Session M59: Matter under Extreme Conditions I

Static Electronic Density Response of Warm Dense Hydrogen: Ab initio Path Integral Monte Carlo Simulations

The properties of hydrogen under extreme conditions are important for many applications, including inertial confinement fusion and astrophysical models. A key quantity is given by the electronic density response to an external perturbation, which is probed in X-ray Thomson scattering (XRTS) experiments---the state of the art diagnostics from which system parameters like the free electron density n_e, the electronic temperature T_e, and the charge state Z can be inferred. In this work, we present highly accurate path integral Monte Carlo (PIMC) results for the static electronic density response of hydrogen. We obtain the static exchange--correlation (XC) kernel K_XC, which is of central relevance for many applications, such as time-dependent density functional theory (TD-DFT). This gives us a first unbiased look into the electronic density response of hydrogen in the warm-dense matter regime, thereby opening up a gamut of avenues for future research.   

Route leading to exotic silicon allotropes and compounds

Over the past decades, diamond silicon has become the fundamental building block in solar cell market due to its high abundance and stability. However, the band gap of d-Si is indirect, therefore the absorbing crystal layer has to be thick and pure, so that the mean-free path of the carriers is comparable with the size of the layer. Due to these well-known limitations, materials with better absorption coefficients have been put forward in the past years. Emphasis has obviously been given to direct band-gap materials with absorption spectra that strongly overlap with the solar spectrum. This would allow for thinner, more flexible, and cheaper silicon solar cells. Using swarm-intelligence-based structure prediction methods, we predict a new Cmcm-SrSi₈ compound that can be synthesized under epitaxial strain conditions at high pressures. The stability of Cmcm-SrSi₈ down to zero pressure has been demonstrated using phonon spectrum calculations. After pressure release, we estimate that using the SrSi₈ clathrate-like structure as a precursor, the Sr degassing process will create a direct bandgap Si phase, designated Si₃₂. Due to the direct bandgap of 1.15 eV, we propose that Si₃₂ could be a potential solar energy absorber. These encouraging results shed fresh light on the developmental approach for direct bandgap semiconductor design. Therefore, there is potential to create a far more diverse landscape for silicon materials with advanced properties.    

New ternary lanthanum superhydrides under pressure

The discovery of room-temperature superconductivity in lanthanum superhydride LaH₁₀ may be understood in terms of the stabilization of a dense hydrogen cage structure with doping of B or N. We report experimental results for La-Y-H, La-Al-H, La-Nd-H, and La-C-H ternary hydrides at megabar pressures. Structures are identified on the basis of x-ray diffraction measurements, along with measurements of T_c from electrical conductivity. In particular, a cubic carbon-doped lanthanum hydride (La_1-nC_nH_x) has been identified at megabar pressures.   

Author not Attending

At pressures ~500 GPa, which corresponds to the deep interior of super-Earths, we can expect the existence of MgSiO₃ post-perovskite (PPv) and MgO. Previous calculations predicted that NaCl-type MgO and MgSiO₃ PPv might combine to form I4^–2d-type Mg₂SiO₄ (pppv).(Umemoto et al., 2017). This phase might be the primary mantle silicate in those massive exoplanets. Therefore, it is essential to understand the properties of this pppv silicate phase, especially in solid-solution with Fe₂SiO₄. Here we present an ab initio study on the properties of Fe-bearing pppv from 400GPa to 1TPa in a range of iron concentrations, x_Fe, varying from ~3% to 12%. Given the strongly correlated nature of iron, LDA + U_sc and conventional DFT methods were used to study the electronic structures. The dependence of U on pressure and spin state is carefully considered in the (Mg_1−xFe_x )₂SiO₄ system. The influence of pressure, temperature, and structure on the spin state has also been discu#@ssed. These results will help model the mantle of super-Earth-type exoplanets.

 

 

K. Umemoto et al., (2017). DOI: 10.1016/j.epsl.2017.08.032

    

Topological Electride Phase of Sodium at High Pressures and Temperatures

Ab initio evolutionary searches coupled with quasiharmonic calculations have solved the structure of a new phase of sodium whose X-ray diffraction pattern was measured during laser-driven ramp-compression experiments [Nat. Commun. 13, 2534 (2022)]. The predicted P6₃/m phase, a topological semimetal with a Dirac nodal surface that is protected by the non-symmorphic symmetry S_2z, is preferred over Na hP4 between 200 GPa at 150 K and 350 GPa at 1900 K. It is characterized by non-nuclear charge localized within 1D honeycomb channels and 0D cages, rendering it an electride. Our results yield new insight on the complexity of warm dense sodium’s electronic structure and rich polymorphism that emerges when ionic cores overlap, and show that the free energies are key for determining the phases that are created in dynamic compression experiments at extreme pressures and temperatures.   

High-Pressure Study of a Novel Cathode Material to 60 GPa

Lithium-rich cobalt oxyfluoride, synthesized as a disordered rock salt, is a novel cathode material for Li-ion batteries that shows high-capacity retention even after long-term cycling, despite high capacity being typically associated with ordered structures. The disordered nature is due to random occupancy of the cations in the octahedral voids and of the anions forming the cubic closed-packed array. Here high pressure is used to further characterize the material and to search for new structures. To that end, x-ray diffraction was performed up to 60 GPa, and Raman and IR spectroscopies were performed up to 50 GPa. The diffraction shows that the ambient structure persists on compression up to 60 GPa at room temperature. The x-ray data give a bulk modulus, K0, of 273(5) GPa, with a K0’ of 1.0 (0.2). A broad Raman peak near 610 cm^-1 shifts to higher wavenumber and increases in intensity under pressure, and a new peak at 660 cm^-1 emerges at 10 GPa, suggesting a change in the structure not apparent in the diffraction data. The IR measurements show that the energy absorption edge increases with pressure indicating an opening of the band gap.   

Raman spectra of boron carbide B4.3C under high pressure from first principles

Despite decades of investigations, the atomic structure of boron carbide below 20% atomic carbon concentration is still subject to debates [1,2]. In particular, the experimental Raman spectrum of B_4.3C is not yet understood: while most of the peaks have for long been explained by the low-energy theoretical ground state structure B₄C with 20% atomic carbon concentration [3], two large bands at low frequencies remain unexplained even when common point defects are introduced into the atomic structure [1,2]. Moreover, the behavior of the two bands under high pressure is uncommon [4]. Finding a theoretical explanation of this behavior certainly requires a detailed understanding of the underlying atomic structure. 

Beside the B₄C unit cell that consists of one  (B₁₁C^p ) icosahedron and one C-B-C chain, new building blocks such as C-B-C-B chains and B<B₂>B pantograph-like intericosahedral blocks have been identified with HRTEM [5]. Two crystalline unit-cells with either building blocks have been confirmed theoretically to be part of the B-C phase diagram at respectively 13.0% (B_6.7CC, named OPO₂) and 8.7% (B_10.5C, named OPO₁) atomic C concentrations [6].  In the present work, the unexplained Raman bands will be discussed in light of calculations under high pressure within the density functional perturbation theory for the lattice vibrational frequencies and with Raman intensityies computed with the 2^nd order response [7]. The respective roles of the building blocks and of disorder will be highlighted. 

    

Excellent thermal stability of silicon carbon under high pressure

A large volume cubic anvil press integrated with synchrotron energy-dispersive x-ray diffraction was employed to study the yielding behavior of powdered beta silicon carbide (SiC) under high pressure and high temperature conditions up to 7.4 GPa and 1400 °C. During compression and heating, the x-ray pattern was collected at each pressure–temperature point, and, then, via assessing the peak width of the x-ray diffraction pattern, the strains/stresses developed inside the sample under varied pressure–temperature conditions were determined. From the constitutive response of the sample as a function of pressure and temperature, we did not observe the yielding occurrence in SiC at cold compression. In contrast, high temperature induces a yielding at 1100 °C with a constant loading pressure of ∼7.4 GPa. By comparison, we found that this material is the most stable, compared with the other three strong ones (diamond, moissanite, and alfa silicon nitride), in terms of the yielding under high pressure and temperature conditions. Along with its much higher pressure and temperature requirements for phase transition and decomposition, SiC is a competent material for the development of novel tools/devices to be used in the harshly extreme working environment, such as deep drilling, high-speed cutting, and aerospace engineering.

    

Accurate Temperature Diagnostics for Matter under Extreme Conditions

The experimental investigation of matter under extreme densities and temperatures as they occur for example in astrophysical objects and nuclear fusion applications constitutes one of the most active frontiers at the interface of material science, plasma physics, and engineering. The central obstacle is given by the rigorous interpretation of the experimental results, as even the diagnosis of basic parameters like the temperature T is rendered highly difficult by the extreme conditions. In this work, we present a simple, approximation-free method to extract the temperature of arbitrarily complex materials from scattering experiments, without the need for any simulations or an explicit deconvolution. This new paradigm can be readily implemented at modern facilities and corresponding experiments will have a profound impact on our understanding of warm dense matter and beyond, and open up a gamut of appealing possibilities in the context of thermonuclear fusion, laboratory astrophysics, and related disciplines.   

Search for metastable A-site ordered perovskite-type ferrites by high-pressure synthesis assisted by first-principles calculations

A perovskite-type iron oxide SrFeO₃ with Fe⁴⁺ in an unusually high valence state has attracted much attention for its versatile helimagnetic phases [1], despite its simple cubic structure with inversion symmetry. Some of the helimagnetic phases are topologically nontrivial multi-q spin textures, which are represented by the superposition of several propagation vectors along the <111> equivalents of the cubic lattice. The number and direction of q can be modulated in various ways by controlling the electron filling [2], Fe-O bond length [3], and dimensionality of the crystal structure [4,5]. In this study, we explored new A-site ordered perovskite-type iron oxides with Fe⁴⁺ by high-pressure synthesis, aiming to modulate the versatile helimagnetic phases of SrFeO₃ by introducing weak anisotropy through A-site ordering.

Since the target materials to synthesize were new compounds without any previous reports, we evaluated the thermodynamic stability under ambient and high pressures. Crystal structures of each candidate were optimized based on DFT calculations, followed by the construction of convex hulls. Two types of new oxygen-deficient A-site ordered perovskites were predicted to be stabilized by applying pressure, and they were successfully obtained by high-pressure synthesis. We also attempted topotactic ozone oxidation to those oxygen-deficient compounds to realize Fe⁴⁺ states. Whereas the oxygen-deficient samples are likely to be antiferromagnetic insulators whose transition temperatures are above 300 K, a potentially novel helimagnetic phase is found far below 300 K in the compounds after ozone oxidation.

[1] S. Ishiwata et al., Phys. Rev. B 101, 134406 (2020).

[2] M. Onose et al., Phys. Rev. Mater. 4, 114420 (2020).

[3] N. Hayashi et al., J. Phys. Soc. Japan 82, 113702 (2013).

[4] J. -H. Kim et al., Phys. Rev. Lett. 113, 147206 (2014).

[5] P. Adler et al., Phys. Rev. B 105, 054417 (2022).   

High-Pressure Behavior Of Isotope Free Boron Arsenide (BAs): A Raman Spectroscopic And Photoluminescence Study

The high-pressure behavior of Boron Arsenide (¹⁰BAs) has been investigated using Raman and photoluminescence (PL) spectroscopy up to 17.26 GPa in a diamond anvil cell (DAC). The Raman spectrum at ambient pressure shows a peak at 726.85 cm ^-1 that corresponds to the vibrational mode of ¹⁰B. Under the application of pressure,¹⁰B exhibits a blueshift. As pressure increased from 8.4 GPa to 15.39 GPa, we observed LO/TO phonon splitting due to different phonon hardening rates. Pressure at 16.63 GPa and 17.26 GPa shows no peak at all. We propose that the flat straight Raman line at and after 16.63 GPa is due to an indirect band gap transition to a direct band gap. The signal at high pressure is washed out by fluorescence. High-pressure Raman spectra also suggest that as pressure increases, the conduction band minimum corresponding to the donor level decreases, affecting the emission of free carriers from defect states. The photoluminescence (PL) measurements show an indirect bandgap of about 1.72eV. The low-temperature PL measurements reveal the presence of carbon and silicon impurities, whereas high-pressure PL spectra display a redshift. Our work establishes a foundation for strain engineering to regulate strain in the transistor channel, enhancing the transport properties of ¹⁰BAs, and ultimately improving device performance.

    

Explaining Certain Elementary Particles (Mu-, Ka-, Sigma-, and Eta-) as Mixed Concepts: 2-Particle Sets Locked at (1d+2d) of Underlying Charge-Only by Mass-Values Cube-Root versus Mass-Units

Expanding on Elementary Particle Clustering data and evaluation by Mac Gregor at Livermore Labs, certain nucleus elementary particle mass-values cube-root become the combination of two core particles a) at different operating distances b) from my limited set of core particles (proton/neutron, Omega then with electron for QED).  For example, muon mass-value as a) for one particle at 1d and 2^nd particle at 2d at 1/d²) the net of Omega versus neutron.

This (r_e/a₀)^V/W universal scaling factor approach provides a geometric method to simplify the elementary particle map to the core four above as definable geometry of multiple core particles.  The R² correlation of mass-values and mass-units using this method is 0.9976.

As such, certain elementary particles need splitting as a mixed combination of two interacting at different position-in-fields.