both spinels up to 15 GPa. Our model has also been applied to the two other Mg2SiO4 polymorphs in order to test its transferability. We find that although it is able to reproduce the structure and physical properties of wadsleyite, the breathing shell description is less successful with forsterite. The Mott-Littleton method has been used to calculate the energy of the intrinsic point defects in both spinels. The results indicate that these phases are likely to have the same defect population with the MgO partial Schottky defect predominating.