Polyamorphic Transformations Research Articles

Polyamorphic transformation of silicon in first-principles molecular-dynamics simulation

Isothermal–isobaric first-principles molecular-dynamics simulations were performed to investigate polyamorphic transformations of silicon. By pressurizing a low-density amorphous form of silicon (normal amorphous silicon with tetrahedral coordination), a new high-density amorphous form that has a strong resemblance to high-density amorphous water is obtained. It is found that the high-density amorphous form is reversely transformed to the low-density amorphous form by decompression and heating. The present simulations have revealed remarkable similarities between silicon and water, which is definitely of use to understand phase relations of polyamorphs of tetrahedrally coordinated substances.

Thermal desorption and IR spectrometric investigation of polyamorphic and polymorphic transformations in cryovacuum condensates of water

Simultaneous thermal desorption and IR spectrometric studies of thermally stimulated transformations in thin films of cryovacuum condensates of water are carried out. It is shown that the temperature interval 12–36K is the existence region of a high-density form of amorphous solid water. The transition of amorphous high-density ice to a low-density amorphous state starts at T≈39K and is completed in the temperature region 60–70K. This process is accompanied by pore formation and reaches its maximum intensity at T≈42K. The transition temperature from the glassy amorphous ice to a super-viscous liquid state is determined: Tg=137±2K. Further increase in temperature leads to transformations of a stepped character in the layer. This may be due to competing processes of crystallization via the growth of cubic and hexagonal nuclei and also to direct crystallization of the super-viscous liquid water formed at Tg and existing together with the crystalline phase up to temperatures ∼200K. Anomalous behavior of the sample at temperatures preceding sublimation is revealed. This confirms that a multicomponent system consisting of amorphous and crystalline samples has different values of the equilibrium pressures of the gas phase, corresponding to partial activation energies of sublimation, i.e., at high temperatures amorphous water evaporates at an earlier stage, and then recondenses on the crystalline components.

Computer modeling of hydrogen and methane transport in cellular nanostructures of amorphous ice

New computer modeling of the condensed state of water is offered. Mechanisms of polyamorphic transformations of high-density amorphous (HDA) and low-density amorphous (LDA) phases of cryogenic ice and their properties to accumulate methane and hydrogen are studied. As have shown essential distinction in the average size of compact clusters (H2O)n in the cryogenic HDA-ice (n ∼105 ) and LDA-ice (n ∼30) allows to change a share of methane accumulation inside clusters of amorphous ice. In HDA-ice large clusters can accumulate a lot more methane molecules than in the case of LDA-ice. Density jumps on polyamorphic transition of ice allows obtaining reversible accumulation of methane inside cellular nanostructures of cryogenic amorphous ice.

Tuning of Tetrahedrality in a Silicon Potential Yields a Series of Monatomic (Metal-like) Glass Formers of Very High Fragility

We obtain monatomic glass formers in simulations by modifying the tetrahedral character in a silicon potential to explore a triple point zone between potentials favoring diamond (dc) and bcc crystals. dc crystallization is always preceded by a polyamorphic transformation of the liquid, and is frustrated when the Kauzmann temperature of the high temperature liquid intersects the liquid-liquid coexistence line. The glass forming liquids are extraordinarily fragile. Our results suggest that Si and Ge liquids may be vitrified at a pressure close to the diamond-beta-tin-liquid triple point.

Temperature-induced structural changes in glassy, supercooled, and molten silica from 77 to 2150 K

In situ polarized and depolarized Raman spectra of glassy, supercooled, and molten SiO2 have been measured over the broad temperature range 77-2150 K in an effort to examine possible structural changes caused by temperature variation. A new experimental setup using a CO2 laser for heating the sample has been designed allowing measurement with controllable blackbody radiation background at temperatures up to 2200 K. Careful and systematic relative intensity measurements and the use of the isotropic and anisotropic Raman representation of the spectra revealed hidden bands in the bending mode region and resolved bands in the stretching region of the spectra. Overall the spectra behavior shows similarities with the spectra of the recently studied tetrahedral glasses/melts of ZnCl2 and ZnBr2. Increasing temperature causes subtle changes of the relative intensities within the silicon-oxygen stretching region at approximately 750-850 cm(-1) and gives rise to a new band at approximately 930 cm(-1). The spectral behavior is interpreted to indicate that the "SiO42" tetrahedra are bound to each other to form the network by apex-bridging and partly by edge-bridging oxygens. The network structure of the glass/melt is formed by mixing a variety of tetrahedra participating in "open" (cristobalitelike), "cluster" (supertetrahedra), and "chain" edge-bridged substructures bound to each other by bridging oxygens. A weak in intensity but strongly polarized composite band is resolved at approximately 1400 cm(-1) and is assigned to Si[Double Bond]O terminal bond frequency. Temperature rise increases the concentration of the terminal bonds by breaking up the network. These structural changes are reminiscent of the polyamorphic transformations occurring in silica as has recently been predicted by computer simulations. At low frequencies the Raman spectra reveal the presence of the Boson peak at approximately 60 cm(-1) which is well resolved even above melting temperature up to 2150 K.

High density amorphous form and polyamorphic transformations of silicon.

Polyamorphic transformations of silicon have been investigated by constant-pressure first-principles molecular-dynamics simulations. By pressurizing a normal amorphous Si with tetrahedral coordination, a new high density amorphous (HDA) form that has a strong resemblance to HDA water is obtained. We find that the HDA form can be also obtained through vitrification of liquid Si under pressure. Both HDA and liquid Si contain deformed tetrahedral configurations with interstitial atoms. These findings indicate that HDA Si is directly connected with liquid Si, which is of particular importance in understanding phase relations of polyamorphs of Si.

Polyamorphic transformations in liquids and glasses

Crystalline solids undergo large changes in their structure and properties as they transform between different polymorphs in response to the pressure and the temperature. It now appears that analogous density- or entropy-driven phase transitions also occur within the liquid state: these represent a new type of general phenomena that occur within liquids, that is only just beginning to be recognised and understood. Such polyamorphic transformations are also reflected within the solid amorphous state, as rapid and often dramatic changes in the structure and properties of glasses and amorphous solids that occur during compression, or as a function of the quench conditions.

Structural transitions and polyamorphism in glass-forming oxides

Brillouin light scattering was used to monitor the structural evolution of glass-forming oxide melts as a function of temperature, including the glass transition range. The storage and loss components of the high-frequency complex mechanical modulus are interpreted in terms of the rigidity resulting from network connectivity and the energy dissipated in aperiodic motions of small structural moieties, respectively. A series of glass-forming oxides with variable fragility was investigated. Most revealing is the fact that thermo-rheological simplicity does not apply to high-frequency processes in these supercooled oxide melts. In order to describe the temperature dependence of the complex moduli consistently for all systems we investigated, we enhanced Maxwell's theory of visco-elasticity to explicitly account for the transition between thermodynamically distinct structural states corresponding to liquid and glass respectively. Using complementary information pertaining to short-range structural characteristics, based on atomic-scale computer simulations and experiments, we have identified mechanisms that are invoked upon transition between structural states. Thereby, we uncovered some attributes of the beginning and ending configurations associated with, what we believe to be, the polyamorphic transformations that underlie the glass transition.