The pressure-induced metallic amorphous state of SnI4. I. A novel crystal-to-amorphous transition studied by X-ray scattering

Melting Is Well-Known, but Is It Also Well-Understood?

Contrary to continuous phase transitions, where renormalization group theory provides a general framework, for discontinuous phase transitions such a framework seems to be absent. Although the thermodynamics of the latter type of transitions is well-known and requires input from two phases, for melting a variety of one-phase theories and models based on solids has been proposed, as a generally accepted theory for liquids is (yet) missing. Each theory or model deals with a specific mechanism using typically one of the various defects (vacancies, interstitials, dislocations, interstitialcies) present in solids. Furthermore, recognizing that surfaces are often present, one distinguishes between mechanical or bulk melting and thermodynamic or surface-mediated melting. After providing the necessary preliminaries, we discuss both types of melting in relation to the various defects. Thereafter we deal with the effect of pressure on the melting process, followed by a discussion along the line of type of materials. Subsequently, some other aspects and approaches are dealt with. An attempt to put melting in perspective concludes this review.

The microscopic transition process from high-density to low-density amorphous state of SnI4

A molecular crystalline SnI₄undergoes pressure-induced solid-state amorphization via molecular dissociation to the high-density amorphous (HDA) state, which we call Am-I. In the present study, we examine the reverse transition process from Am-I to the low-density amorphous (LDA) state, called Am-II. We first measure the structure factor on decompression from 30 GPa down to 1.1 GPa at room temperature, usingin situangle-dispersive synchrotron x-ray measurement and a diamond anvil cell. We then estimate the density, which exhibits an abrupt change between 3.3 and 3.0 GPa, indicating the HDA(Am-I)-to-LDA(Am-II) transition. We use the density and the molecular configuration generated from a molecular dynamics simulation as input to a reverse Monte Carlo fit. The fit vividly visualizes gradual molecular reassociation between 18 and 14 GPa within the Am-I region. The Am-I state can thus be divided into two states: the high-pressure Am-I state containing isolated Sn atoms and the low-pressure Am-I state consisting of deformed molecules connected by metallic I₂bonds. In the latter state, the molecular shape becomesC_3v-like just before the transition to Am-II, in which molecules recover the originalT_dsymmetry. This local symmetry change has been detected on the liquid-liquid transition of SnI₄, suggesting the strong coupling between the local symmetry and the global order parameter of density.

Do SnI4 molecules deform on heating and pressurization in the low-pressure crystalline phase?

A SnI₄ molecule lowers its symmetry from T _d to [Formula: see text] on the liquid-liquid transition. Because it is possible to lower the molecular symmetry without violating the crystalline symmetry, it is worth examining whether the deformation occurs in the crystalline phase field. Extended x-ray absorption fine structure (EXAFS) measurements on the crystalline state were carried out to investigate the change in the environment around a Sn atom at high pressures and temperatures. We could not find clear evidence on the symmetry change of molecules even close to the melting points, where the melting curve becomes abnormally flat against pressure. Indeed, no inconsistency was found when we assumed that the coordination number of a Sn atom remains unchanged in the temperature and pressure range examined. The situation remains true when the system entered the high-pressure crystalline phase on compression. We can propose a consistent scenario as to the structural change on the phase transformation. The incompressibility of a SnI₄ molecule could be suitably quantified. The procedure enabled us to conclude the molecule is more than an order of magnitude incompressible than the lattice.

Isotactic poly(4-methyl-1-pentene) melt as a porous liquid: Reduction of compressibility due to penetration of pressure medium

A pressure-induced structural change of a polymer isotactic poly(4-methyl-1-pentene) (P4MP1) in the melted state at 270 °C has been investigated by high-pressure in situ x-ray diffraction, where high pressures up to 1.8 kbar were applied using helium gas. The first sharp diffraction peak (FSDP) position of the melt shows a less pressure dependence than that of the normal compression using a solid pressure transmitting medium. The contraction using helium gas was about 10% at 2 kbar, smaller than about 20% at the same pressure using a solid medium. The result indicates that helium entered the interstitial space between the main chains. The helium/monomer molar ratio was estimated to be 0.3 at 2 kbar from the FSDP positions. These results suggest that the compressibility of the P4MP1 melt can be largely dependent on the pressure transmitting media. As the pore size is reversibly and continuously controllable by compression, we suggest that the P4MP1 melt can be an ideal porous liquid for investigating a novel mechanical response of the pores in a non-crystalline substance.

Reversible switching between pressure-induced amorphization and thermal-driven recrystallization in VO2(B) nanosheets

Pressure-induced amorphization (PIA) and thermal-driven recrystallization have been observed in many crystalline materials. However, controllable switching between PIA and a metastable phase has not been described yet, due to the challenge to establish feasible switching methods to control the pressure and temperature precisely. Here, we demonstrate a reversible switching between PIA and thermally-driven recrystallization of VO₂(B) nanosheets. Comprehensive in situ experiments are performed to establish the precise conditions of the reversible phase transformations, which are normally hindered but occur with stimuli beyond the energy barrier. Spectral evidence and theoretical calculations reveal the pressure–structure relationship and the role of flexible VO_x polyhedra in the structural switching process. Anomalous resistivity evolution and the participation of spin in the reversible phase transition are observed for the first time. Our findings have significant implications for the design of phase switching devices and the exploration of hidden amorphous materials.

Pressure can either make materials more disordered, like amorphization, or more thermodynamic stable, yet the switching between the two metastable phases has not been described. Wang et al. study it in vanadium oxide nanosheets and highlight the role played by spin and charge during the transition.

Characterization of melt-quenched and milled amorphous solids of gatifloxacin

The objectives of this study were to characterize and investigate the differences in amorphous states of gatifloxacin. We prepared two types of gatifloxacin amorphous solids coded as M and MQ using milling and melt-quenching methods, respectively. The amorphous solids were characterized via X-ray diffraction (XRD), nonisothermal differential scanning calorimetry (DSC) and time-resolved near-infrared (NIR) spectroscopy. Both the solids displayed halo XRD patterns, the characteristic of amorphous solids; however, in the non-isothermal DSC profiles, these amorphous solids were distinguished by their crystallization and melting temperatures. The Kissinger-Akahira-Sunose plots of non-isothermal crystallization temperatures at various heating rates indicated a lower activation energy of crystallization for the amorphous solid M than that of MQ. These results support the differentiation between two amorphous states with different physical and chemical properties.

Structural Memory in Pressure-Amorphized AlPO4

Molecular dynamics simulations reveal the mechanism of the structural memory effect in alpha-berlinite (AlPO(4)), where a single crystal transforms to an amorphous solid by compression and then reverts to the original crystalline structure upon release of pressure. The enhanced oxygen coordination around the aluminum atoms in AlPO(4) at high pressure leads to a mechanical instability that causes the phase transformation. The difference in the structural memory behavior between AlPO(4) and isostructural alpha-quartz, for which the pressure-induced amorphized phase is recoverable, can be attributed to the presence of the PO(4) units, which remain essentially four-coordinated even when severely distorted.

Deep-Focus Earthquakes and Recycling of Water into the Earth's Mantle

For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside Earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces. Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle. Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet.

Towards an Understanding of the Structurally Based Potential for Mechanically Activated Disordering of Small Molecule Organic Crystals

The potential for various small molecule organic crystals to undergo complete mechanically induced disordering is investigated. A model is proposed, which considers changes in free energy required for lattice incorporation of a critical dislocation density. Application requires knowledge of a few physical properties, namely the elastic shear modulus, Burgers vector magnitude, molar volume, melting temperature, and heat of fusion. The model was tested using seven compounds; acetaminophen, aspirin, gamma-indomethacin, salicylamide, sucrose, and two proprietary drug compounds, PFZ1 and PFZ2. Crystalline solids were subjected to high shear, controlled temperature comminution for various durations, after which the samples were examined using powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC). The results verified that acetaminophen, aspirin, and salicylamide, which were suggested by the model to be resistant to complete mechanical disordering, remained fully crystalline, even after 5 h of milling. Sucrose and gamma-indomethacin were both predicted to be susceptible to amorphization, which was confirmed by physical characterization. Single, 3-h grinding experiments were performed on two proprietary compounds, PFZ1 and PFZ2. The model indicated that each should be resistant to complete disordering, a trend held by PFZ1. Evidence of partial disordering of PFZ2 was unexpected and is discussed with respect to possible temperature effects.

Determination of low-pressure crystalline–liquid phase boundary of SnI4

The location of the liquidus in the low-pressure crystalline phase of SnI(4) was determined utilizing in situ x-ray diffraction measurements under pressures up to approximately 3.5 GPa. The liquidus is not well fitted to a monotonically increasing curve such as Simon's equation, but breaks near 1.5 GPa and then becomes almost flat. The results are compared to those from molecular dynamics simulations. Ways to improve the model potential adopted in the simulations are discussed.

Effects of high pressure on molecules

Recent high-pressure studies reveal a wealth of new information about the behavior of molecular materials subjected to pressures well into the multimegabar range (several hundred gigapascal), corresponding to compressions in excess of an order of magnitude. Under such conditions, bonding patterns established for molecular systems near ambient conditions change dramatically, causing profound effects on numerous physical and chemical properties and leading to the formation of new classes of materials. Representative systems are examined to illustrate key phenomena, including the evolution of structure and bonding with compression; pressure-induced phase transitions and chemical reactions; pressure-tuning of vibrational dynamics, quantum effects, and excited electronic states; and novel states of electronic and magnetic order. Examples are taken from simple elemental molecules (e.g. homonuclear diatomics), simple heteronuclear species, hydrogen-bonded systems (including H2O), simple molecular mixtures, and selected larger, more complex molecules. There are many implications that span the sciences.

Pressure-Induced Amorphization of R -Al 5 Li 3 Cu: A Structural Relation Among Amorphous Metals, Quasi-Crystals, and Curved Space

A central question in the study of amorphous materials is the extent to which they are ordered. When the crystalline intermetallic R-Al(5)Li(3)Cu is compressed to 23.2 gigapascals at ambient temperature, an amorphous phase is produced whose order can be described as defects in a curved-space crystal. This result supports a structural relation between quasi-crystals and amorphous metals based on icosahedral ordering. This result also shows that a metallic crystal can be made amorphous by compression.

Memory Glass: An Amorphous Material Formed from AlPO4

A glass exhibiting structural memory has been produced through the compression of a single crystal of AlPO(4) berlinite to 18 gigapascals at 300 kelvin. The unique and extraordinary characteristic of this glass is that upon decompression below 5 gigapascals it transforms back into a single crystal with the same orientation as the starting crystal. This glass has a "memory" of the previous crystallographic orientation of the crystal from which it forms.