Spin-State Transition and Metal-Insulator Transition in La1−x_{1-x}Eux_xCoO3_3}

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of La$_{1-x}$Eu$_x$CoO$_3$. LaCoO$_3$ shows a temperature-induced spin-state transition around 100 K and a metal-insulator transition around 500 K. Partial substitution of La$^{3+}$ by the smaller Eu$^{3+}$ causes chemical pressure and leads to a drastic increase of the spin gap from about 190 K in LaCoO$_3$ to about 2000 K in EuCoO$_3$, so that the spin-state transition is shifted to much higher temperatures. A combined analysis of thermal expansion and susceptibility gives evidence that the spin-state transition has to be attributed to a population of an intermediate-spin state with orbital order for $x<0.5$ and without orbital order for larger $x$. In contrast to the spin-state transition, the metal-insulator transition is shifted only moderately to higher temperatures with increasing Eu content, showing that the metal-insulator transition occurs independently from the spin-state distribution of the Co$^{3+}$ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all $x$ and approaches a doping-independent value around 1000 K indicating that well above the metal-insulator transition the same spin state is approached for all $x$.Comment: 10 pages, 6 figure

A multi-scale approach to study Solid Oxide Fuel Cells: from Mechanical Properties and Crystal Structure of the Cell\u27s Materials to the Development of an Interactive and Interconnected Educational Tool

Solid Oxide Fuel Cells are energy conversion devices that convert chemical energy of a fuel directly into electrical energy. They are known for being fuel-flexible, have minimal harmful emissions, ideal for combined heat and power applications, highly energy-efficient when combined with gas or steam turbines. The current challenges facing the widespread adoption these fuel cells include cost reduction, long-term testing of fully integrated systems, improving the fuel cell stack and system performance, and studies related to reliability, robustness and durability. The goal of this dissertation is to further the understanding of the mechanical properties and crystal structure of materials used in the cathode and electrolyte of solid oxide fuel cells, as well as to report on the development of a supplementary educational tool that could be used in course related to fuel cells. The first part of the dissertation relates to the study of LaCoO3 based perovskites that are used as cathode material in solid oxide fuel cells and in other energy-related applications. In-situ neutron diffraction of LaCoO3 perovskite during uniaxial compression was carried out to study crystal structure evolution and texture development. In this study, LaCoO3 was subjected to two cycles of uniaxial loading and unloading with the maximum stress value being 700-900 MPa. The in-situ neutron diffraction revealed the dynamic crystallographic changes occurring which is responsible for the non-linear ferroelastic deformation and the appearance of hysteresis in LaCoO3. At the end of the first cycle, irreversible strain was observed even after the load was removed, which is caused by non-recoverable domain reorientation and texture development. At the end of the second cycle, however, no irreversible strain was observed as domain reorientation seemed fully recovered. Elastic constants were calculated and Young\u27s modulus was estimated for LaCoO3 single crystals oriented along different crystallographic directions. The high temperature mechanical behavior study of LaCoO3 based perovskites is also of prime importance as solid oxide fuel cells operate at high temperatures. Incidentally, it was observed that as opposed to the behavior of most materials, LaCoO3 exhibits stiffening between 700 oC to 900 oC, with the Young\u27s modulus going from a value of ~76 GPa at room temperature to ~120 GPa at 900 oC. In-situ neutron diffraction, XRD and Raman spectroscopy were used to study structural changes occurring in the material as it was heated. The results from these experiments will be discussed. The next portion of the dissertation will focus on electrolytes. Numerical simulation was carried out in order to predict the non-linear load-stress relationship and estimation of biaxial flexure strength in layered electrolytes, during ring-on-ring mechanical testing. Finally, the development of an interactive and inter-connected educational software is presented that could serve as a supplementary tool to teach fuel cell related topics

Local electronic structure and magnetic properties of LaMn0.5Co0.5O3 studied by x-ray absorption and magnetic circular dichroism spectroscopy

We have studied the local electronic structure of LaMn0.5Co0.5O3 using soft-x-ray absorption spectroscopy at the Co-L_3,2 and Mn-L_3,2 edges. We found a high-spin Co^{2+}--Mn^{4+} valence state for samples with the optimal Curie temperature. We discovered that samples with lower Curie temperatures contain low-spin nonmagnetic Co^{3+} ions. Using soft-x-ray magnetic circular dichroism we established that the Co^{2+} and Mn^{4+} ions are ferromagnetically aligned. We revealed also that the Co^{2+} ions have a large orbital moment: m_orb/m_spin ~ 0.47. Together with model calculations, this suggests the presence of a large magnetocrystalline anisotropy in the material and predicts a non-trivial temperature dependence for the magnetic susceptibility.Comment: 8 pages, 7 figure

On the thermal and mechanical properties of Mg0.2_{0.2}Co0.2_{0.2}Ni0.2_{0.2}Cu0.2_{0.2}Zn0.2_{0.2}O across the high-entropy to entropy-stabilized transition

As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek further understanding of property changes across the phase boundary between \enquote{high-entropy} and \enquote{entropy-stabilized}. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent X-ray diffraction, and nano-indentation. Thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ~2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24 % from 10.8 to 14.1 x 10$^{-6}$ K$^{-1}$. Mechanical softening is also observed across the transition.Comment: 14 pages, 4 figures, to be published in APL Material

Thermal Transport in Cuprates, Cobaltates, and Manganites

The subject of this thesis is the investigation of the thermal transport properties of three classes of transition-metal oxides: Cuprates, cobaltates, and manganites. The layered cuprates R2CuO4 with R=La, Pr, Nd, Sm, Eu, and Gd show an anomalous thermal conductivity k. Two maxima of k are observed as a function of temperature for a heat current within the CuO2 planes, whereas for a heat current perpendicular to the CuO2 planes only a conventional phononic low-temperature maximum of k is present. Evidence is provided that the high-temperature maximum is caused by heat-carrying excitations on the CuO2 square lattice. Moreover, it is shown that the complex low-temperature and magnetic-field behavior of k in Nd2CuO2 is most likely caused by additional phonon scattering rather than by heat-carrying Nd magnons, as it was proposed in the literature. In the cobaltates RCoO3 with R=La, Pr, Nd, and Eu, a temperature-induced spin-state transition of the Co(3+) ions occurs. It is shown that the additional lattice disorder caused by the random distribution of populated higher spin states causes a large suppression of the thermal conductivity of LaCoO3 for T>25K. The effect is much weaker in PrCoO3 and NdCoO3 due to the increased spin gap. A quantitative analysis of the responsible mechanisms based on EuCoO3 as a reference compound is provided. A main result is that the static disorder is sufficient to explain the suppression of k. No dynamical Jahn-Teller distortion, as proposed in the literature, is necessary to enhance the scattering strength. Below 25K k is mainly determined by resonant phonon scattering on paramagnetic impurity levels, e.g. caused by oxygen non-stoichiometry. Such a suppression of the thermal conductivity by resonant scattering processes is e.g. known from Holmium ethylsulfate. This effect is most pronounced in LaCoO3, presumably due to magnetic polaron formation. In the doped compounds LaxSr1-xCoO3 with 0<=x<=0.25, a large thermopower, a low thermal conductivity, and a considerable large thermoelectric figure of merit is found. Here, k is strongly suppressed by the Sr-induced magnetic polarons, whereas the large thermopower arises from a large entropy contribution due to the different spin states of Co(3+) and Co(4+). In the orthorhombic manganites NdMnO3 and TbMnO3 complex temperature and field dependencies of k are observed. In combination with magnetic-field dependent thermal expansion measurements it is shown that the dominating effect determining k is resonant phonon scattering by the 4f orbitals of the R(3+) ions. The complicated magnetic structure of TbMnO3 at low temperature as well as the ferroelectricity has only a minor influence on the thermal conductivity

Size-dependent magnetic properties of LaCoO₃ and La₁₋ₓSrₓMnO₃ nanoparticles

Systematische Untersuchungen an Perowskit-Nanopartikeln erfordern mehrere Chargen von m¨oglichst monodispersen, einkristallinen Nanopartikeln unterschiedlicher Größe, aber mit vergleichbarer Kristallg¨ute und Sauerstoffstöchiometrie. Durch die Mikroemulsionssynthese ist es möglich, die Nanopartikelgröße durch das Wasser/Tensid-Verhältnis zu regeln. Somit können Chargen mit unterschiedlicher Partikelgröße unter gleichen Kalzinierungsbedingungen entstehen. Anhand der so hergestellten Chargen wurden die magnetischen Eigenschaften von LaCoO3 und La1-xSrxMnO3 Nanopartikeln untersucht. In beiden Materialien führt die Reduzierung der Nanopartikelgröße zu einerVergrößerung der Einheitszelle und einer Verl¨angerung der Übergangsmetall-Sauerstoff-Bindungslänge. Diese Ver¨anderungen skalieren linear mit dem Oberfläche/Volumen-Verhältnis (S/V ) und sind wahrscheinlich eine Konsequenz von Wasseradsorption an der Oberfläche. Da die Bindungsenergie der Adsorbate sowohl vom Übergangsmetallion als auch dessen Valenz abh¨angig ist, dehnt sich das Kristallgitter in La1-xSrxMnO3 st¨arker aus als in LaCoO3. Durch Verringerung der Kristallfeldaufspaltung am Co3+ ion f¨uhrt die Vergrößerung des Kobalt-Sauerstoff-Abstands zur Stabilisierung des High-Spin-Zustandes (HS, S=2). Demzufolge steigt das magnetische Moment in LaCoO3 linear mit S/V an. Im Gegensatz zu Studien an d¨unnen Schichten von LaCoO3 [1] ist die Co3+-HS-Konzentration in Nanopartikeln zu gering, um langreichweitige ferromagnetische Ordnung zu etablieren, und die Nanopartikel zeigen paramagnetische Verhalten. In La1-xSrxMnO3 Nanopartikeln schw¨acht die Verl¨angerung der Mn-O-Bindungsl¨ange und Verringerung des Mn- O-Mn-Bindungswinkels den Doppelaustausch und reduziert somit die ferromagnetische Ordnungstemperatur TC. TC wird weiter verringert durch den intrinsischen finite-size- Effekt. The beobachteten größeninduzierten Änderungen der magnetischen Eigenschaften könnten die kontrollierte Ver¨anderung der magnetischen Eigenschaften in LaCoO3- und La1-xSrxMnO3-Nanopartikeln gewährleisten

Space Electrochemical Research and Technology (SERT)

The conference provided a forum to assess critical needs and technologies for the NASA electrochemical energy conversion and storage program. It was aimed at providing guidance to NASA on the appropriate direction and emphasis of that program. A series of related overviews were presented in the areas of NASA advanced mission models (space stations, low and geosynchronous Earth orbit missions, planetary missions, and space transportation). Papers were presented and workshops conducted in a variety of technical areas, including advanced rechargeables, advanced concepts, critical physical electrochemical issues, and modeling

Hydrogen-Induced Topotactic Phase Transformations of Cobaltite Thin Films

Manipulating physical properties through ion migration in complex oxide thin films is an emerging research direction to achieve tunable materials for advanced applications. While the reduction of complex oxides has been widely reported, few reports exist on the modulation of physical properties through a direct hydrogenation process. Here, we report an unusual mechanism for hydrogen-induced topotactic phase transitions in perovskite La0.7Sr0.3CoO3 thin films. Hydrogenation is performed upon annealing in a pure hydrogen gas environment, offering a direct understanding of the role that hydrogen plays at the atomic scale in these transitions. Topotactic phase transformations from the perovskite (P) to hydrogenated-brownmillerite (H-BM) phase can be induced at temperatures as low as 220 °C, while at higher hydrogenation temperatures (320–400 °C), the progression toward more reduced phases is hindered. Density functional theory calculations suggest that hydroxyl bonds are formed with the introduction of hydrogen ions, which lower the formation energy of oxygen vacancies of the neighboring oxygen, enabling the transition from the P to H-BM phase at low temperatures. Furthermore, the impact on the magnetic and electronic properties of the hydrogenation temperature is investigated. Our research provides a potential pathway for utilizing hydrogen as a basis for low-temperature modulation of complex oxide thin films, with potential applications in neuromorphic computing

Interplay between electronic structure and catalytic activity in transition metal oxide model system

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 109-125).The efficiency of many energy storage and conversion technologies, such as hydrogen fuel cells, rechargeable metal-air batteries, and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen electrochemical reactions. Transition-metal oxides can exhibit high catalytic activity for oxygen electrochemical reactions, which can be used to improve efficiency and cost of these devices. Identifying a catalyst "design principle" that links material properties to the catalytic activity can accelerate the development of highly active, abundant transition metal oxide catalysts fore more efficient, cost-effective energy storage and conversion system. In this thesis, we demonstrate that the oxygen electrocatalytic activity for perovskite transition metal oxide catalysts primarily correlates to the a* orbital ("eg") occupation. We further find that the extent of B-site transition metal-oxygen covalency can serve as a secondary activity descriptor. We hypothesize that this correlation reflects the critical influences of the a* orbital and transition metal-oxygen covalency on the ability of the surface to displace and stabilize oxygen-species on surface transition metals. We further propose that this ability to stabilize oxygen-species reflect as the rate-limiting steps of the oxygen electrochemical reactions on the perovskite oxide surfaces, and thus highlight the importance of electronic structure in controlling the oxide catalytic activity.by Jin Suntivich.Sc.D