Defect Chemistry Research Articles

Electrochemical Study of La0.31Sr0.58Ti0.97Ni0.03O3-δ Thin Film and Pt-10Sc1CeSZ Electrodes

La and Ni-doped Sr-titanates, LaxSr1-xTi1-yNiyO3- d, are considered to be promising solid oxide fuel cell anode materials with Ni exsolution possibilities [1]. In this work two different types of La0.31Sr0.58Ti0.97Ni0.03O3-δ (LSTN) thin film microelectrodes were studied. Electrodes with photolitographycally microstructured Pt current collectors under ceramic electrode layer [2] and microelectrodes without current collectors were made using pulsed laser deposition and measured using EIS at different temperatures around 650 ℃ in water-hydrogen atmosphere at reduced pressures (0.5, 1 and 1 mbar) and at 1 atm ambient pressure. Measurements were carried out using single chamber 2-electrode configuration where large area porous GDC-Pt electrode was used as counter electrode [3]. Main aim of this study was to collect additional information to understand electrochemical data collected during in situ NAP-XPS measurements of LSTN thin film electrode and also link the data collected at ambient pressure with data measured at pressures where NAP-XPS experiments are performed. The measurements showed differences between electrodes with Pt current collectors and microelectrodes without Pt current collectors. LSTN electrodes without Pt showed weak relationship between anodic overpotential and polarization resistance suggesting that overpotential acted mainly on LSTN defect chemistry while electrodes with Pt current collectors were activated more by anodic overpotential. Oxide ion transport resistance from electrolyte to thin film | gas phase surface seems to be the main contributor to the resistance of the electrodes. J. T. S. Irvine et al., Nature Energy, 1, 15014 (2016).A. Nenning, A. K. Opitz, T. M. Huber, and J. Fleig, Phys. Chem. Chem. Phys., 16, 22321–22336 (2014).A. Nenning and J. Fleig, Surface Science, 680, 43–51 (2019).

High-Temperature Piezoelectric Resonators: Electroacoustic Properties and Applications of Langasite-Type Crystals

Piezoelectric devices based on high-temperature stable single crystals are of increasing interest due to a wide range of potential applications, such as actuators and resonant gravimetric sensors. The latter require highly accurate resonant-frequency measurements at temperatures as high as 1000 °C, and this can only be achieved with narrow resonance peaks and correspondingly low acoustic loss at these temperatures. Relevant atomistic defects in langasite (La3Ga5SiO14, LGS) and catangasite (Ca3TaGa3Si2O14, CTGS) are reviewed with respect to their impact on electrical conductivity and acoustic loss. The focus on these crystals also enables a comparison of electroacoustic properties of partially disordered (LGS) and ordered (CTGS) crystals and the formulation of approaches for reducing acoustic loss and enhancing device performance.Starting with defect chemistry models for LGS and CTGS, electrical conductivities as a function of temperature and oxygen partial pressure are considered. One result of this analysis is that contributions of oxygen ions to the total conductivity are determined to be relatively small in the studied crystals. Temperature-dependent acoustic loss of LGS and CTGS crystals is also studied. Two independent techniques, contact resonant piezoelectric spectroscopy, and a contactless tone-burst excitation technique, are applied. Contributions to the measured acoustic loss are determined by fitting loss-related functions for point defects and conductivity-related relaxations. The extracted fit parameters, in particular the activation energies of the processes, are discussed and compared with that of the electrical conductivity. The losses are caused by a superposition of several mechanisms including conductivity-related relaxations, point-defect relaxations and intrinsic phonon-phonon loss.The long-term stability of the crystals is also evaluated at 1000 °C with respect to fundamental materials properties including electrical conductivity and resonant frequency.After one year of thermal treatment, the resonance frequency of CTGS resonators from different crystal sources is found to decrease by no more than 0.4 %. Thus, the applicability of CTGS as a piezoelectric resonator that operates for long periods at extreme temperatures, such as a resonant nanobalance, is demonstrated.Application examples to be presented include resonant nanobalances for in-situ monitoring of non-stoichiometry and phase transformations in thin films during growth at high temperatures. In this context, the key application properties of the sensors at high temperatures, such as mass sensitivity and resolution, are discussed. As an application example, the attached figure shows the oxygen non-stoichiometry x of thin praseodymium-cerium oxide films (Pr0.1Ce0.9O2-x) at 700 °C as a function of oxygen partial pressure determined by LGS and CTGS nanobalances, which agrees very well with literature data (chemical capacitance: D. Chen et al., Chem. Mater. 2014, 26, 22, 6622–6627). Figure 1

In-Situ Raman Spectroscopy of Defined Copper Oxide Surfaces Formed by Electrochemically Controlled High-Temperature Oxidation

The oxidation processes of copper are of great importance since its occurrence leads to various problems, both in further processing and in operation of electronic devices and other technical applications. Extensive research on the thermal oxidation of copper has demonstrated that temperature and oxygen partial pressure affect the formation of oxides, oxidation rates and the chemical composition.In this work powerful electrochemical methods for controlled oxidation are presented, where the oxygen partial pressure on the metal surface can be varied over a range of 10-25 to 100 bar in a single experiment. A specific arrangement of the setup extends its capabilities to enable in-situ Raman spectroscopy measurements of the produced oxide. Controllable oxidation states are achieved in a solid-state electrochemical cell by polarizing a copper sample in contact with a single crystal of yttria-stabilized zirconia, which is conductive for oxygen ions at high temperatures. Optical transparency of the single crystal allows the Raman laser to pass through the crystal to measure the copper oxide formed on the copper-electrolyte interface. Cyclic voltammograms contain kinetic and thermodynamic information about the oxidation processes. Thus, the various oxidation reactions are controlled by the potential that corresponds to the oxygen partial pressure, while the current measures the rate of layer growth as well as the thickness of the oxide layer. In situ Raman measurements allow more detailed analyses of the oxides. The combined data will be used to build a model for oxide layer growth, with particular attention to the defect chemistry of the copper oxide.

Linear-scaling density functional theory (DFT) simulations of point, Frenkel and Schottky defects in CeO2

CeO2 (ceria) is a material of significant industrial and technological importance, used in solid oxide fuel cells and catalysis. Here, we explore the usage of linear-scaling density functional theory as implemented in the ONETEP code, which allows to use larger simulation cells. By using DFT+U calculations we revise the defect chemistry of ceria, including point defects, Frenkel and Schottky defects.We found that the ground state of an oxygen vacancy is associated to two neighbouring reduced cerium sites. A cerium vacancy is the least favourable point defect, where holes localise on neighbouring oxygen sites. It is more favourable to displace an oxygen interstitial defect away from the octahedral interstitial site, with the formation of a stable peroxide species. Our simulations show that a cerium interstitial is best accommodated in the octahedral interstitial site, as this minimises the distortion of the lattice.Placing a vacancy and an interstitial defect at a separation of 5.18 Å for the OF¡110¿ and 4.77 Å for the CeF¡111¿, stable Frenkel defects can be formed. We also studied the effect of different supercell size on the energetic ordering of Schottky defects, where the S¡111¿ is more favourable than the S¡110¿ for a given simulation cells containing 324 or more atoms.

Efficient photodegradation of micropollutants by tubular carbon nitride: The role of nitrogen defects

Defect chemistry possessed the significant effect for the performance of heterogeneous photocatalysts, so the construction of defect structures in photocatalysts has attracted attention. In this paper, tubular g-C3N4 (TCN) with an excellent specific surface area was obtained by morphological regulation, and the defect chemistry in TCN and its photocatalytic mechanism were explored. The elemental composition and chemical states verified the formation of nitrogen defects during preparation process, and the nitrogen defects would adjust the electronic and band structures of TCN according to the results of density functional theory calculation, resulted in its excellent optical properties. The photocatalytic activity of the obtained TCN was evaluated through the elimination of some typical micropollutants. The results indicated that more than 90% of micropollutants could be degraded after 30 min visible light exposure, and TCN still maintained superior photocatalytic performance after reuse. Meanwhile, the oxygen adsorption model also indicated that the nitrogen defect would promote the adsorption of dissolved oxygen on TCN to produce ·O2-, which acted as the main reactive oxygen species during atrazine degradation process. Finally, the toxicity of treated solution and intermediates were proved to be reduced by the bioluminescence inhibition tests and Ecological Structure Activity Relationships (ECOSAR) software.

Oxygen sensing with SrTi1−xFexO3-δ thin films on sapphire optical fiber at extreme temperatures

Optical fiber-based sensors are uniquely suited for in-situ sensing applications that involve high operational temperature and harsh chemical conditions, such as solid oxide fuel cell (SOFC) monitoring, chemical manufacturing, and power generation. Single crystal optical fiber, such as α-alumina (sapphire), provides a substrate that exhibits improved stability over off-the-shelf silica fiber at temperatures above 800 °C. In this work, single crystal sapphire fiber is paired with a high-temperature stable perovskite oxide sensing layer, SrTi0.65Fe0.35FeO3-δ (STF35), to demonstrate all-optical oxygen sensing from 0.01 to 0.19 atm at extreme temperatures (800–1000 °C). The defect chemistry associated with the optical oxygen sensing response is also considered within the context of the current literature and modeled under relevant sensor conditions. Stability is evaluated under dry and humid conditions, and low cross-sensitivity with CO2 is demonstrated, indicating viability for operation on the cathode stream within an operational SOFC or within a post-combustion environment.

Composition-driven morphological evolution of BaTiO3 nanowires for efficient piezocatalytic hydrogen production

Hydrogen production from water by piezocatalysis is very attractive owing to its high energy efficiency and novelty. BaTiO3, a highly piezoelectric material, is particularly suitable for this application due to its high piezoelectric potential, non-toxic nature, and physicochemical stability. Owing to the critical role of morphology on properties, one-dimensional (1D) materials are expected to exhibit superior water-splitting performance and thus there is a need to optimise the processing conditions to develop outstanding piezocatalysts. In the present work, piezoelectric BaTiO3 nanowires (NWs) were hydrothermally synthesised with precursor Ba:Ti molar ratios of 1:1, 2:1, and 4:1. The morphology, defect chemistry, and hydrogen evolution reaction (HER) efficiency of the as-synthesised BaTiO3 NWs were systematically investigated. The results showed that the morphological features, aspect ratio, structural stability and defect contents of the 1D morphologies collectively have a significant impact on the HER efficiency. The morphological evolution mechanism of the 1D structures were described in terms of ion exchange and dissolution-growth processes of template-grown BaTiO3 NWs for different Ba:Ti molar ratios. Notably, the BaTiO3 NWs synthesised with Ba:Ti molar ratio of 2:1 displayed high crystallinity, good defect concentrations, and good structural integrity under ultrasonication, resulting in an outstanding HER efficiency of 149.24 μmol h−1g−1 which is the highest obtained for nanowire morphologies. These results highlight the importance of synthesis conditions for BaTiO3 NWs for generating excellent piezocatalytic water splitting performance. Additionally, post-ultrasonication tested BaTiO3 NWs demonstrated unexpected photocatalytic activity, with the BTO-1 sample (1:1 Ba:Ti) exhibiting 56% photodegradation of RhB in 2 h of UV irradiation.

Electronic Structure and Surface Chemistry of Hexagonal Boron Nitride on HOPG and Nickel Substrates.

The effect of point defects and interactions with the substrate are shown by density functional theory calculations to be of significant importance for the structure and functional properties of hexagonal boron nitride (h-BN) films on highly ordered pyrolytic graphite (HOPG) and Ni(111) substrates. The structure, surface chemistry, and electronic properties are calculated for h-BN systems with selected intrinsic, oxygen, and carbon defects and with graphene hybrid structures. The electronic structure of a pristine monolayer of h-BN is dependent on the type of substrate, as h-BN is decoupled electronically from the HOPG surface and acts as bulk-like h-BN, whereas on a Ni(111) substrate, metallic-like behavior is predicted. These different film/substrate systems therefore show different reactivities and defect chemistries. The formation energies for substitutional defects are significantly lower than for intrinsic defects regardless of the substrate, and vacancies formed during film deposition are expected to be filled by either ambient oxygen or carbon from impurities. Significantly lower formation energies for intrinsic and oxygen and carbon substitutional defects were predicted for h-BN on Ni(111). In-plane h-BCN hybrid structures were predicted to be terminated by N-C bonding. Substitutional carbon on the boron site imposes n-type semiconductivity in h-BN, and the n-type character increases significantly for h-BN on HOPG. The h-BN film surface becomes electronically decoupled from the substrate when exceeding monolayer thickness, showing that the surface electronic properties and point defect chemistry for multilayer h-BN films should be comparable to those of a freestanding h-BN layer.

Defect and migration energies of oxygen vacancies in ABO2N (A – Ba, Ca, Sr and B – Ta, Nb) perovskite oxynitrides

ABO2N (A = Ca, Ba, Sr & B = Ta, Nb) perovskite oxynitrides are gaining attention as they have band gaps (Eg ∼ 1.8–2 eV) in the visible region. However, these systems are reported to have defects which significantly impact their catalytic behavior. Hence it is essential to understand the defect chemistries of ABO2N systems. In this work, the Mott-Littleton (M-L) method is used to calculate the defect and migration energies associated with oxygen vacancies. For this study, Buckingham potential along with core-shell model has been used for describing the defects in the systems. It is shown that BaTaO2N, BaNbO2N, SrTaO2N and SrNbO2N have low defect energy for oxygen vacancy. Hence, these systems can be explored for solid oxide fuel cell and supercapacitor applications. This is since the presence of oxygen vacancies is reported to improve the ionic conductivity in materials. But for rapid ion/vacancy migration, a low migration barrier is also essential. Hence investigations on oxygen migration energetics are pertinent. The migration energy calculations on ABO2N systems identifies SrTaO2N to be most favorable for applications involving ionic conductivity (e.g. SOFC, gas sensors, supercapacitors). Overall, the work presented here provides specific quasi-chemical pointers relevant for functional applications using oxynitrides.

Improved High Temperature Thermoelectric Properties in Misfit Ca3Co4O9 by Thermal Annealing

Ca3Co4O9, a p-type thermoelectric material based on transition-metal oxides, has garnered significant interest due to its potential in thermoelectric applications. Its unique misfit-layered crystal structure contributes to low thermal conductivity and a high Seebeck coefficient, leading to a thermoelectric figure of merit (zT) of ≥1 at 1000 K. Conventionally, it has been believed that thermopower reaches its upper limit above 200 K. However, our thermopower measurements on polycrystalline Ca3Co4O9 samples have revealed an unexpected increase in thermopower above 380 K. In this study, we investigate the effects of high oxygen pressure annealing on Ca3Co4O9 and provide an explanation based on the mixed oxide states of cobalt and carrier hopping. Our results demonstrate that annealing induces modifications in the defect chemistry of Ca3Co4O9, leading to a decrease in electron hopping probability and the emergence of a thermal activation-like behavior in thermopower. These findings carry significant implications for the design and optimization of thermoelectric materials based on misfit cobaltates, opening new avenues for enhanced thermoelectric performance.

Arsenic-Doped CdSeTe Solar Cells Achieve World Record 22.3% Efficiency

For more than three decades, Cu has been critical to dope CdSeTe solar cells, form effective contacts, and maximize efficiency. At the same time, Cu defect chemistry has limited stability, carrier concentration, and further efficiency improvements. In this article, 22.3% world record efficiency is demonstrated without Cu by implementing As doping, which also improves stability, temperature coefficient, and energy yield. The efficiency crossing point of Group V technology relative to Cu has been driven by steady improvements in the open-circuit voltage. Here, the certified record cell reaches open-circuit voltage of 899 mV while retaining high photocurrent values of 31.4 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ; the fill factor is relatively low at 78.9%. Coupling 80% fill factor with top open-circuit voltage values of 917 mV reported here offers a near-term path to 23% efficiency. Characterization indicates reducing recombination and improving activation provide viable paths to 25% efficiency.

A Comprehensive Review of Defects Genealogy in Olivines: From the Mineral World to Modern Electrode Materials

The “Learning from nature” strategy is currently going through a renaissance period in modern materials science. Valuable experience gained by observing existing natural materials—minerals—paves the way towards design and modification of prospective functional materials for energy storage, which typically inherit the peculiarities of the parental minerals. The faults and flaws of the crystal structure—its defects—play a crucial role in determining both mechanical and electrochemical properties of the electrode materials. In this review, we endeavored to rethink the defect chemistry in triphylite-type positive electrode materials for metal-ion batteries and reflected on it from the perspective of their mineral olivine counterparts, thus establishing important correlations between point defects in olivine minerals and related electrode materials, their origin and formation processes. This work is meant to review geoscience and materials science perceptions of defects in triphylite-type electrode materials for Li- and Na-ion batteries.

Thermochemical Expansion and Protonic and Electronic Hole Conductivity of Grain Interior and Grain Boundaries in 10 Mole% Y-Substituted SrZrO3.

Proton conducting acceptor-doped SrZrO3 has a history as long as that of BaZrO3 , but has attracted less interest. Inspired by its higher transport number of ionic conduction in wet oxygen revealed by our recent work, we here explore further aspects of doped SrZrO3 as electrolyte in proton ceramic electrochemical cells. In-situ high temperature XRD (HT-XRD) analysis of SrZr0.9 Y0.1 O3-δ (SZY10) indicated an anisotropic chemical expansion of hydration, stronger along the b than the a direction, and negative in the c direction. A systematic electromotive force (EMF) and impedance spectroscopy study as a function of and allowed determination of partial conductivities of electron holes and ions (mainly protons) in bulk (grain interior) and grain boundaries. Enthalpies and preexponentials were determined and interpreted for bulk and grain boundary partial conductivities based on defect chemistry and a brick layer model. The hole conductivity in bulk is modest and ensures high ionic transport numbers in oxidizing atmospheres, while grain boundaries exhibit lower ionic transport numbers from a relatively higher hole conductivity attributed primarily to tunnelling past the deepest part of the space charge region. Y-doped SrZrO3 (SZY) materials exhibit lower proton conductivities but excel over Y-doped BaZrO3 (BZY) in terms of thermal expansion compatibility with electrode materials and higher ionic transport numbers in oxidizing atmospheres and may hence be candidates for functional layers between BZY-based electrolytes and positrodes in proton ceramic electrochemical cells.

Near-Infrared Fluorescent Biosensors Based on Covalent DNA Anchors.

Semiconducting single-walled carbon nanotubes (SWCNTs) are versatile near-infrared (NIR) fluorophores. They are noncovalently modified to create sensors that change their fluorescence when interacting with biomolecules. However, noncovalent chemistry has several limitations and prevents a consistent way to molecular recognition and reliable signal transduction. Here, we introduce a widely applicable covalent approach to create molecular sensors without impairing the fluorescence in the NIR (>1000 nm). For this purpose, we attach single-stranded DNA (ssDNA) via guanine quantum defects as anchors to the SWCNT surface. A connected sequence without guanines acts as flexible capture probe allowing hybridization with complementary nucleic acids. Hybridization modulates the SWCNT fluorescence and the magnitude increases with the length of the capture sequence (20 > 10 ≫ 6 bases). The incorporation of additional recognition units via this sequence enables a generic route to NIR fluorescent biosensors with improved stability. To demonstrate the potential, we design sensors for bacterial siderophores and the SARS CoV-2 spike protein. In summary, we introduce covalent guanine quantum defect chemistry as rational design concept for biosensors.

Conduction Mechanism in Acceptor- or Donor-Doped ZrO2 Bulk and Thin Films.

The leakage current in capacitors in future electronics should be highly suppressed to achieve low power consumption, high reliability, and fast data processing. Although considerable efforts have been directed at reducing the leakage current, fundamental studies on the effects of doping on bulk and thin-film materials have rarely been conducted. Herein, we investigated the effects of doping with acceptor and donor elements on the conduction of bulk and thin-film ZrO2 and elucidated the underlying charge conduction mechanism. In the case of bulk ZrO2, the electrical conductivity was reliably modulated by the type of dopant element, which is highly consistent with defect chemistry theory. However, unlike in the bulk material, in acceptor- and donor-doped thin-film ZrO2, the leakage current was suppressed, indicating that the factors determining the electrical property in thin films are different from those in bulk materials.

AiiDA-defects: an automated and fully reproducible workflow for the complete characterization of defect chemistry in functional materials

Functional materials that enable many technological applications in our everyday lives owe their unique properties to defects that are carefully engineered and incorporated into these materials during processing. However, optimizing and characterizing these defects is very challenging in practice, making computational modelling an indispensable complementary tool. We have developed an automated workflow and code to accelerate these calculations (AiiDA-defects), which utilises the AiiDA framework, a robust open-source high-throughput materials informatics infrastructure that provides workflow automation while simultaneously preserving and storing the full data provenance in a relational database that is queryable and traversable. This paper describes the design and implementation details of AiiDA-defects, the models and algorithms used, and demonstrates its use in an application to fully characterize the defect chemistry of the well known solid-state Li-ion conductors LiZnPS4. We anticipate that AiiDA-defects will be useful as a tool for fully automated and reproducible defect calculations, allowing detailed defect chemistry to be obtained in a reliable and high-throughput way, and paving the way toward the generation of defects databases for accelerated materials design and discovery.

Alleviating Defect and Oxidation in Tin Perovskite Solar Cells Using a Bidentate Ligand

Tin-perovskite solar cells (Sn-PSCs) have low energy conversion efficiency and stability due to facile oxidation of Sn2+ during precursor solution preparation and film growth. Herein, we introduced formohydrazide (FHZ) as a bidentate ligand into the Sn-halide perovskite (Sn-HaP) to improve the optoelectronic properties. This approach is found to be effective for the suppression of Sn-oxidation and interfacial energy band modulation. The depth profile distribution confirmed that the FHZ additive is primarily located on surfaces and the hole transport layer (HTL)/Sn-HaP interface with partly capping at the grain boundaries, which offers a reducing ambient in the Sn-HaP film. Therefore, the device with FHZ demonstrated a device efficiency of 12.87% (9.93% control) with enhanced open circuit voltage from ∼0.734 to 0.874 V and improved operational device stability. The device analysis suggests that the FHZ additive alleviates the bulk and interface defect in the Sn-PSC with −NH2 and −O═C bidentate bonding to Sn-HaP, which is supported by theoretical calculations. Thus, this work corroborates the importance of multidentate ligands for modulating the film morphology and defect chemistry in the Sn-perovskite for high efficiency and superior device stability.

Defect Chemistry Engineering Promotes the Dopability of Cu for an Extraordinary Thermoelectric Performance of Hole-Doped PbSe with Resonant States

Cu substitution for Pb (CuPb) has been theoretically predicted to introduce resonant states near the valence band edge of PbSe. However, experimentally, it has not been verified yet due to the extremely low solubility (∼0.3 atom %) of CuPb. In this study, we demonstrate that the solubility limit of CuPb is extended to at least 1.5 atom % in PbSe by pairing with an identical amount of Cu interstitials (Cui), while excess Cu precipitates out as a Cu2Se phase at grain boundaries. As a result of the increased dopability of CuPb, a significant enhancement of the Seebeck coefficient is realized in hole-doped PbSe because of CuPb-induced resonant states. This is also testified by first-principles calculations. Moreover, the rich point defects (CuPb, Cui), line defects (dislocations), and Cu2Se precipitates remarkably frustrate the phonon propagation of PbSe, leading to ∼45% reduction of lattice thermal conductivity (κlat) at room temperature. At elevated temperatures (>623 K), there is a dynamic migration of Cu atoms from Cu2Se precipitates to tetrahedral interstices of the PbSe matrix as evidenced by the temperature-variant Hall study and thermal expansion coefficient measurement. This diffusion-like process of Cu further drops the κlat to ∼0.27 W m–1 K–1 at 823 K. Consequently, a peak ZT of ∼1.8 at 873 K is achieved in the (Cui)0.01Pb0.97(CuPb)0.01Na0.02Se sample. This work highlights the potential of the defect structure design in innovating new functional materials, particularly high-performance thermoelectrics.

Yttrium and Niobium Elements Co-Doping and the Formation of Double Perovskite Structure Ba2YNbO6 in BCZT.

The (Ba0.85Ca0.15) (Ti0.90Zr0.10)O3 + x Y3+ + x Nb5+ (abbreviated as BCZT-x(Nb + Y), x = 0 mol%, 0.05 mol%, 0.1 mol%, 0.2 mol%, 0.3 mol%) lead-free piezoceramics samples were prepared by a traditional solid-state sintering method. And the effects of Yttrium and Niobium elements (Y3+ and Nb5+) co-doping on the defect, phase and structure, microstructure, and comprehensive electrical properties have been investigated. Research results show that the Y and Nb elements co-doping can dramatically enhance piezoelectric properties. It is worth noting that XPS defect chemistry analysis, XRD phase analysis and TEM results together show that a new phase of double perovskite structure Barium Yttrium Niobium Oxide (Ba2YNbO6) is formed in the ceramic, and the XRD Rietveld refinement and TEM results show the coexistence of the R-O-T phase. Both these two reasons together lead to significant performance improvements of piezoelectric constant (d33) and planar electro-mechanical coupling coefficient (kp). The functional relation between temperature and dielectric constant testing results present that the Curie temperature increases slightly, which shows the same law as the change of piezoelectric properties. The ceramic sample reaches an optimal performance at x = 0.1% of BCZT-x(Nb + Y), where d33 = 667 pC/N, kp = 0.58, εr = 5656, tanδ = 0.022, Pr = 12.8 μC/cm2, EC = 2.17 kV/cm, TC =92 °C, respectively. Therefore, they can be used as potential alternative materials to lead based piezoelectric ceramics.

Study of the defect chemistry in Ag2Q (Q = S, Se, Te) by first-principles calculations

Ag2Q-based (Q = S, Se, Te) silver chalcogenides show great potential in thermoelectrics due to their suitable band gaps, high electron mobilities, and even remarkable ductility. Particularly, Ag2S and Ag2S/Se/Te solid solutions have been reported with both good ductility and thermoelectric performance, which are extremely suitable for the application in flexible wearables. However, the underlying mechanism of the native n-type conduction and p-type undopability for Ag2Q remains elusive. Herein, we use first-principles calculations based on density functional theory combined with GW correction to investigate the defect chemistry in Ag2Q. It is found that the site potential and Voronoi volume deviations resulting from Ag interstitials are noticeably smaller than those caused by Ag vacancies, which makes Ag interstitials with low formation energy more desirable during preparation, contributing to the native n-type conduction. The small and even negative dopability windows, on the other hand, account for the p-type undopability of Ag2Q. The calculated carrier concentrations of pristine Ag2Q are well consistent with the experimental observations, validating the reliability of our defect calculations. This work provides valuable guidance for first-principles calculation of defect chemistry in other narrow-gap semiconductors.