Structure-property correlation in oxide-ion and proton conductors for clean energy applications: recent experimental and computational advancements

In the last decade, the field of oxide-ion and proton conductors continued to trigger a significant amount of basic research aimed at improving the properties and the comprehension of actual materials, as well as at discovering novel phases. This need comes from the current and future urgent requests of changing our energy management towards cleaner solutions such as solid oxide fuel cells. In this review article, we highlight the most recent advancements in this exciting field by putting particular emphasis on the structure-property correlations in oxide-ion and proton conductors both from an experimental and a computational perspective. Special focus is laid on developments in the area of operando and in situ spectroscopy, machine learning and high-throughput approaches to accelerate the discovery of new and advanced materials

Structure of glassy lithium sulfate films sputtered in nitrogen (LISON): Insight from Raman spectroscopy and ab initio calculations

Raman spectra of thin solid electrolyte films obtained by sputtering a lithium sulfate target in nitrogen plasma are measured and compared to ab initio electronic structure calculations for clusters composed of 28 atoms. Agreement between measured and calculated spectra is obtained when oxygen atoms are replaced by nitrogen atoms and when the nitrogen atoms form bonds with each other. This suggests that the incorporation of nitrogen during the sputtering process leads to structures in the film, which prevent crystallization of these thin film salt glasses.Comment: 5 pages, 4 figure

The effect of cation substitution on the local coordination of protons in Ba2In1.85M0.15O6H2 (M = In, Ga, Sc and Y)

We report on an investigation of the local structure and vibrational dynamics in the brownmillerite-based proton conductors Ba2In1.85M0.15O6H2 with M = In, Ga, Sc and Y. The aim is to determine the effect of the cation (M) substitution on the local coordination environment of the protons. The techniques used are infrared spectroscopy and inelastic neutron scattering. The materials are characterized by two main types of proton sites, denoted as H (1) and H(2), which are featured by different local structures. We establish that the relative population of these two proton sites varies as a function of M. Specifically, it is found that, with respect to Ba2In2O6H2, the relative population of H(1) protons increases upon the substitution of In with any of the three different cations. The strongest effect is observed for M = Ga and Sc, whereas the effect observed for M = Y is minor. This new insight motivates efforts to unravel the mobility of the two types of protons, since then cation modification would offer a rational route for improving the proton conductivity of these types of materials

Phosphor Thermometry of Alumina-Forming High-Temperature Alloys Using Luminescent Rare-Earth Ions in YAG: Proof of Concept Using a Dispersion of Ce3+ -Doped YAG Particles in a FeCrAl Alloy

Most high-temperature processes require monitoring and controlling temperature, preferably with high precision and good lateral resolution. Here we evaluate the use of the technique commonly known as phosphor thermometry, which exploits the temperature dependent photoluminescence from an inorganic phosphor, for the determination of the temperature of a composite material consisting of the metallic alloy FeCrAl dispersed with phosphor particles of yttrium aluminum garnet (Y3Al5O12, YAG) doped with a small amount of luminescent Ce3+ ions (YAG:Ce3+). The results show that with some optimization and by changing the dopant ion, YAG based phosphor particles offer a unique opportunity to measure the surface temperature of metal alloys with high precision and high lateral resolution, all the way up to the maximum working temperature of alumina-forming high temperature alloys at ca. 1300 \ub0C

Photoluminescence Properties and Fabrication of Red-Emitting LEDs based on Ca9Eu(VO4)(7) Phosphor

We study the photoluminescence properties of the red-emitting phosphor Ca9Eu(VO4)(7) and establish a strong red emission centered at 613 nm under excitation at 395 nm (near ultra violet light, near-UV light) due to the intra-configurational D-5(0) -> F-7(2) transition within the 4f(6) configuration of the Eu3+ ions. The intensity of the emitted light decreases with increasing temperature and at T = 470 K about 50% of the intensity of the emitted light at room temperature is lost. Five different red-LED prototypes were constructed by applying a mixture of Ca9Eu(VO4)(7) phosphor and silicone gel on the headers of near-UV LED chips. The prototypes showed a color output from violet for the lowest phosphor concentration (133 g phosphor /l silicone gel), reaching an almost pure red-light output for the highest phosphor concentration (670 g phosphor /l silicone gel). The luminous efficiency of optical radiation (LER) was found to decrease slightly with increasing applied current. For the highest phosphor concentration, the LER decreases from 238 lmW(-1) for 1 mA current supply to 235 lmW(-1) for 18 mA current supply. The external quantum efficiency decreased from 7.33% for the lowest phosphor containing LED prototype to 4.13% for the highest one. (C) The Author(s) 2019. Published by ECS

Unraveling the impact of different thermal quenching routes on the luminescence efficiency of the Y3Al5O12:Ce3+ phosphor for white light emitting diodes

Cerium doped yttrium aluminium garnet, Y3-zCezAl5O12, is the prototype material for solid-state white lighting and it still is an important white LED phosphor. However, fundamental understanding of the thermal quenching of luminescence, which leads to a pronounced reduction of the emission intensity under high-power light-emitting diode operation, remains to be obtained. Here we show, through a multitechnique approach based on photoluminescence, thermoluminescence and mode-selective vibrational excitation experiments that thermal quenching of luminescence in Y3-zCezAl5O12 is caused by a combined effect of thermal ionization, thermally activated concentration quenching, and thermally activated 5d → 4f crossover relaxation via electron-phonon coupling, and establish the general trends upon variation of the Ce3+ concentration and temperature. Thermal quenching below 600 K is primarily the result of concentration quenching and crossover relaxation, which can be suppressed by keeping the Ce3+ dopant concentration far below 0.7 mol%, whereas for temperatures above 600 K thermal ionization is the predominating quenching process. This new insight into the interplay between different thermal quenching processes provides design principles for optimizing the light emittance and colour stability of new phosphor materials used in white lighting devices characterized by certain operating temperatures. This journal i

Rotational Dynamics of Organic Cations in Formamidinium Lead Iodide Perovskites

We report results from quasi-elastic neutron scattering studies on the rotational dynamics of formamidinium (HC[NH2]2+, FA) and methylammonium (CH3NH3+, MA) cations in FA1-xMAxPbI3 with x = 0 and 0.4 and compare it to the dynamics in MAPbI3. For FAPbI3, the FA cation dynamics evolve from nearly isotropic rotations in the high-temperature (T > 285 K) cubic phase through reorientations between preferred orientations in the intermediate-temperature tetragonal phase (140 K < T ⩽ 285 K) to an even more complex dynamics, due to a disordered arrangement of the FA cations, in the low-temperature tetragonal phase (T ⩽ 140 K). For FA0.6MA0.4PbI3, the dynamics of the respective organic cations evolve from a relatively similar behavior to FAPbI3 and MAPbI3 at room temperature to a different behavior in the lower-temperature phases where the MA cation dynamics are a factor of 50 faster as compared to those of MAPbI3. This insight suggests that tuning the MA/FA cation ratio may be a promising approach to tailoring the dynamics and, in effect, optical properties of FA1-xMAxPbI

Proton Diffusion Mechanism in Hydrated Barium Indate Oxides

We report on quasielastic neutron scattering (QENS) andab initiomolecular dynamics (AIMD) simulations of the mechanism of proton diffusionin the partially and fully hydrated barium indate oxide proton conductorsBa(2)In(2)O(5)(H2O)( x ) (x = 0.30 and 0.92). Structurally,these materials are featured by an intergrowth of cubic and "pseudo-cubic"layers of InO6 octahedra, wherein two distinct proton sites,H(1) and H(2), are present. We show that the main localized dynamicsof these protons can be described as rotational diffusion of O-H(1)species and H(2) proton transfers between neighboring oxygen atoms.The mean residence times of both processes are in the order of picosecondsin the two studied materials. For the fully hydrated material, Ba2In2O5(H2O)(0.92), we also reveal the presence of a third proton site, H(3), whichbecomes occupied upon increasing the temperature and serves as a saddlestate for the interexchange between H(1) and H(2) protons. Crucially,the occupation of the H(3) site enables long-range diffusion of protons,which is highly anisotropic in nature and occurs through a two-dimensionalpathway. For the partially hydrated material, Ba2In2O5(H2O)(0.30), the occupationof the H(3) site and subsequent long-range diffusion are not observed,which is rationalized by hindered dynamics of H(2) protons in thevicinity of oxygen vacancies. A comparison to state-of-the-art proton-conductingoxides, such as barium zirconate-based materials, suggests that thegenerally lower proton conductivity in Ba2In2O5(H2O)( x ) is dueto a large occupation of the H(1) and H(2) sites, which, in turn,means that there are few sites available for proton diffusion. Thisinsight suggests that the chemical substitution of indium by cationswith higher oxidation states offers a novel route toward higher protonconductivity because it reduces the proton site occupancy while preservingan oxygen-vacancy-free structure