Polyhydric alcohols under high pressure: comparative ultrasonic study of elastic properties.

We carried out an experimental ultrasonic study of polyhydric alcohols with the general chemical formula CH(OH) with an increasing number of OH groups: glycerol ( = 3), erythritol ( = 4), xylitol ( = 5), sorbitol ( = 6). The baric and temperature dependences of the elastic characteristics of these substances in the crystalline and glassy states were studied both under isothermal compression up to 1 GPa and during the isobaric heating of 77-295 K. For glycerol, glasses were obtained at different cooling rates, glass-liquid transitions were studied at different pressures.

Remarkably Stable Glassy GeS Densified at 8.3 GPa: Hidden Polyamorphism, Contrasting Optical Properties, Raman and DFT Studies, and Advanced Applications.

Glassy GeS, densified at 8.3 GPa, exhibits a strongly reduced bandgap, predominantly tetrahedral Ge environment, enhanced chemical disorder and partial 3-fold coordination of both germanium and sulfur, assuming two possible reaction paths under high pressure: (i) a simple dissociation 2Ge-S ⇄ Ge-Ge + S-S and (ii) a chemical disproportionation GeS ⇄ GeS + S. The observed electronic and structural changes remain intact for at least seven years under ambient conditions but are gradually evolving upon heating.

High-Pressure Synthesis of Cubic ZnO and Its Solid Solutions with MgO Doped with Li, Na, and K.

The possibility of doping ZnO in its metastable rock salt structure with Li, Na, and K intended to act as acceptor dopants was investigated. For the first time, MgZnO alloys and pure ZnO with a rock salt structure doped with Li, Na, and K metals was obtained by high-pressure synthesis from pure oxides with the addition of carbonates or acetates of the corresponding metals as dopant sources. Successful stabilization of the metastable rock salt structure and phase purity were confirmed by X-ray diffraction.

Benzophenone glass, supercooled liquid, and crystals: elastic properties and phase transitions.

The elastic properties of glass α- and β-modifications of benzophenone (CH)CO are determined for the first time by the ultrasonic method at high pressures up to 1 GPa and in the temperature range of 77 K< < 293 K. Four states of benzophenone are experimentally observed in the investigated temperature range of 77-293 K: glass, supercooled liquid, and α- and β-crystalline phases. The boundaries of phase transitions during isobaric heating are determined.

Transient Mesoscopic Immiscibility, Viscosity Anomaly, and High Internal Pressure at the Semiconductor-Metal Transition in Liquid GaTe.

Gallium tellurides appear to be promising phase-change materials (PCMs) of the next generation for brain-inspired computing and reconfigurable optical metasurfaces. They are different from the benchmark PCMs because of sp gallium hybridization in both cubic GaTe and amorphous pulsed laser deposition (PLD) films. Liquid GaTe also shows a viscosity η() anomaly just above melting when η() first and only then starts decreasing.

Ultrasonic study of 1-X adamantane (X = F, Cl, Br) compounds at high pressure and at order-disorder transitions.

We present an ultrasonic study of the elastic properties of 1-X adamantane (X = F, Cl, Br) during order-disorder and order-quasi-order transitions at various temperatures (77-305 K) and high pressures (up to 1 GPa). On the basis of our ultrasonic experiments, we studied for the first time the high-temperature (HT) 3, medium-temperature (MT) 4/, and low-temperature (LT) 42 phases of 1-fluoroadamantane at high pressures. The elastic properties of these phases at pressures up to 1 GPa at = 293 and 77 K, as well as at isobaric heating from 77 to 293 K, have been determined.

Phase transitions in 1-bromoadamantane compared to 1-chloroadamantane: similarities and unique features.

The elastic properties of 1-chloroadamantane and 1-Bromoadamantane in order-disorder and order-quasi-order phase transitions at temperatures in the range of 77-305 K and high pressures up to 1.1 GPa are studied by the ultrasonic method. The elastic moduli of halogenated adamantanes clearly indicate these transitions, demonstrating high capabilities of the ultrasonic method.

Comparative study of the elastic properties of adamantane and 1-chloroadamantane at high pressure and different temperatures and at order-disorder transitions.

We present a comparative ultrasonic study of the elastic properties of adamantane and 1-chloroadamantane at high pressure (up to 1.4 GPa) and different temperatures (77-293 K) and at order-disorder transitions. The ultrasonic method provides complementary pictures of the order-disorder transitions in diamondoids under pressure.

Universal Effect of Excitation Dispersion on the Heat Capacity and Gapped States in Fluids.

The change in dispersion of high-frequency excitations in fluids, from an oscillating solidlike to a monotonic gaslike one, is shown for the first time to affect thermal behavior of heat capacity and the q-gap width in reciprocal space. With in silico study of liquified noble gases, liquid iron, liquid mercury, and model fluids, we established universal bilinear dependence of heat capacity on q-gap width, whereas the crossover precisely corresponds to the change in the excitation spectra. The results open novel prospects for studies of various fluids, from simple to molecular liquids and melts.

WB : Synthesis, Properties, and Crystal Structure-New Insights into the Long-Debated Compound.

The recent theoretical prediction of a new compound, WB, has spurred the interest in tungsten borides and their possible implementation in industry. In this research, the experimental synthesis and structural description of a boron-rich tungsten boride and measurements of its mechanical properties are performed. The ab initio calculations of the structural energies corresponding to different local structures make it possible to formulate the rules determining the likely local motifs in the disordered versions of the WB structure, all of which involve boron deficit.

Experimental and modeling evidence for structural crossover in supercritical CO_{2}.

The physics of supercritical states is understood to a much lesser degree compared to subcritical liquids. Carbon dioxide, in particular, has been intensely studied, yet little is known about the supercritical part of its phase diagram. Here, we combine neutron scattering experiments and molecular dynamics simulations and demonstrate the structural crossover at the Frenkel line.

Extended short-range order determines the overall structure of liquid gallium.

Polyvalent metal melts (gallium, tin, bismuth, etc.) have microscopic structural features, which are detected by neutron and X-ray diffraction and which are absent in simple liquids. Based on neutron and X-ray diffraction data and the results of ab initio molecular dynamics simulations for liquid gallium, we examine the structure of this liquid metal at the atomistic level.

Direct Experimental Evidence of Longitudinal and Transverse Mode Hybridization and Anticrossing in Simple Model Fluids.

A significant number of key properties of condensed matter are determined by the spectra of elementary excitations and, in particular, collective vibrations. However, the behavior and description of collective modes in disordered media (e.g.

Pressure-Driven Chemical Disorder in Glassy AsS up to 14.7 GPa, Postdensification Effects, and Applications in Materials Design.

A small difference in energy between homopolar and heteropolar bonds and the glass-forming ability of pure chalcogens leads to unexpected trends in densification mechanisms of glassy chalcogenides compared to vitreous oxides. Using high-precision compressibility measurements and in situ high-energy X-ray diffraction up to 14.7 GPa, we show a new densification route in a canonical glass AsS.

Excitation spectra in fluids: How to analyze them properly.

Although the understanding of excitation spectra in fluids is of great importance, it is still unclear how different methods of spectral analysis agree with each other and which of them is suitable in a wide range of parameters. Here, we show that the problem can be solved using a two-oscillator model to analyze total velocity current spectra, while other considered methods, including analysis of the spectral maxima and single mode analysis, yield rough results and become unsuitable at high temperatures and wavenumbers. To prove this, we perform molecular dynamics (MD) simulations and calculate excitation spectra in Lennard-Jones and inverse-power-law fluids at different temperatures, both in 3D and 2D cases.

Anticrossing of Longitudinal and Transverse Modes in Simple Fluids.

If interacting modes of the same symmetry cross, they repel from each other and become hybridized. This phenomenon is called anticrossing and is well-known for mechanical oscillations, electromagnetic circuits, waveguides, metamaterials, polaritons, and phonons in crystals, but it still remains poorly understood in simple fluids. Here, we show that structural disorder and anharmonicity, governing properties of fluids, lead to the anticrossing of longitudinal and transverse modes, which is accompanied by their hybridization and strong redistribution of excitation spectra.

Bizarre behavior of heat capacity in crystals due to interplay between two types of anharmonicities.

The heat capacity of classical crystals is determined by the Dulong-Petit value C ≃ D (where D is the spatial dimension) for softly interacting particles and has the gas-like value C ≃ D/2 in the hard-sphere limit, while deviations are governed by the effects of anharmonicity. Soft- and hard-sphere interactions, which are associated with the enthalpy and entropy of crystals, are specifically anharmonic owing to violation of a linear relation between particle displacements and corresponding restoring forces. Here, we show that the interplay between these two types of anharmonicities unexpectedly induces two possible types of heat capacity anomalies.

Structure and topology of three-dimensional hydrocarbon polymers.

A new family of three-dimensional hydrocarbon polymers which are more energetically favorable than benzene is proposed. Although structurally these polymers are closely related to well known diamond and lonsdaleite carbon structures, using topological arguments we demonstrate that they have no known structural analogs. Topological considerations also give some indication of possible methods of synthesis.

Vivid Manifestation of Nonergodicity in Glassy Propylene Carbonate at High Pressures.

As glasses are nonergodic systems, their properties should depend not only on external macroparameters, such as P and T, but also on the time of observation and thermobaric history. In this work, comparative ultrasonic studies of two groups of molecular propylene carbonate glasses obtained by quenching from a liquid at pressures of 0.1 and 1 GPa have been performed.

Pressure-Induced Amorphization and a New High Density Amorphous Metallic Phase in Matrix-Free Ge Nanoparticles.

Over the last two decades, it has been demonstrated that size effects have significant consequences for the atomic arrangements and phase behavior of matter under extreme pressure. Furthermore, it has been shown that an understanding of how size affects critical pressure-temperature conditions provides vital guidance in the search for materials with novel properties. Here, we report on the remarkable behavior of small (under ~5 nm) matrix-free Ge nanoparticles under hydrostatic compression that is drastically different from both larger nanoparticles and bulk Ge.

Diamond monohydride: the most stable three-dimensional hydrocarbon.

Most of the hydrocarbons are either molecular structures or linear polymeric chains. Discovery of graphene and manufacturing of its monohydride - graphane have incited interest in the search for three-dimensional hydrocarbon polymers. However, up to now all hypothetical hydrocarbon lattices significantly have lost in terms of energy to stacked graphane sheets and solid benzene.