Development and validation of a computational approach to predicting the synthesis of inorganic materials

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 159-191).The concept of computational materials design envisions the identification of chemistries and structures with desirable properties through first-principles calculations, and the downselection of these candidates to those experimentally accessible using available synthesis methods. While first-principles property screening has become routine, the present lack of a robust method for the identification of synthetically accessible materials is an obstacle to true materials design. In this thesis, I develop a general approach for evaluating synthesizeability, and where possible, identifying synthesis routes towards the realization of target materials. This approach is based on a quasi- thermodynamic analysis of synthesis methods, relying on the assumption that phase selection is guided by transient thermodynamic stability under the conditions relevant to phase formation. By selecting the thermodynamic handles relevant to a growth procedure and evaluating the evolution of thermodynamic boundary conditions throughout the reaction, I identify potential metastable end-products as the set of ground state phases stabilized at various stages of the synthesis. To validate this approach, I derive the quasi-thermodynamic influence of adsorption-controlled finite- size stability and bulk off-stoichiometry on phase selection in the aqueous synthesis of polymorphic FeS2 and MnO2 systems, rationalizing the results of a range of synthesis experiments. To enable this analysis, I develop and benchmark the methodology necessary for the reliable first-principles evaluation of structure-sensitive bulk and interfacial stability in aqueous media. Finally, I describe a manganese oxide oxygen evolution catalyst, whose high activity is controlled by metastable, tetrahedrally- coordinated Mn3+ ions as an example of materials functionality enabled by structural metastability. The framework for the first-principles analysis of synthesis proposed and validated in this thesis lays the groundwork for the development of computational synthesis prediction and holds the potential to greatly accelerate the design and realization of new functional materials.by Daniil A. Kitchaev.Ph. D

Constructing and proving the ground state of a generalized Ising model by the cluster tree optimization algorithm

Generalized Ising models, also known as cluster expansions, are an important tool in many areas of condensed-matter physics and materials science, as they are often used in the study of lattice thermodynamics, solid-solid phase transitions, magnetic and thermal properties of solids, and fluid mechanics. However, the problem of finding the global ground state of generalized Ising model has remained unresolved, with only a limited number of results for simple systems known. We propose a method to efficiently find the periodic ground state of a generalized Ising model of arbitrary complexity by a new algorithm which we term cluster tree optimization. Importantly, we are able to show that even in the case of an aperiodic ground state, our algorithm produces a sequence of states with energy converging to the true ground state energy, with a provable bound on error. Compared to the current state-of-the-art polytope method, this algorithm eliminates the necessity of introducing an exponential number of variables to counter frustration, and thus significantly improves tractability. We believe that the cluster tree algorithm offers an intuitive and efficient approach to finding and proving ground states of generalized Ising Hamiltonians of arbitrary complexity, which will help validate assumptions regarding local vs. global optimality in lattice models, as well as offer insights into the low-energy behavior of highly frustrated systems

Phase behavior and superprotonic conductivity in the Cs_(1-x)Rb_xH_2PO_4 and Cs_(1-x)K_xH_2PO_4 systems

The solid acid compound CsH_2PO_4 (CDP) adopts a cubic structure of high conductivity above 228 °C, rendering it attractive as a fuel cell electrolyte for intermediate temperature operation. This superprotonic phase is stable from the phase transition temperature, T_s, to the dehydration temperature, T_d, where the latter depends on water vapor pressure (e.g. T_d = 290 °C at p_(H_2O) = 0.8 atm). In this work we examine the possibility of modifying these temperatures and thereby, amongst other characteristics, fuel cell operating conditions by introduction of Rb and K as substituents for Cs in CDP. The phase behavior of the Cs_(1−x)Rb_xH_2PO_4 and Cs_(1−x)K_xH_2PO_4 pseudo-binary systems are determined by in situ X-ray diffraction (XRD) and thermal analysis. It is found that RbH_2PO_4 (RDP) and CDP are entirely miscible both below and above the transition to the cubic phase. With increasing Rb concentration, T_s increases and T_d decreases. In contrast, K has limited solubility in CDP, with a 27 at.% solubility limit in the cubic phase, and both T_s and T_d decrease with K content. The eutectoid temperature in the Cs_(1−x)K_xH_2PO_4 system is 208 ± 2 °C and the K solubility decreases sharply below this temperature. In both systems, conductivity decreases monotonically with increasing substituent concentration. Furthermore, even after normalization for cation size, the impact of K is greater than that of Rb, suggesting local disruptions to the proton migration pathway, beyond global changes in unit cell volume. Although this investigation shows unmodified CDP to remain the optimal fuel cell electrolyte material, the study provides a possible framework for elucidating proton transport mechanisms in superprotonic conductors

Finding and proving the exact ground state of a generalized Ising model by convex optimization and MAX-SAT

This paper was supported primarily by the US Department of Energy (DOE) under Contract No. DE-FG02-96ER45571. In addition, some of the test cases for ground states were supported by the Office of Naval Research under contract N00014-14-1-0444.Lattice models, also known as generalized Ising models or cluster expansions, are widely used in many areas of science and are routinely applied to the study of alloy thermodynamics, solid-solid phase transitions, magnetic and thermal properties of solids, fluid mechanics, and others. However, the problem of finding and proving the global ground state of a lattice model, which is essential for all of the aforementioned applications, has remained unresolved for relatively complex practical systems, with only a limited number of results for highly simplified systems known. In this paper, we present a practical and general algorithm that provides a provable periodically constrained ground state of a complex lattice model up to a given unit cell size and in many cases is able to prove global optimality over all other choices of unit cell. We transform the infinite-discrete-optimization problem into a pair of combinatorial optimization (MAX-SAT) and nonsmooth convex optimization (MAX-MIN) problems, which provide upper and lower bounds on the ground state energy, respectively. By systematically converging these bounds to each other, we may find and prove the exact ground state of realistic Hamiltonians whose exact solutions are difficult, if not impossible, to obtain via traditional methods. Considering that currently such practical Hamiltonians are solved using simulated annealing and genetic algorithms that are often unable to find the true global energy minimum and inherently cannot prove the optimality of their result, our paper opens the door to resolving longstanding uncertainties in lattice models of physical phenomena. An implementation of the algorithm is available at https://github.com/dkitch/maxsat-isingPublisher PDFPeer reviewe

The interplay between thermodynamics and kinetics in the solid-state synthesis of layered oxides.

In the synthesis of inorganic materials, reactions often yield non-equilibrium kinetic byproducts instead of the thermodynamic equilibrium phase. Understanding the competition between thermodynamics and kinetics is a fundamental step towards the rational synthesis of target materials. Here, we use in situ synchrotron X-ray diffraction to investigate the multistage crystallization pathways of the important two-layer (P2) sodium oxides Na0.67MO2 (M = Co, Mn). We observe a series of fast non-equilibrium phase transformations through metastable three-layer O3, O3' and P3 phases before formation of the equilibrium two-layer P2 polymorph. We present a theoretical framework to rationalize the observed phase progression, demonstrating that even though P2 is the equilibrium phase, compositionally unconstrained reactions between powder precursors favour the formation of non-equilibrium three-layered intermediates. These insights can guide the choice of precursors and parameters employed in the solid-state synthesis of ceramic materials, and constitutes a step forward in unravelling the complex interplay between thermodynamics and kinetics during materials synthesis

Magnetoentropic mapping and computational modeling of cycloids and skyrmions in the lacunar spinels GaV4_4S8_8 and GaV4_4Se8_8

We report the feasibility of using magnetoentropic mapping for the rapid identification of magnetic cycloid and skyrmion phases in uniaxial systems, based on the GaV4S8 and GaV4Se8 model skyrmion hosts with easy-axis and easy-plane anisotropies respectively. We show that these measurements can be interpreted with the help of a simple numerical model for the spin Hamiltonian to yield unambiguous assignments for both single phase regions and phase boundaries. In the two lacunar spinel chemistries, we obtain excellent agreement between the measured magnetoentropic features and a minimal spin Hamiltonian built on Heisenberg exchange, single-ion anisotropy, and anisotropic Dzyaloshinskii-Moriya interactions. In particular, we identify characteristic high-entropy behavior in the cycloid phase that serves as a precursor to the formation of skyrmions at elevated temperatures and is a readily-measurable signature of this phase transition. Our results demonstrate that rapid magnetoentropic mapping guided by numerical modeling is an effective means of understanding the complex magnetic phase diagrams innate to skyrmion hosts. One notable exception is the observation of an anomalous, low-temperature high-entropy state in the easy-plane system GaV$_4$Se$_8$, which is not captured in the numerical model. Possible origins of this state are discussed.Comment: 10 pages and 7 figure

Structural evolution and skyrmionic phase diagram of the lacunar spinel GaMo4Se8

In the $AB_4Q_8$ lacunar spinels, the electronic structure is described on the basis of inter- and intra-cluster interactions of tetrahedral $B_4$ clusters, and tuning these can lead to myriad fascinating electronic and magnetic ground states. In this work, we employ magnetic measurements, synchrotron X-ray and neutron scattering, and first-principles electronic structure calculations to examine the coupling between structural and magnetic phase evolution in GaMo$_4$Se$_8$, including the emergence of a skyrmionic regime in the magnetic phase diagram. We show that the competition between two distinct Jahn-Teller distortions of the room temperature cubic $F\overline{4}3m$ structure leads to the coexistence of the ground state $R3m$ phase and a metastable $Imm2$ phase. The magnetic properties of these two phases are computationally shown to be very different, with the $Imm2$ phase exhibiting uniaxial ferromagnetism and the $R3m$ phase hosting a complex magnetic phase diagram including equilibrium N\'eel--type skyrmions stable from nearly $T$ = 28 K down to $T$ = 2 K, the lowest measured temperature. The large change in magnetic behavior induced by a small structural distortion reveals that GaMo$_4$Se$_8$ is an exciting candidate material for tuning unconventional magnetic properties $via$ mechanical means

Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes

The discovery of facile Li transport in disordered, Li-excess rocksalt materials has opened a vast new chemical space for the development of high energy density, low cost Li-ion cathodes. We develop a strategy for obtaining optimized compositions within this class of materials, exhibiting high capacity and energy density as well as good reversibility, by using a combination of low-valence transition metal redox and a high-valence redox active charge compensator, as well as fluorine substitution for oxygen. Furthermore, we identify a new constraint on high-performance compositions by demonstrating the necessity of excess Li capacity as a means of counteracting high-voltage tetrahedral Li formation, Li-binding by fluorine and the associated irreversibility. Specifically, we demonstrate that 10–12% of Li capacity is lost due to tetrahedral Li formation, and 0.4–0.8 Li per F dopant is made inaccessible at moderate voltages due to Li–F binding. We demonstrate the success of this strategy by realizing a series of high-performance disordered oxyfluoride cathode materials based on Mn²+/⁴+ and V⁴+/⁵+ redox.Vehicle Technologies Program (U.S.) (Contract No. DE-AC02-05CH11231)United States. Department of Energy. Office of Energy Efficiency and Renewable Energy. Advanced Battery Materials Research Program (Subcontract No. 7056411)National Science Foundation (U.S.) (Reward No. OCI-1147503)National Science Foundation (U.S.) (grant number ACI- 105357)National Science Foundation (U.S.) (NSF DMR 172025)United States. Department of Energy (Contract No. DE-AC02-06C H11357)United States. Department of Energy. Office of Science (contract no. DE-AC02-05CH11231

Hidden structural and chemical order controls lithium transport in cation-disordered oxides for rechargeable batteries

This work was supported by the Robert Bosch Corporation, Umicore Specialty Oxides and Chemicals, and the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technologies Office of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 under the Advanced Battery Materials Research (BMR) Program. The research conducted at the NOMAD Beamline at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Sciences, U.S. Department of Energy. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The computational analysis was performed using computational resources sponsored by the Department of Energy’s Office of Energy Efficiency and Renewable Energy and located at the National Renewable Energy Laboratory, as well computational resources provided by Extreme Science and Engineering Discovery Environment (XSEDE), which was supported by the National Science Foundation grant number ACI-1053575.Structure plays a vital role in determining materials properties. In lithium ion cathode materials, the crystal structure defines the dimensionality and connectivity of interstitial sites, thus determining lithium ion diffusion kinetics. In most conventional cathode materials that are well-ordered, the average structure as seen in diffraction dictates the lithium ion diffusion pathways. Here, we show that this is not the case in a class of recently discovered high-capacity lithium-excess rocksalts. An average structure picture is no longer satisfactory to understand the performance of such disordered materials. Cation short-range order, hidden in diffraction, is not only ubiquitous in these long-range disordered materials, but fully controls the local and macroscopic environments for lithium ion transport. Our discovery identifies a crucial property that has previously been overlooked and provides guidelines for designing and engineering cation-disordered cathode materials.Publisher PDFPeer reviewe