Session H38: Materials in Extremes: Phase Transitions II

Metastable, Unstable and ground-state crystal structure of lithium

Lattice of light atomic nuclei remains restless at low temperature due to their quantum mechanical zero point energy. Combination of a quantum lattice and a free electron system is predicted to have exotic properties. Lithium is the lightest metal and has a nearly free electron system and therefore has been studied extensively for evidence of quantum mechanical contribution to its lattice. Here we present a detailed structural study of two lithium isotopes and present evidence that the thermodynamic ground state of the lithium lattice is face-centered-cubic (fcc). However, the known ambient pressure martensitic transition of body-centered-cubic (bcc) to 9R in Li, is a kinetically favored transition that prevails the transition to fcc structure. Most surprisingly, we find that unlike 7Li, 6Li does not show any evidence of a martensitic transition at low pressure and temperature (T ~ 20 K) and remains in bcc structure due to its lattice quantum-dynamics.   

Probing quantum effects in lithium

In periodic table lithium is the first element innediately after helium and the lightest metal. While fascinating quantum nature of condensed helium is suppressed at high densities, lithium is expected to adape more quantum solid behavior under compression. Physics of dense lithium offers a rich playgroumd to look for new emergent quantum phenomena in condensed matter. We will display recent progress in studying the quantum nature of dense lithium.   

B1-B2 phase transition of MgO by ab-initio molecular dynamics

Usually, thermal vibrations of atoms can be taken into account via the so-called quasiharmonic approximation (QHA). In this framework, the phonon dispersion relations are calculated at 0 K using density functional perturbation theory (DFPT) and the temperature is included only via the thermal dilatation, i.e., by computing the phonon spectrum at different volumes. The main drawback of this approximation is that its accuracy is difficult to assess since anharmonic contributions beyond quasiharmonicity are difficult to estimate. But if the harmonic part of the atomic vibrations dominate the free energy at elevated temperatures, the neglect or an inaccurate evaluation of seemingly minor contribution as the anharmonic one can result in falsely predicted phase stabilities or inaccurate phase transition temperatures.
Here we will present a recent method, called the temperature dependent energy potential (TDEP) that provides a consistent way to extract the best possible harmonic (or higher order) potential energy surface at finite temperatures. Then we will show some results on the phase transition between the B1 and B2 structures in MgO.   

Phase transition between trivial and non-trivial states in BiTeBr under high pressure

We report the topological phase transition at ~3 GPa in BiTeBr. The transition from a trivial state to a non-trivial one in BiTeI was theoretically predicted at a few gigapascals under hydrostatic condition. Both the layered structure with a symmetry of P3m1 and the electronic state of BiTeBr are similar to those of BiTeI. This transition was therefore also expected to occur in BiTeBr. We investigated the transition in combination with experiments up to 5 GPa (XRD, resistivity) under hydrostatic condition and the first-principles calculations utilizing experimental structural parameters. The P3m1 structure remains stable up to pressures of 5 GPa with a minimum of the ratio of lattice constants at 2.5-3 GPa. In the same range, the temperature dependence of resistivity changes from metallic to semiconducting at 3 GPa and has a plateau region between 50 and 150 K in the semiconducting state. Meanwhile, the pressure variation of band structure shows that the bulk band-gap energy closes at 2.9 GPa and re-opens at higher pressures. Furthermore, Wilson loop analysis revealed that the topological nature of electronic states at 0 and 5 GPa are trivial and non-trivial, respectively. These results suggest that topological phase transition in BiTeBr occurs at 2.9 GPa.   

In-situ x-ray diffraction of SiO2 and TiO2 under shock compression

SiO₂ and TiO₂ are known to undergo phase transitions under shock compression. The nature of these transitions and the resulting high-pressure phases are of fundamental interest for understanding the dynamic response of minerals, for characterizing natural impact sites, and for understanding high-pressure phases of planetary interiors. In this study, in situ x-ray diffraction experiments on shock-compressed SiO₂ and TiO₂ were conducted at the Dynamic Compression Sector of the Advanced Photon Source. The lattice-level structure was investigated through time-resolved x-ray diffraction measurements on samples reaching a peak stress ranging from 12 to 70 GPa. For SiO₂, we have examined natural low-porosity polycrystals as well as fused silica. For TiO₂, we have studied single crystals in different orientations. Our results provide direct constraints on the nature of the high-pressure phases formed under shock compression in these materials.   

Measurements of High Strain Rate Rayleigh-Taylor Growth in Solid Pb at the National Ignition Facility

We report on recent National Ignition Facility experiments investigating high pressure (~3.5 Mbar), high strain rate (> 10⁶ s^-1) deformation of Pb. Face-on X-ray radiography is used to monitor the growth of pre-formed sinusoidal ripples, which seed Rayleigh-Taylor growth as the driven material flows. Our samples are driven by a ramped pressure wave that is generated by an expanding plasma drive from a low-density foam reservoir that achieves these high pressures while maintaining Pb in the solid state. Pb makes an interesting study case at it transforms from the ambient face-centered-cubic to the body-centered-cubic phase at the conditions in our experiments. Phase transformations are expected to influence material strength and could contribute to the high strength seen in high strain rate Fe using multiple platforms [1-2]. The dynamic strength of Pb is inferred by comparing our data to hydrodynamics simulations that include strength models, including a newly developed Improved-Steinberg-Guinan model for body-centered-cubic Pb [3].

[1] CM Huntington et al., APS SCCM Proc., 2015
[2] Y Ping et al. PRL 111, 065501 (2013)
[3] Robert E. Rudd et al., APS SCCM Proc., 2017.   

Dynamic X-ray Diffraction to examine the Shock-Induced Phase Transitions and Melting in Cerium Metal

There is a scientific need to understand the dynamic response of materials at extreme conditions which is relevant for the understanding of phenomena related to condensed matter physics, general solid flow behavior, and planetary science. Dynamic experiments are needed to locate phase boundaries, to obtain equation-of-state information on pure phases, and to understand the evolution of the microstructure during loading. Cerium is ideal for such studies because it exhibits a rich phase diagram that includes four solid phase at zero pressure, additional solid phases at high pressure, and an anomalous melt boundary. Of particular interest to the current work is the well-known isostructural (γ-α) phase transition that occurs at low-pressures. This boundary terminates in the solid phase at a critical point and is accompanied by a large volume collapse that leads to a low-pressure melt transition at approximately 10.2 GPa. The current work takes advantage of this low-pressure melt transition to use dynamic X-ray diffraction to study shock melting of metals. XRD data were obtained for impact stresses that spanned the melt transition which provided information on the high pressure phase pointing toward a complete melt transition near 16 GPa.   

High pressure and temperature phases of magnesium

Understanding the high-pressure behavior of Mg is important of the role that it plays in planetary science. Computational studies have also predicted exotic, electride-like solid structures of compressed Mg. Using first-principles theory, we have investigated the high-pressure phases of Mg, both solid and liquid. New findings for its finite-temperature stability and melting curve will be reported and explained. The implications of these results for other elements will be briefly discussed.   

Quantitative insights into the mechanisms of nucleation during crystallization

Nucleation during first order phase transitions, such as a lake freezing in winter, formation of snowflakes, and bio-mineralization, occur ubiquitously in nature. Traditionally nucleation is understood in terms of the classical nucleation theory (CNT) in which the rate of nucleation is given by J = Bexp(−G/kT), where B is a pre-exponential factor and G is the barrier to form a nucleus of critical size which depends on the interfacial free energy γ and the chemical potential difference between the solid and liquid. Many factors that control J are determined by a material's electronic structure. However, experimental evidence suggests that depending on growth conditions (i.e. extrinsic factors such as temperature, pressure, chemistry of solution, and concentration of solute) nucleation can involve kinetically favored pathways that deviate from homogeneous nucleation in CNT. For instance, crystallization can involve the formation of crystallites (i) with lower γ and lattice symmetries that are different from the final crystallite (e.g. two-step nucleation), or (ii) by the coalescence of smaller crystallites by oriented attachment (e.g. crystal growth by particle attachment). Thus, in practice the predicted nucleation rates are usually off by orders of magnitude. Our ability to predict crystallization therefore hinges on how various kinetic pathways are affected by growth factors. To this end, I will present new insights on two aspects of nucleation under highly driven conditions; (a) polymorph selection and (b) the competition between diffusion and dislocation activity during crystal growth by particle attachment. I will illustrate how these evidences challenge the traditional interpretations of how crystals nucleate and growth.   

Nanosecond Freezing of Water at the Limits of Metastability and Classical Nucleation Theory

The importance and ubiquity of water is universally recognized. The phase diagram of water, which encapsulates its equilibrium behavior, is still not firmly established. Understanding its non-equilibrium behavior presents an even greater challenge. One of the most familiar non-equilibrium processes is freezing, and of particular interest is freezing at pressures above 2 GPa. These are conditions found in the icy sheets of some Jovian moons, as well as extrasolar super-Earths. Several studies have performed dynamic compression experiments to drive water well into the ice VII region of the phase diagram. The studies report a number of findings, including an apparent limit of metastability at around 7 GPa, where water undergoes homogeneous nucleation and freezes to ice VII on a nanosecond timescale. Here we show how a theoretical/computational modeling framework based on classical nucleation theory can provide both a qualitative and quantitative explanation for many of these observations. This is the first demonstration that such a framework can accurately capture nanosecond freezing kinetics under high-pressure dynamic compression.   

New Phase Diagram of Water under Negative Pressure: Rise of The Lowest-Density Ice Clathrates

Using Monte Carlo packing algorithm and dispersion-corrected density functional theory, we predict a crystalline clathrate of cubic structure III (s-III) with mass density of 0.593 g/cm³ composed of two large icosihexahedral cavities (8⁶6⁸4¹²) and six small decahedral cavities (8²4⁸) per unit cell [Sci. Adv. 2016, 2, e1501010]. A new phase diagram of water ice with TIP4P/2005 model potential is constructed, where the s-III clathrate emerges as the most stable ice polymorph in the pressure region below −5834 bar at 0 K and below −3411 bar at 300 K. Inspired by this discovery, another cubic crystalline phase of s-IV with even lower density of 0.506 g/cm³ is found a stable phase in the phase diagram. Given that s-III and s-IV can be related to the silica analog of ROH and FAU, respectively, we explore all the possible clathrates from database of zeolite structures and find that a FAU ice clathrate with ultralow density (0.5 g/cm³) in the phase diagram. Finally, the new phase diagram under negative pressure is updated. With pressure decreasing, ice Ih emerges firstly, then s-II ice clathrate dominates. Below s-II, s-IV arises at low temperature and FAU dominates at high temperature.