Origin of the Spin-Orbital Liquid State in a Nearly J=0 Iridate Ba3ZnIr2O9

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba3ZnIr2O9 is a realization of a novel spin-orbital liquid state. Our results reveal that Ba3ZnIr2O9 with Ir5+ (5d(4)) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J = 0 state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir2O9 dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK

Numerical tools for atomistic simulations.

The final report for a Laboratory Directed Research and Development project entitled 'Parallel Atomistic Computing for Failure Analysis of Micromachines' is presented. In this project, atomistic algorithms for parallel computers were developed to assist in quantification of microstructure-property relations related to weapon micro-components. With these and other serial computing tools, we are performing atomistic simulations of various sizes, geometries, materials, and boundary conditions. These tools provide the capability to handle the different size-scale effects required to predict failure. Nonlocal continuum models have been proposed to address this problem; however, they are phenomenological in nature and are difficult to validate for micro-scale components. Our goal is to separately quantify damage nucleation, growth, and coalescence mechanisms to provide a basis for macro-scale continuum models that will be used for micromachine design. Because micro-component experiments are difficult, a systematic computational study that employs Monte Carlo methods, molecular statics, and molecular dynamics (EAM and MEAM) simulations to compute continuum quantities will provide mechanism-property relations associated with the following parameters: specimen size, number of grains, crystal orientation, strain rates, temperature, defect nearest neighbor distance, void/crack size, chemical state, and stress state. This study will quantify sizescale effects from nanometers to microns in terms of damage progression and thus potentially allow for optimized micro-machine designs that are more reliable and have higher fidelity in terms of strength. In order to accomplish this task, several atomistic methods needed to be developed and evaluated to cover the range of defects, strain rates, temperatures, and sizes that a material may see in micro-machines. Therefore we are providing a complete set of tools for large scale atomistic simulations that include pre-processing of realistic material configurations, processing under different environments, and post-processing with appropriate continuum quantities. By running simulations with these tools, we are able to determine size scale effects that correlate microstructure and defect configurations with mechanical properties of materials

Jahn-Teller driven electronic instability in thermoelectric tetrahedrite

Tetrahedrite, Cu12Sb4S13, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal-to-semiconductor transition, remains largely unknown. This work presents the first determination of the low-temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, our results show that the trigonal-planar copper cations remain in planar coordination below the metal-to-semiconductor transition. The atomic displacement parameters of the trigonal-planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the metal-to-semiconductor transition. The phase transition is consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found inside the sodalite cages in the cubic phase. This study reveals that a Jahn-Teller electronic instability leads to the formation of “molecular-like” Cu57+ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper-copper interactions. Our first-principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. The present results provide insights on the interplay between phonon transport, electronic properties and crystal structure in mixed-valence compounds

Argonne National Laboratory Report ANL-80-12

Analysis of refractories exposed to slag attack during the last 500-h test run shows that complex spinels formed at the slag-refractories

Argonne National Laboratory Report ANL-79-93

Quarterly report on the activities of the Argonne National Laboratory Materials Science Division regarding economical conversion of coal into clean and usable fuels through the use of durable materials systems. This project is designed to provide part of the materials information necessary for successful operation of coal-conversion systems

Argonne National Laboratory Report ANL-79-22

Quarterly report on the activities of the Argonne National Laboratory Materials Science Division regarding studies on ceramic (refractory) and metallic materials presently being used or intended for use in coal conversion processes. The program entails nondestructive testing, failure analysis, and studies of erosive wear, corrosion and refractory degradation