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Register a new userPolymorphism of superionic ice
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Added by
Alexander Goncharov
Publication date
2020
fields
Physics
and research's language is
English
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Water is abundant in natural environments but the form it resides in planetary interiors remains uncertain. We report combined synchrotron X-ray diffraction and optical spectroscopy measurements of H2O in the laser-heated diamond anvil cell up to 150 gigapascals (GPa) and 6500 kelvin (K) that reveal first-order transitions to ices with body-centered cubic (bcc) and face-centered cubic (fcc) oxygen lattices above 900 (1300) K and 20 (29) GPa, respectively. We assigned these structures to theoretically predicted superionic phases based on the distinct density, increased optical conductivity, and greatly decreased enthalpies of fusion. Our measurements address current discrepancies between theoretical predictions and various static/dynamic experiments on the existence and location of melting curve and superionic phase(s) in the pressure-temperature phase diagram indicating a possible presence of the conducting fcc-superionic phase in water-rich giant planets, such as Neptune and Uranus.
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Using direct atomic simulations, the vibration scattering time scales are characterized, and then the nature and the quantitative weight of thermal excitations are investigated in an example system Li2S from its amorphous solid state to its partial-solid partial-liquid and, liquid states. For the amorphous solid state at 300 K, the vibration scattering time ranges a few femtoseconds to several picoseconds. As a result, both the progagons and diffusons are the main heat carriers and contribute largely to the total thermal conductivity. The enhancement of scattering among vibrations and between vibrations and free ions flow due to the increase of temperature, will lead to a large reduction of the scattering time scale and the acoustic vibrational thermal conductivity, i.e., 0.8 W/mK at 300 K to 0.56 W/mK in the partial solid partial liquid Li2S at 700 K. In this latter state, the thermal conductivity contributed by convection increases to the half of the total, as a result of the usually neglected cross-correlation between the virial term and the free ions flow. The vibrational scattering time can be as large as ~ 1.5 picoseconds yet, and the vibrational conductivity is reduced to a still significant 0.42 W/mK highlighting the unexpected role of acoustic transverse and longitudinal vibrations in liquid Li2S at 1100 K. At this same temperature, the convection heat transfer takes overreaching 0.63 W/mK. Our study provides a fundamental understanding of the thermal excitations at play in amorphous materials from solid to liquid.
Solid-state materials with high ionic conduction are necessary to many technologies including all-solid-state Li-ion batteries. Understanding how crystal structure dictates ionic diffusion is at the root of the development of fast ionic conductors. Here, we show that LiTi2(PS4)3 exhibits a Li-ion diffusion coefficient about an order of magnitude higher than current state-of-the-art lithium superionic conductors. We rationalize this observation by the unusual crystal structure of LiTi2(PS4)3 which offers no regular tetrahedral or octahedral sites for lithium to favorably occupy. This creates a smooth, frustrated energy landscape resembling more the energy landscapes present in liquids than in typical solids. This frustrated energy landscape leads to a high diffusion coefficient combining low activation energy with a high pre-factor.
Superionic hydrogen was previously thought to be an exotic state predicted and confirmed only in pure H2O ice. In Earths deep interior, H2O exists in the form of O-H groups in ultra-dense hydrous minerals, which have been proved to be stable even at the conditions of the core-mantle boundary (CMB). However, the superionic states of these hydrous minerals at high P-T have not been investigated. Using first-principles calculations, we found that pyrite structured FeO2Hx (0 <= x <= 1) and d-AlOOH, which have been proposed to be major hydrogen-bearing phases in the deep lower mantle (DLM), contain superionic hydrogen at high P-T conditions. Our observations indicate a universal pathway of the hydroxyl O-H at low pressure transforming to symmetrical O-H-O bonding at high-P low-T, and a superionic state at high-P high-T. The superionicity of hydrous minerals has a major impact on the electrical conductivity and hydrogen transportation behaviors of Earths lower mantle as well as the CMB.
We investigate the high-pressure behaviour of beryllium, magnesium and calcium difluorides using ab initio random structure searching and density functional theory (DFT) calculations, over the pressure range 0-70 GPa. Beryllium fluoride exhibits extensive polymorphism at low pressures, and we find two new phases for this compound - the silica moganite and CaCl2 structures - which are stable over the wide pressure range 12-57 GPa. For magnesium fluoride, our searching results show that the orthorhombic `O-I TiO2 structure (Pbca, Z=8) is stable for this compound between 40 and 44 GPa. Our searches find no new phases at the static-lattice level for calcium difluoride between 0 and 70 GPa; however, a phase with P62m symmetry is close to stability over this pressure range, and our calculations predict that this phase is stabilised at high temperature. The P62m structure exhibits an unstable phonon mode at large volumes which may signal a transition to a superionic state at high temperatures. The Group-II difluorides are isoelectronic to a number of other AB2-type compounds such as SiO2 and TiO2, and we discuss our results in light of these similarities.
Polymorphism, which describes the occurrence of different lattice structures in a crystalline material, is a critical phenomenon in material science and condensed matter physics. It has emerged as a major focus for industry and regulatory agencies respectively. Thermomicroscopy, infrared spectroscopy and thermal analysis, especially differential scanning calorimetry (DSC) is used to characterize polymorphism to provide a powerful to isolate and identify of crystalline modification. Enantiotropic and monotropic with reversible endothermic and irreversible exothermic phase transition is representative classifications of polymorphism. Recently, Dirac metamaterial based on pyrene derivatives is attracting great attention. It succeeded in forming a periodic and regular structure using the unique {pi}-{pi} interaction of the pyrene derivative, namely HYLION-12. The phase transition between modifications is not classified into the existing polymorphism system. Here, we propose a new kind of polymorphism by identifying and analyzing thermodynamic functions such as heat capacity, enthalpy, entropy and, Gibbs free energy between modifications from DSC. This not only allows us to better understand the formation of Dirac materials at the molecular level, but also to think about the condition for new types of polymorphism.
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