This report is the final documentation for the one-year LDRD project 226360: Simulated X-ray Diffraction and Machine Learning for Optimizing Dynamic Experiment Analysis. As Sandia has successfully developed in-house X-ray diffraction tools for study of atomic structure in experiments, it has become increasingly important to develop computational analysis methods to support these experiments. When dynamically compressed lattices and orientations are not known a priori, the identification requires a cumbersome and sometimes intractable search of possible final states. These final states can include phase transition, deformation and mixed/evolving states. Our work consists of three parts: (1) development of an XRD simulation tool and use of traditional data science methods to match XRD patterns to experiments; (2) development of ML-based models capable of decomposing and identifying the lattice and orientation components of multicomponent experimental diffraction patterns; and (3) conducting experiments which showcase these new analysis tools in the study of phase transition mechanisms. Our target material has been cadmium sulfide, which exhibits complex orientation-dependent phase transformation mechanisms. In our current one-year LDRD, we have begun the analysis of high-quality c-axis CdS diffraction data from DCS and Thor experiments, which had until recently eluded orientation identification.
Abstract: Here, we report the high pressure phase and morphology behavior of ordered anatase titanium dioxide (TiO2) nanocrystal arrays. One-dimensional TiO2 nanorods and nanorices were synthesized and self-assembled into ordered mesostructures. Their phase and morphological transitions at both atomic scale and mesoscale under pressure were studied using in situ synchrotron wide- and small-angle x-ray scattering (WAXS and SAXS) techniques. At the atomic scale, synchrotron WAXS reveals a pressure-induced irreversible amorphization up to 35 GPa in both samples but with different onset pressures. On the mesoscale, no clear phase transformations were observed up to 20 GPa by synchrotron SAXS. Intriguingly, sintering of TiO2 nanorods at mesoscale into nano-squares or nano-rectangles, as well as nanorices into nanowires, were observed for the first time by transmission electron microscopy. Such pressure-induced nanoparticle phase-amorphization and morphological changes provide valuable insights for design and engineering structurally stable nanomaterials. Impact statement: The high pressure behavior of nanocrystals (NCs) continues to be of interest, as previous studies have demonstrated that an externally applied pressure can serve as an efficient tool to induce structural phase transitions of NC assemblies at both the atomic scale and mesoscale without altering any chemistry by manipulating NC interatomic and interparticle distances. In addition, the high pressure generated deviatoric stress has been proven to be able to force adjacent NCs to connect and fuse into new crystalline nanostructures. Although the atomic structural evolution of TiO2 NCs under pressure has been widely investigated in the past decades, open questions remain regarding the mesoscale phase transition and morphology of TiO2 NC assemblies as a function of pressure. Therefore, in this work, systemic high pressure experiments on ordered arrays of TiO2 nanorods and nanorices were conducted by employing wide/small angle x-ray scattering techniques. The sintering of TiO2 assemblies at mesoscale into various nanostructures under pressure were revealed by transmission electron microscopy. Overall, this high pressure work fills the current gap in research on the mesoscale phase behavior of TiO2 assemblies. The observed morphology tunability attained by applying pressure opens new pathways for engineering nanomaterials and optimizing their collective properties through mechanical compression stresses. Graphical abstract: [Figure not available: see fulltext.].
Pulsed-power generators using the magnetic loading technique are able to produce well-controlled continuous ramp compression of condensed matter for high-pressure equation-of-state studies. X-ray diffraction (XRD) data from dynamically compressed samples provide direct measurements of the elastic compression of the crystal lattice, onset of plastic flow, strength-strain rate dependence, structural phase transitions, and density of crystal defects such as dislocations. Here, we present a cost effective, compact X-ray source for XRD measurements on pulsed-power-driven ramp-loaded samples. This combination of magnetically-driven ramp compression of materials with single, short-pulse XRD diagnostic will be a powerful capability for the dynamic materials community. The success in fielding this new XRD diagnostic dramatically improves our predictive capability and understanding of rate-dependent behavior at or near phase transition. As Sandia plans the next-generation pulse-power driver platform, a key element needed to deliver new state-of-the-art experiments will be having the necessary diagnostic tools to probe new regimes and phenomena. These diagnostics need to be as versatile, compact, and portable as they are powerful. The development of a platform-independent XRD diagnostic gives Sandia researchers a new window to study the microstructure and phase dynamics of materials under load. This project has paved the way for phase transition research in a variety of materials with mission interest.
Here, we describe recent efforts to improve our predictive modeling of rate-dependent behavior at, or near, a phase transition using molecular dynamics simulations. Cadmium sulfide (CdS) is a well-studied material that undergoes a solid-solid phase transition from wurtzite to rock salt structures between 3 and 9 GPa. Atomistic simulations are used to investigate the dominant transition mechanisms as a function of orientation, size and rate. We found that the final rock salt orientations were determined relative to the initial wurtzite orientation, and that these orientations were different for the two orientations and two pressure regimes studied. The CdS solid-solid phase transition is studied, for both a bulk single crystal and for polymer-encapsulated spherical nanoparticles of various sizes.
Recent dynamic compression experiments [M. D. Knudson et al., Science 348, 1455 (2015); P. M. Celliers et al., Science 361, 677 (2018)] have observed the insulator-metal transition in dense liquid deuterium, but with an approximately 95-GPa difference in the quoted pressures for the transition at comparable estimated temperatures. It was claimed in the latter of these two papers that a very large latent heat effect on the temperature was overlooked in the first, requiring correction of those temperatures downward by a factor of 2, thereby putting both experiments on the same theoretical phase boundary and reconciling the pressure discrepancy. We have performed extensive path-integral molecular dynamics calculations with density functional theory to directly calculate the isentropic temperature drop due to latent heat in the insulator-metal transition for dense liquid deuterium and show that this large temperature drop is not consistent with the underlying thermodynamics.
The properties of silica (SiO 2) at extreme conditions have important applications for planetary processes and for high pressure research. We report the results of 125 plate impact shock compression experiments on fused silica spanning 200-1100 GPa using the Z machine at Sandia National Laboratories. Additionally, we present a complementary set of density functional theory based molecular dynamics calculations based on an amorphous reference state that extend the Hugoniot to 2500 GPa. We find good agreement between the Z data, extant laser driven shock compression experiment data, and computational results over most of the pressure range. With these results, fused silica can be used as a new impedance matching standard for shock compression experiments.
