Chemical strain in perovskite-like materials

Pseudo-cubic perovskites based upon substituted oxides RBO3-δ as well as double perovskites RBaB2O6-δ and Sr2BMoO6, where R=rare-earth element and B=3d-transition metal, with A-site and B-site, respectively, cation ordering are very promising materials for a variety of different devices for moderate high temperature applications. The unique feature of the oxides is their ability to undergo both thermal strain and that induced by the defects of oxygen nonstoichiometry in the oxide crystal lattice. The latter is called as chemical or defect-induced strain, which is extremely sensitive to the defect structure of the oxide material. This property was shown recently to be isotropic for pseudo-cubic perovskites unlike that of double perovskites. The crystal lattice of a double perovskite expands along a-axis and simultaneously contracts along c-axis with the decreasing lattice oxygen content. The model of the oxide lattice chemical strain based on a change of mean ionic radius due to reduction of most reducible cation has been recently developed by us. In this work we introduced the new feature in the model such as change of preferable coordination of cations caused by change of oxygen content in the oxide. The modified model was shown to enable correct prediction of chemical expansion upon increasing oxygen nonstoichiometry along a-axis for both pseudo-cubic and double perovskite oxides and simultaneous lattice contraction along c-axis in double perovskites. Thus most important finding is that simultaneous lattice contraction along c-axis in double perovskites is caused by aforementioned change of preferable coordination

Experimental thermochemical verification of trends in thermodynamic stability of hybrid perovskite-type organic-inorganic halides

Hybrid perovskite-type methylammonium lead halides have received great attention in recent years due to high conversion efficiency obtained in solar cells based on such materials. Since the time of the first demonstration photovoltaic devices based on the hybrid perovskites CH3NH3PbX3 (X = Cl, Br, I) have showed huge progress in increase of conversion efficiency reaching currently 20.1%. However, despite very promising achievements fundamental chemistry and physics of hybrid organic-inorganic (HOIP) perovskites is far from being completely understood. In particular it is true for thermodynamic properties of HOIP perovskite-type halides ABX3 and A2BX4 (A=CH3NH3, formamidinium, Cs, Rb, etc; B=Sn, Pb, 3d-element; X = Cl, Br, I). Moreover, reported results of DFT calculations aiming at estimating the stability of these materials often give controversial results. In addition, some of the HOIP perovskites (for example, CH3NH3PbX3 (X = Cl, Br, I)) are known to be entropy-stabilized phases. Therefore experimental verification of the stability trends in HOIP perovskite-type halide systems is strongly required. This is especially important for assessment of the stability of these materials under particular working conditions. Therefore, the main aim of this work was to study the thermodynamics of formation of HOIP perovskite-type halides ABX3 and A2BX4 (A=CH3NH3, formamidinium, Cs, Rb, etc; B=Sn, Pb, 3d-element; X = Cl, Br, I). Their standard formation enthalpy at 298 K was measured by solution calorimetry. Heat capacity was measured in the temperature range 2-298 K using PPMS system. Standard entropy was obtained by integration of the Cp/T vs T curve. Standard Gibbs free energy of ABX3 and A2BX4 (A=CH3NH3, formamidinium, Cs, Rb, etc; B=Sn, Pb, 3d-element; X = Cl, Br, I) was evaluated using measured formation enthalpy and entropy. Trends in variation of the thermodynamic functions with chemical composition and crystal structure of HOIP perovskite-type halides were analyzed and compared with available results of DFT calculations. This work was supported by the Russian Science Foundation (grant No. 18-73-10059)

Hydration thermodynamics of proton-conducting perovskite Ba4Ca2Nb2O11

The oxygen nonstoichiometry index δ, i.e. number of oxygen vacancies per formula unit, in perovskite-type BaCa(1+y)/3Nb(2-y)/3O3–δ (BCNy) oxides can be tailored by varying the Ca–Nb ratio y, and equals . These oxygen vacancies can be hydrated under humid atmosphere, providing nonstoichiometric BCNy oxides with good proton conductivity. It makes them promising materials for proton-conducting solid oxide fuel cell (SOFC) electrolytes and high-temperature humidity sensors. The present work aimed to partly address the lack of fundamental thermodynamic studies on BCNy by investigating the heat of low-temperature hydration-induced phase transition as well as the higher-temperature thermodynamics of hydration and related defect chemistry of BCN50 oxide. Please click Additional Files below to see the full abstract

In situ and ex situ study of cubic La0.5Ba0.5CoO3–δ to double perovskite LaBaCo2O6– δ transition

Double perovskites REBaCo2O6–δ (RE – rare-earth element) have received great attention in past decades as promising materials for various electrochemical devices because of their high mixed ionic-electronic conductivity and catalytic activity for the reaction of oxygen reduction. Among these compounds, cobaltite LaBaCo2O6–δ can serve as a good example of structural flexibility, since it is able to form either A-site disordered cubic “simple” perovskite or layered A-site ordered double perovskite. However, the exact limits of the thermodynamic stability of LaBaCo2O6–δ double perovskite with respect to temperature (T) and oxygen partial pressure (pO2) have not been determined so far. Furthermore, synthesis and study of selected properties of either cubic or layered LaBaCo2O6–δ oxide were mostly of interest for researchers, whereas the transition from “simple” to double perovskite was not addressed in detail so far. At the same time, it is generally recognized that such transition significantly improves oxide ion transport in the perovskite-type oxides and, therefore, understanding this order-disorder transition is of key importance for successful development of new materials for practical application. Therefore, the present work aims at providing some insights into the nature of the aforementioned order-disorder transformation of LaBaCo2O6–δ, as well as into thermodynamic stability of both ordered and disordered phases, using a set of complementary techniques such as transmission electron microscopy, in situ X-ray diffraction and solid state coulometric titration. As a result, formation of complex domain textured intermediate products during the phase transition “ordered LaBaCo2O6–δ – disordered La0.5Ba0.5CoO3–δ” was revealed. These products were found to exhibit strong affinity to oxygen and fast oxygen exchange with ambient atmosphere even at temperature as low as 70 °C. This particularity seems to provide a unique possibility to develop a new class of advanced materials for IT SOFCs, ceramic membranes and catalysis. The thermodynamic stability limits of the cubic and double perovskites were determined as log(pO2) = f(1/T) dependencies. The stability diagram of the LaBaCo2O6–δ – La0.5Ba0.5CoO3–δ system was plotted as a result. Oxygen nonstoichiometry of the thermodynamically stable cubic perovskite La0.5Ba0.5CoO3–δ was measured as a function of pO2 in temperature range between 1000 and 1100 °C using coulometric titration technique. Acknowledgement: This study was supported by the Russian Foundation for Basic Research (Grant No. 18-33-20243)

Crystal structure, oxygen nonstoichiometry, hydration and conductivity BaZr1- xMxO3-d (M=Pr, Nd, Y, Co)

Partially substituted perovskite-like barium zirconates with general formula BaZr1-хМхО3-d possess oxygen-ion and proton conductivity and, therefore, may be promising as electrolytes for intermediate temperature solid oxide fuel cells. The aim of this work was to study the crystal structure, thermal and chemical expansion, water uptake, oxygen nonstoichiometry, total conductivity and Seebeck coefficient of zirconates BaZr1-xMxO3-d (M=Pr, Nd, Y, Co) in the atmospheres with different levels of humidity (log(pH2O/atm.) = -1.75; -2.5; -3.5) as a function of oxygen partial pressure (log(pO2/atm) = -20 - -0.67) and temperature (T = 25 – 1050 °C). Synthesis of the samples was carried out by glycerol-nitrate method. The phase composition of the as-prepared powders was analyzed by the X-ray diffraction (XRD). Room temperature and high temperature XRD studies were carried out using Shimadzu XRD-7000 diffractometer equipped with high temperature chamber HTK 16N (Anton Paar GmbH). Thermal and chemical expansion was also measured using DIL 402 C dilatometer (Netzsch GmbH). Oxygen nonstoichiometry was studied by solid state coulometric titration and thermogravimetry. Electrical conductivity and Seebeck coefficient were measured simultaneously in the same setup. This work was supported by the Russian Science Foundation (project No.18-73-00022)

Potential of Core-Collapse Supernova Neutrino Detection at JUNO

JUNO is an underground neutrino observatory under construction in Jiangmen, China. It uses 20kton liquid scintillator as target, which enables it to detect supernova burst neutrinos of a large statistics for the next galactic core-collapse supernova (CCSN) and also pre-supernova neutrinos from the nearby CCSN progenitors. All flavors of supernova burst neutrinos can be detected by JUNO via several interaction channels, including inverse beta decay, elastic scattering on electron and proton, interactions on C12 nuclei, etc. This retains the possibility for JUNO to reconstruct the energy spectra of supernova burst neutrinos of all flavors. The real time monitoring systems based on FPGA and DAQ are under development in JUNO, which allow prompt alert and trigger-less data acquisition of CCSN events. The alert performances of both monitoring systems have been thoroughly studied using simulations. Moreover, once a CCSN is tagged, the system can give fast characterizations, such as directionality and light curve

Detection of the Diffuse Supernova Neutrino Background with JUNO

As an underground multi-purpose neutrino detector with 20 kton liquid scintillator, Jiangmen Underground Neutrino Observatory (JUNO) is competitive with and complementary to the water-Cherenkov detectors on the search for the diffuse supernova neutrino background (DSNB). Typical supernova models predict 2-4 events per year within the optimal observation window in the JUNO detector. The dominant background is from the neutral-current (NC) interaction of atmospheric neutrinos with 12C nuclei, which surpasses the DSNB by more than one order of magnitude. We evaluated the systematic uncertainty of NC background from the spread of a variety of data-driven models and further developed a method to determine NC background within 15\% with {\it{in}} {\it{situ}} measurements after ten years of running. Besides, the NC-like backgrounds can be effectively suppressed by the intrinsic pulse-shape discrimination (PSD) capabilities of liquid scintillators. In this talk, I will present in detail the improvements on NC background uncertainty evaluation, PSD discriminator development, and finally, the potential of DSNB sensitivity in JUNO

Real-time Monitoring for the Next Core-Collapse Supernova in JUNO

Core-collapse supernova (CCSN) is one of the most energetic astrophysical events in the Universe. The early and prompt detection of neutrinos before (pre-SN) and during the SN burst is a unique opportunity to realize the multi-messenger observation of the CCSN events. In this work, we describe the monitoring concept and present the sensitivity of the system to the pre-SN and SN neutrinos at the Jiangmen Underground Neutrino Observatory (JUNO), which is a 20 kton liquid scintillator detector under construction in South China. The real-time monitoring system is designed with both the prompt monitors on the electronic board and online monitors at the data acquisition stage, in order to ensure both the alert speed and alert coverage of progenitor stars. By assuming a false alert rate of 1 per year, this monitoring system can be sensitive to the pre-SN neutrinos up to the distance of about 1.6 (0.9) kpc and SN neutrinos up to about 370 (360) kpc for a progenitor mass of 30$M_{\odot}$ for the case of normal (inverted) mass ordering. The pointing ability of the CCSN is evaluated by using the accumulated event anisotropy of the inverse beta decay interactions from pre-SN or SN neutrinos, which, along with the early alert, can play important roles for the followup multi-messenger observations of the next Galactic or nearby extragalactic CCSN.Comment: 24 pages, 9 figure

PrBaCo₂O_6−δ-Ce_0.8Sm_0.2O_1.9 Composite Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells: Stability and Cation Interdiffusion

The single-phase oxide PrBaCo2O6−δ and composites (100 − y)PrBaCo2O6−δ-yCe0.8Sm0.2O1.9 (y = 10⁻30 wt.%) were investigated as cathode materials for intermediate-temperature solid oxide fuel cells. The chemical compatibility, cation interdiffusion, thermal expansion and dc conductivity were studied. As a result, strong interdiffusion of Pr and Sm was found between PrBaCo2O6−δ and Ce0.8Sm0.2O1.9. This leads to only insignificantly decreasing thermal expansion coefficient of composite with increasing fraction of Ce0.8Sm0.2O1.9 and, thus, mixing PrBaCo2O6−δ with Ce0.8Sm0.2O1.9 does not improve thermal expansion behavior of the cathode material. Moreover, formation of poorly-conducting BaCeO3, caused by chemical interaction between the double perovskite and doped ceria, was shown to lead to pronounced drop in the electrical conductivity of the composite cathode material with increasing Ce0.8Sm0.2O1.9 content

Thermochemical Study of CH3NH3Pb(Cl1−xBrx)3 Solid Solutions

Hybrid organic–inorganic perovskite halides, and, in particular, their mixed halide solid solutions, belong to a broad class of materials which appear promising for a wide range of potential applications in various optoelectronic devices. However, these materials are notorious for their stability issues, including their sensitivity to atmospheric oxygen and moisture as well as phase separation under illumination. The thermodynamic properties, such as enthalpy, entropy, and Gibbs free energy of mixing, of perovskite halide solid solutions are strongly required to shed some light on their stability. Herein, we report the results of an experimental thermochemical study of the CH3NH3Pb(Cl1−xBrx)3 mixed halides by solution calorimetry. Combining these results with molecular dynamics simulation revealed the complex and irregular shape of the compositional dependence of the mixing enthalpy to be the result of a complex interplay between the local lattice strain, hydrogen bonds, and energetics of these solid solutions