Polyamorphic Transition Research Articles

Pressure-Induced Polyamorphism in Salty Water

We investigated the metastable phase diagram of an ionic salt aqueous solution, LiCl:6D₂O, at high pressure and low temperature by neutron diffraction measurements and computer simulations. We show that the presence of salt triggers a stepwise transformation, under annealing at high pressure, to a new very high-density amorphous form. The transition occurs abruptly at 120 K and 2 GPa, is reversible, and is characterized by a sizeable enthalpy release. Simulations suggest that the polyamorphic transition is linked to a local structural reorganization of water molecules around the Li ions.

The structure and terahertz dynamics of water confined in nanoscale pools in salt solutions

The behaviour of liquid water below its melting point is of great interest as it may hold clues to the properties of normal liquid water and of water in and on the surfaces of biomolecules. A second critical point, giving rise to a polyamorphic transition between high and low density water, may be hidden in the supercooled region but cannot be observed directly. Here it is shown that water can be locked up in nano-pools or worm-like structures using aqueous LiCl salt solutions and can be studied with terahertz spectroscopies. Very high dynamic range ultrafast femtosecond optical Kerr effect (OKE) spectroscopy is used to study the temperature-dependent behaviour of water in these nano-pools on timescales from 10 fs to 4 ns. These experiments are complemented by temperature-dependent nuclear magnetic resonance (NMR) diffusion measurements, concentration-dependent Fourier-transform infrared (FTIR) measurements, and temperature-dependent rheology. It is found that liquid water in the nanoscale pools undergoes a fragile-to-strong transition at about 220 K associated with a sharp increase in the inhomogeneity of translational dynamics.

Evidence for a simple monatomic ideal glass former: The thermodynamic glass transition from a stable liquid phase

Under cooling, a liquid can undergo a transition to the glassy state either as a result of a continuous slowing down or by a first-order polyamorphous phase transition. The second scenario has so far always been observed in a metastable liquid domain below the melting point where crystalline nucleation interfered with the glass formation. We report the first observation of the liquid-glass transition by a first-order polyamorphous phase transition from the equilibrium stable liquid phase. The observation was made in a molecular dynamics simulation of a one-component system with a model metallic pair potential. In this way, the model, demonstrating the thermodynamic glass transition from a stable liquid phase, may be regarded as a candidate for a simple monatomic ideal glass former. This observation is of conceptual importance in the context of continuing attempts to resolve the long-standing Kauzmann paradox. The possibility of a thermodynamic glass transition from an equilibrium melt in a metallic system also indicates a new strategy for the development of bulk metallic glass-forming alloys.

Polyamorphism and the universal liquid–liquid critical point in the supercooled state

The physics of critical phenomena is well established in systems as diverse as molecular fluids, crystalline alloys and magnetic materials. As the critical point is approached, the susceptibility increases anomalously and fluctuations give rise to dramatic opalescence. Evidence has recently emerged for the existence of a second critical point in the liquid state at supercooled temperatures, below which polyamorphic phases coexist differing in density but sharing the same composition. Whilst much attention has been paid to supercooled water, polyamorphic phases have been observed in many elemental and oxide liquids potentially offering routes to low-entropy glasses. Our recent direct observation of a polyamorphic phase transition in levitated molten yttria–alumina offers the first opportunity to study the associated critical point in a real supercooled system.In situsmall-angle X-ray scattering records sharp rises in the average correlation length of density fluctuations and in the compressibility as the transition is approached. Both increases approximate to the universal power-law relations predicted by the three-dimensional Ising model in common with all critical point phenomena. The observation brings the second critical point predicted in liquids into line with other critical phenomena.

Low-density to high-density transition inCe75Al23Si2 metallic glass

Using in situ high-pressure x-ray diffraction (XRD), we observed a pressure-induced polyamorphictransition from the low-density amorphous (LDA) state to the high-density amorphous (HDA) statein Ce75Al23Si2 metallic glass at about 2 GPa and 300 K. The thermal stabilities of both LDA and HDAmetallic glasses were further investigated using in situ high-temperature and high-pressureXRD, which revealed different pressure dependences of the onset crystallization temperature(Tx) between them with a turning point at about 2 GPa. Compared withCe75Al25 metallic glass, minor Si doping shifts the onset polyamorphic transition pressure from 1.5 to2 GPa and obviously stabilizes both LDA and HDA metallic glasses with higherTx and changestheir slopes dTx/dP. The results obtained in this work reveal another polyamorphous metallic glass system byminor alloying (e.g. Si), which could modify the transition pressure and also properties ofLDA and HDA metallic glasses. The minor alloying effect reported here is valuable for thedevelopment of more polyamorphous metallic glasses, even multicomponent bulkmetallic glasses with modified properties, which will trigger more investigations inthis field and improve our understanding of polyamorphism and metallic glasses.

Properties of polyamorphousCe75Al25metallic glasses

The thermal stability and electronic transport properties of polyamorphous ${\text{Ce}}_{75}{\text{Al}}_{25}$ metallic glass (MG) have been investigated using in situ high-pressure, high-temperature, energy-dispersive synchrotron x-ray diffraction and in situ high-pressure and low-temperature, four-probe resistance measurements. The results are compared with the properties of ${\text{La}}_{75}{\text{Al}}_{25}$ MG. The pressure dependence of the crystallization temperature and resistance of the ${\text{Ce}}_{75}{\text{Al}}_{25}$ MG exhibited turning points at the polyamorphic transition pressure, 1.5 GPa, and they clearly presented different behaviors below and above 1.5 GPa. In contrast, no turning points were observed in the ${\text{La}}_{75}{\text{Al}}_{25}$ MG (La has no $4f$ electron). Additionally, the pressure-tuned temperature coefficient of resistance of the ${\text{Ce}}_{75}{\text{Al}}_{25}$ MG was observed. These results revealed switchable properties in the polyamorphous ${\text{Ce}}_{75}{\text{Al}}_{25}$ MG that are linked with $4f$ electron delocalization.

Hydrogen induced phase transitions in amorphous alloys

Hydrogen induced structure transformations in amorphous alloys ZrPd, ZrRh and ZrRhBe were studied by neutron and X-ray-diffraction technique under gas pressure up to 2 kbar. It was shown that the amorphous alloy Zr 0.7Pd 0.3 which was hydrogenated at temperature below the temperature of crystallization decomposes into metastable hyper-stoichiometric Pd-hydride with bcc lattice and amorphous hypo-stoichiometric Zr-hydride. The polyamorphous transition (transition between the amorphous phases) was reversible while the formation of metastable phase was not.

Pressure-induced transformations and superconductivity of amorphous germanium

The effect of pressure on amorphous Ge was probed by Raman spectroscopy combined with molecular-dynamics simulations. A large jump occurs in the principal peak position due to nearest-neighbor Ge-Ge vibrations at 11--12 GPa and 7--5.5 GPa, respectively, during increasing/decreasing pressure due to a polyamorphic transition occurring between the low-density amorphous semiconductor and a metallic high-density polyamorph (HDA). We measured the superconducting transition temperature $({T}_{c})$ using magnetic susceptibility measurements in the diamond anvil cell and determined that ${T}_{c}$ for the high-density HDA polyamorph was higher than that for the $\ensuremath{\beta}\text{-Sn}$ structured Ge-II crystalline phase.

Polyamorphic transition in amorphous fullerites

Samples of amorphous fullerites have been prepared by mechanoactivation (grinding in a ball mill), and their structure has been studied using neutron diffraction. It has been found that the amorphous fullerites subjected to high-temperature (600–1700°C) annealing undergo a polyamorphic transition from the molecular glass to the atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles.

Elasticity and pressure-induced structural changes in vitreous MgSiO 3-enstatite to lower mantle pressures

Knowledge of the pressure–density relations and elastic properties of silicate melts at high pressure and temperature conditions is fundamental for a better understanding of the dynamics and properties of the Earth's interior. Here we report the sound velocities and elastic properties of vitreous MgSiO 3-enstatite, a simple analog for melts in the Earth's mantle, determined up to 33.4 ± 1.2 GPa in a diamond anvil cell by Brillouin scattering spectroscopy. The adiabatic bulk and shear moduli of MgSiO 3 glass at ambient conditions are K S0 = 78.4(6) GPa and µ 0 = 41.1(3) GPa, corresponding to compressional and shear wave velocities of V P = 6.969(30) km/s and V S = 3.868(20) km/s. On compression both longitudinal V P and transverse V S modes exhibit anomalous change in slope at 8 GPa and between 15.0(1) and 17.9(2) GPa, that are associated with polyamorphic phase transitions in the glass. The acoustic data were used to determine the density and compressibility of MgSiO 3 glass at high pressure in both compression and decompression experiments. In contrast to velocities and elastic moduli, the density increases smoothly without singularities upon compression over the investigated pressure range. Densities of recovered glasses at room pressure are 1.5 to 2.4% higher than uncompressed glasses, indicating pressure-induced irreversible densification. The anomalous changes in velocities and compressibility with pressure are consistent with structural modifications that have been identified as the main densification mechanisms of the glass by in situ spectroscopic studies and molecular dynamic calculations. The results suggest that the residual densification of recovered glasses is associated with irreversible structural changes in the intermediate-range order (i.e., ring statistics) rather than with changes in the coordination number of Si upon pressurization. If the changes in compressibility of MgSiO 3 glasses identified in the present Brillouin study also occurs in the analog melts, they may have important implications for the present-day dynamics of the mantle and early evolution of the Earth during a magma ocean phase.

Thermal properties and Brillouin-scattering study of glass, crystal, and “glacial” states in n-butanol

We investigated through noncommercial calorimetry and elastoacoustic Brillouin experiments the phase diagram of n-butanol and measured the specific heat and the thermal conductivity in a wide low-temperature range for its three different states, namely, glass, crystal, and the so-called "glacial" states. The main aim of the work was to shed light on the controversial issue of these allegedly polyamorphic transitions found in some molecular glass-forming liquids, first reported to occur in triphenyl phosphite and later in n-butanol. Our experimental results show that the obtained glacial state in n-butanol is not a homogenous, amorphous state, but rather a mixture of two different coexisting phases, very likely the (frustrated) crystal phase embedded in a disordered, glassy phase.

High-pressure phase transformations, pressure-induced amorphization, and polyamorphic transition of the clathrateRb6.15Si46

The type-I clathrate ${\text{Rb}}_{6.15}{\text{Si}}_{46}$ with partly empty cage sites has been studied up to 36 GPa using Raman spectroscopy, synchrotron x-ray diffraction in diamond-anvil cells, and ab initio total-energy and lattice-dynamics calculations. A first phase transition is observed at $13\ifmmode\pm\else\textpm\fi{}1\text{ }\text{GPa}$ and a ``volume collapse'' transition within the clathrate structure is then observed at $24\ifmmode\pm\else\textpm\fi{}1\text{ }\text{GPa}$. Pressure-induced amorphization into a high-density amorphous (HDA) state occurs above $P=33\ifmmode\pm\else\textpm\fi{}1\text{ }\text{GPa}$. The HDA form transforms into a low-density amorphous polymorph during decompression. During the compression study using angle dispersive synchrotron x-ray diffraction techniques we measured bulk modulus parameters for rocksalt-structured TaO, included adventitiously in the clathrate sample [${K}_{0}=293(3)\text{ }\text{GPa}$ and ${K}_{0}^{\ensuremath{'}}=5.4(3)$].

High pressure effects on liquid viscosity and glass transition behaviour, polyamorphic phase transitions and structural properties of glasses and liquids

Since the pioneering work of Bridgman it has been known that pressure affects the glass transition of polymers and liquid state viscosities. Usually the Tg and viscosity both increase as a function of pressure as expected from ‘free volume’ theories. However, H2O provided a notable exception in that the viscosity passes through a minimum at low temperature. It was thought that this might be linked to the anomalous thermal expansion behavior. However further research on geologically important aluminosilicate liquids revealed that they could show anomalous viscosity decreases with increasing pressure and this behavior is given a structural interpretation as five-fold coordinated Si4+ and Al3+ species are formed. Also the existence of polyamorphism or density-driven liquid–liquid phase transitions in certain systems can lead to anomalies in the Tg or η vs. P relations. This may be the case for H2O, for example. Current research is focusing on investigating structural changes in liquids and glasses at high pressure as the rich variety of behavior is becoming recognized. Both experimental studies and computer simulations are important as the underlying phenomonology is linked to changes in the glass or liquid structure as a function of densification.

Differences between pressure-induced densification ofLiCl–H2O glass andpolyamorphic transition of H2O

We perform volumetric measurements of LiCl aqueous solution up to 1.00 GPa in the100–170 K range, examine the pressure-induced vitrification and densification, and draw thepressure–temperature–volume surface. The pressure-induced vitrification of the solutioncorresponds to the cooling-induced vitrification of the liquid. We found that thevolumetric decrease of glassy solution during the densification is continuous and thisbehavior depends on the glassy state before the compression. Raman profiles of theglassy solutions before and after the densification are similar. In contrast, thepolyamorphic transition from low-density amorphous ice (LDA) to high-densityamorphous ice (HDA) is discontinuous and their Raman profile before and after thetransition is distinct. These results suggest that the densification relates to thestructural relaxation and differs intrinsically from the polyamorphic transition.Furthermore, the densification of HDA is observed under high pressure, suggesting thatvery high-density amorphous ice (VHDA) may be the densified HDA. In order to recognizea polyamorphic transition under a non-equilibrium condition correctly, evidence of not onlylarge volume change but also some distinct structural changes in glassy state isnecessary.

Polyamorphic transitions in vitreousB2O3 under pressure

We have studied the nature of structural transitions inB2O3 glass under pressure using molecular dynamics simulations, based on a newly developedcoordination-dependent charge transfer potential, to complement the results from ourearlier Brillouin and Raman scattering experiments and to interpret these findings. Thisinteraction model allows for charges to re-distribute between atoms upon the formation andrupture of chemical bonds, and accommodates multiple coordination states for a given speciesin the course of the simulation. The macroscopic observables of the simulated vitreousB2O3, such as the variation of density and elastic modulus with pressure, agree well withthose seen in experiments. The compaction of simulated structures is based on apolyamorphic transition that involves transitory four-coordinated boron atoms athigh pressures. While the coordination of boron completely reverts to trigonalupon pressure release, without this transitory coordination increase permanentdensification would not be manifest in the recovered glass. The response of vitreousB2O3 to pressure is virtually independent of the concentration of boroxol rings in the structure.In simulated glass, boroxol rings dissolve when subject to pressure, which explainsthe disappearance of the breathing mode in the Raman spectrum of compressedB2O3 glass.

High pressure elastic and plastic deformations of silica: In situ diamond anvil cell Raman experiments

Normal silica glass is usually referred to as low density amorphous silica as it can be converted to high density amorphous silica by a hydrostatic pressure (polyamorphic transition). In this work in situ Raman experiments are performed in a diamond anvil cell up to 18GPa. The pressure effects on the structure of silica after successive compression decompression experiments are analyzed. The mode Grüneisen parameters corresponding to the elastic compression of high density amorphous silica are obtained and compared with those of normal silica. A reorganization of the high density amorphous silica below 3GPa is evidenced.

Isothermal transformation of supercooled liquidn-butanol near the glass transition: Polyamorphic transitions in molecular liquids investigated using Raman scattering

Raman investigations were carried out during an isothermal transformation of deeply supercooled liquid $n$-butanol, recently interpreted as a polyamorphic transition. Raman spectra indicate that the newly emerging state is a mixed crystal-liquid state, and not another amorphous state. The long-range order in crystallites was identified as reflecting that of the crystal. The time dependence of the O-H stretch spectrum indicates that the transformation is driven by the formation of a two-dimensional H-bonded network, according to the archetypal-layered structures of normal alcohols. The origin of the frustration of the crystallization in $n$-butanol can be explained from the consideration that the rearrangement of the hydrogen-bonded molecules is unavoidably prevented by the very low molecular mobility of the system only two degrees above ${T}_{g}$.

Vitrification of a monatomic metallic liquid

Although the majority of glasses in use in technology are complex mixtures of oxides or chalcogenides, there are numerous examples of pure substances-'glassformers'-that also fail to crystallize during cooling. Most glassformers are organic molecular systems, but there are important inorganic examples too, such as silicon dioxide and elemental selenium (the latter being polymeric). Bulk metallic glasses can now be made; but, with the exception of Zr50Cu50 (ref. 4), they require multiple components to avoid crystallization during normal liquid cooling. Two-component 'metglasses' can often be achieved by hyperquenching, but this has not hitherto been achieved with a single-component system. Glasses form when crystal nucleation rates are slow, although the factors that create the slow nucleation conditions are not well understood. Here we apply the insights gained in a recent molecular dynamics simulation study to create conditions for successful vitrification of metallic liquid germanium. Our results also provide micrographic evidence for a rare polyamorphic transition preceding crystallization of the diamond cubic phase.

High-pressure x-ray scattering and computer simulation studies of density-induced polyamorphism in silicon

A low- to high-density pressure-driven phase transition in amorphous silicon is investigated by synchrotron x-ray diffraction in the diamond anvil cell. Complementary atomistic molecular dynamics computer simulations provide insight into the underlying structural transformations and allow us to interpret the structure factors obtained from experiment. During compression the form of the scattering function $S(Q)$ changes abruptly at $13.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, indicating significant structural rearrangement in the amorphous solid. In particular, the first peak in $S(Q)$ shifts to larger $Q$ values. The changes are correlated with the occurrence of a low- to high-density (LDA-HDA) polyamorphic transition observed previously using Raman scattering and electrical conductivity measurements. The data are analyzed to provide real space (pair distribution function) information. The experimental data are compared with results from molecular dynamics (MD) simulations using a modified Stillinger-Weber many-body potential energy function in order to extract structural information on the densified amorphous material. We deduce that the polyamorphic transition involves an abrupt increase in the proportion of 5- and 6-coordinate Si atoms. The overall structure of the HDA polyamorph can be related to that of the LDA form by creation of highly-coordinated ``defects'' within the tetrahedrally-bonded LDA network. However classical and quantum MD simulations indicate that an even higher density amorphous state might exist, based on structures that resemble the densely-packed metallic polymorphs of crystalline Si.

Phase separation in dilute LiCl–H2O solution related to the polyamorphism of liquid water

When an emulsified 4.8 mol % LiCl-H2O solution was cooled under a pressure of 0.35 or 0.45 GPa and decompressed to 0.1 GPa at 142 K, slightly above its glass transition temperature (approximately 140 K at 0.1 GPa), its volume increased suddenly. This was regarded as an appearance of the low-density amorphous ice in the liquid solution as suggested by x-ray and Raman measurements, and this appearance corresponded to the high-to-low-density polyamorphic transition of pure H2O. Hysteresis was considered to accompany this volumetric change. The hysteresis of the liquid transition proves its first-order nature and, as for the solution, this suggests that the transition is a polyamorphic phase separation.