Session F23: Matter at Extreme Conditions I

Ammonium Azide under High Pressure -- a combined Theoretical and Experimental Study

Efforts to synthesize, characterize and recover novel polynitrogen energetic materials have driven attempts to subject high nitrogen content precursor materials (in particular metal and non-metal azides) to elevated pressures. Here we present a combined theoretical and experimental study of the high pressure behavior of ammonium azide (NH$_{4}$N$_{3})$. Using density functional theory we have considered the relative thermodynamic stability of the material with respect to two other crystal phases, namely trans-tetrazene (TTZ), and also a novel hydronitrogen solid (HNS) of the form (NH)$_{4}$, that was recently predicted to become relatively stable under high pressure. Experimentally we have measured the Raman spectra of NH$_{4}$N$_{3}$ up to 71 GPa at room temperature. Our calculations demonstrate that the HNS becomes stable only at pressures much higher (89.4 GPa) than previously predicted (36 GPa). Our Raman spectra are consistent with earlier reports up to lower pressures, and at higher pressures, while some additional subtle behavior is observed (e.g. mode splitting) there is again no evidence of a phase transition to either TTZ or the HNS.   

Calculations and experimental studies of bis-triaminoguanidium azotetrazolate (TAGzT) under high pressure

Nitrogen-rich organic compounds may offer distinct advantages over conventional energetic materials for applications relating to gas generators, low-signature propellants, and additives to pyrotechnics and explosives. We have performed plane-wave, density functional theory calculations of TAGzT, an energetic, nitrogen-rich salt, up to 40 GPa, and report the pressure dependences of polarizability, x-ray diffraction patterns, and dipole moments. These results are compared to those we obtain experimentally from Raman Spectroscopy,$^{\mathrm{1}}$ and x-ray diffraction analysis and infrared spectroscopy. Our results suggest TAGzT does not undergo any phase transitions within this pressure range. Mulliken and Hirshfeld population analysis of TAGzT at ambient and high pressure yields the change of charge distribution with an increase in pressure. We report and discuss this trend at the meeting. Also, we report trends in the pressure-induced modifications of both bond lengths and angles of TAGzT, and reveal how hydrogen bonding contributes to the stability of TAGzT under pressure. $^{\mathrm{1}}$K.D. Behler, J.A. Ciezak-Jenkins, R.C. Sausa, J. Phys. Chem A. 117(8), 1737 (2013)   

Many-body Green's function calculations of optical properties of LiF in extreme conditions

We present Density Functional Theory (DFT) + quasiparticle ($G_0W_0$) + Bethe-Salpeter calculations of the real and imaginary parts of the long-wavelength dielectric function of LiF between ambient pressure and $P$= 5 Mbars. While the optical absorption spectrum is predicted to show dramatic pressure-dependent features above the optical gap, the index of refraction well below the gap is shown to exhibit the same trends as that seen in both DFT calculations and experiment: a linear increase with $P$. This increase does not result from a decrease in the band gap, but rather follows from the increase in oscillator strength which counteracts a smaller increase in band gap with $P$. Our calculations also suggest that the index of refraction (for visible and near-UV light) of the higher-$T$ B2-phase should be close enough to that of the B1 (ambient crystalline) phase that a transition from B1 to B2 is not likely to present a substantial change in index. These findings may be of interest to researchers who use LiF as a window material in dynamic compression experiments.   

Strongly correlated valence electrons and core-level chemical bonding of Lithium at terapascal pressures

As the simplest pure metal, lithium exhibits some novel properties on electrical conductivity and crystal structures under high pressure. All-electron density functional theory simulations, recently developed by using the linear combination of localized Slater atomic orbitals, revealed that the bandwidth of its valence bands remains almost unchanged within about 3.5 eV even up to a terapascal pressure range. This indicates that the development from delocalized to strongly correlated electronic systems takes place under compression, resulting in metal-semiconductor and superconductivity transitions together with a sequence of new high-pressure crystal phases, discovered experimentally. In contrast to the valence bands, the core-level bands become broadening up to about 10 eV at terapascal pressures. It means the transformation from chemical non-bonding to bonding for core electrons. Thus, dense lithium under compression can be characterized as core-level chemical bonding and a completely new class of strongly correlated materials with narrow bands filled in s-electron shells only.   

Equation of state, thermodynamic and transport properties in liquid indium and iron under high pressure

We apply a general equation of state of liquid [1] to study thermodynamic properties in liquid indium and iron under high temperature and high pressure. In particular, density, isothermal bulk modulus, and internal pressure are then analyzed in liquid indium and iron. Molar volume of molten indium is calculated along the isothermal line within good precision comparing with the known data from experiments in an externally heated diamond anvil cell. For liquid indium at given parameters, density anomaly, i.e. the minimum volume, is observed along certain isobaric paths. In liquid iron, the entropy scaling law of self --diffusion coefficient and viscosity under high pressure (up to 350GPa) and high temperature (up to 8000K) are investigated. Comparisons are made with experimental data and other EOS models for liquid iron. \\[4pt] [1] V. G. Baonza , M. Caceres and J. Nunez, Phys. Rev. B 51, 28(1995).   

Stable Xenon Nitride at High Pressures

Nitrogen is the most abundant element on Earth and exists as inert N$_{2}$ molecules in the atmosphere. Noble gas nitrides are missing in nature because N$_{2}$ molecules do not interact with noble gases at ambient conditions, greatly impeding the understanding of physics and chemistry of such nitrides. We report here a pressure-induced chemical reaction of N$_{2}$ with xenon predicted using a swarm-structure searching calculation as implemented in the CALYPSO code [1-2]. This reaction leads to the formation of a hitherto unexpected Xe nitride at megabar pressure accessible to high-pressure experiments. The high-pressure phase with a hypervalent state of Xe by accepting unprecedented Xe-N covalent bonds appears to be the most stable stoichiometry. The Xe bonding situation in this new phase is substantially different from earlier high-pressure examples of ionic Xe bonding or van der Waals interactions. \\[4pt] [1] Wang, Y., Lv, J., Zhu, L. {\&} Ma, Y. Crystal structure prediction via particle-swarm optimization. Phys. Rev. B 82, 094116 (2010).\\[0pt] [2] Wang, Y., Lv, J., Zhu, L. {\&} Ma, Y. CALYPSO: A method for crystal structure prediction. Comput. Phys. Commun. 183, 2063--2070 (2012).   

The phase diagram of solid hydrogen at high pressure: A challenge for first principles calculations

We present comprehensive results for the high-pressure phase diagram of solid hydrogen. We focus on the energetically most favorable molecular and atomic crystal structures. To obtain the ground-state static enthalpy and phase diagram, we use semi-local and hybrid density functional theory (DFT) as well as diffusion quantum Monte Carlo (DMC) methods. The closure of the band gap with increasing pressure is investigated utilizing quasi-particle many-body calculations within the GW approximation. The dynamical phase diagram is calculated by adding proton zero-point energies (ZPE) to static enthalpies. Density functional perturbation theory is employed to calculate the proton ZPE and the infra-red and Raman spectra. Our results clearly demonstrate the failure of DFT-based methods to provide an accurate static phase diagram, especially when comparing insulating and metallic phases. Our dynamical phase diagram obtained using fully many-body DMC calculations shows that the molecular-to-atomic phase transition happens at the experimentally accessible pressure of 374 GPa. We claim that going beyond mean-field schemes to obtain derivatives of the total energy and optimize crystal structures at the many-body level is crucial.   

Novel chemistry of matter under high pressure

The periodicity of the elements and the non-reactivity of the inner-shell electrons are two related principles of chemistry, rooted in the atomic shell structure. Within compounds, Group I elements, for example, invariably assume the $+$1 oxidation state, and their chemical properties differ completely from those of the p-block elements. These general rules govern our understanding of chemical structures and reactions. Using first principles calculations, we demonstrate that under high pressure, the above doctrines can be broken. We show that both the inner shell electrons [1] and the outer shell empty orbitals [2] of Cs and other elements can involve in chemical reactions. Furthermore, we show that the quantized orbitals of the enclosed interstitial space may play the same role as atomic orbitals, an unprecedented view that led us to a unified theory for the recently observed high-pressure electride phenomenon [3]. [1] M. S. Miao, Nature Chemistry, 5, 846 (2013). [2] J. Botana and M. S. Miao, Nature Communications, 5, 4861 (2014). [3] M. S. Miao and R. Hoffmann, Accounts of Chemical Research, 47, 1311 (2014).   

Multi-center semi-empirical quantum models for carbon under extreme thermodynamic conditions

We report on the development of many-body density functional tight binding (DFTB) models for carbon accurate over thousands of GPa and tens of thousands of Kelvin. DFTB holds promise as a fast quantum simulation approach that can yield several orders of magnitude increase in computational efficiency over Kohn-Sham Density Functional Theory (DFT) while retaining most of its accuracy. However, standard DFTB can yield large errors for materials under high pressures and temperatures, where electrons can become delocalized. Here, we overcome these limitations by computing the environmental dependence of the DFTB Hamiltonian matrix elements directly from DFT. We include these results in DFTB calculations by either explicitly calculating three-center terms in the Hamiltonian, or by implicitly incorporating them in the diagonal matrix elements. We then determine a three-body repulsive energy for the implicit approach, which yields accurate equation of state and structural properties for both solid and metallic liquid states of carbon. Our new models exhibit a straightforward method by which many-body effects can be included in DFTB, thus extending it to the time scales of current compression experiments, where physical and chemical properties can be difficult to interrogate directly.   

Decomposition reactions in RDX at elevated temperatures and pressures

Mechanisms and rates of elementary reactions controlling condensed-phase decomposition of RDX under elevated temperatures (up to 2000 K) and pressures (up to a few GPa) are not known. Global decomposition kinetics in RDX below 700 K has been measured; however, the observed global pathways result from complex manifolds of elementary reactions and are likely to be altered by elevated temperatures. Elevated pressures can further affect the condensed-phase kinetics and compete with elevated temperatures in promoting some elementary reactions and suppressing others. This presentation will describe density functional theory (DFT) based molecular dynamics simulations of crystalline and molten RDX aimed to delineate the effects of elevated temperatures and pressures on the mechanism of initial dissociation and the resulting secondary reactions.   

Atomistic picture of the shock to deflagration transition in a solid explosive: ultra-fast chemistry under non-equilibrium

We use large-scale molecular dynamics (MD) simulations to describe the chemical reactions following the shock-induced collapse of cylindrical pores in the high-energy density material RDX. For shocks with particle velocities of 2km/s, we find that the collapse of a 40 nm diameter pore leads to a deflagration wave, resulting in the first atomic-level description of this process. Pore collapse leads to ultra-fast, multi-step chemical reactions that occur under non-equilibrium conditions. The formation of exothermic product molecules within a few picoseconds of the pore collapse prevents the nanoscale hot spot from quenching. Within 30 ps, a local deflagration wave develops which propagates at speeds of $\sim$ 0.25 km/s and consists of an ultra-thin reaction zone of only $\sim$ 5 nm, thus involving large temperature and composition gradients. These results provide insight into the initiation of detonation, which is critical to understanding the performance and safety of this class of materials.   

Electride-like phases at extreme compression: towards bridging the gap between theory and experiment

The transformation of materials into electride-like structures under the application of extreme pressure has attracted a lot of interest recently. Theoretical studies have predicted the existence of low-coordinated crystal phases, where the conduction electrons are localized in the interstitial atomic regions, for a number of elements at high density. Most of these works have been limited to static lattice calculations. The pressures where such transformations are projected to occur are accessible in dynamically-driven experiments, but at elevated temperatures. In this talk I will discuss the temperature dependence of electride structures, both solids and liquids, as well as the requirements for their accurate simulation.   

The effect of core level crossing on the high-pressure equation of state of osmium

The equation of state of the 5d transition metal osmium has been studied with a combination of experiment and theory at pressures up to 500 GPa. The experimental results show a c/a ratio increasing by approximately 1 percent over this pressure range and displaying anomalies at pressures near 180 GPa and near 400 GPa. We have use all-electron fully relativistic density functional theory (DFT) calculations to study the cold equation of state and structural parameters of osmium at pressures up to 500 GPa, using one LDA and two GGA functionals. The increase in the c/a ratio agrees well with experiment, and we find anomalies, although less extreme, near the experimentally observed pressures. We find that the high pressure anomaly coincides with the crossing and hybridization of the 4f(7/2) and 5p(3/2) semi-core levels. In this talk we discuss the theoretical results and methodology and the possible implication for the equations of state of the 5d transition and actinide metals.   

Dynamic compression experiments and first-principles simulations on liquid deuterium above the melt boundary to investigate the insulator-to-metal transition

Important phenomena at high pressure, for example in planetary science, occur at conditions that cannot be reached in shock impact experiments. Different techniques have therefore been developed at Sandia's Z-machine. One new approach is shock-ramp loading. The accelerator delivers a two-step current pulse that accelerates the electrode, creating a well-defined shock, and subsequently produces ramp compression from the shocked state. The technique makes it possible to achieve cool (1000-2000 K), high pressure (above 300 GPa), high compression states (10-15 fold) in hydrogen, thus allowing experimental access to the region of phase space where hydrogen is predicted to undergo a first-order phase transition from an insulating molecular liquid to a conducting atomic fluid. Knowing the behavior of hydrogen under these conditions is of pivotal importance to understanding the physics of giant planets. We will survey theoretical predictions for the liquid-liquid insulator-to-metal transition in hydrogen and present the results of experiments on Z.