Chemical short-range order in liquid Ni-Cu

Neutron diffraction in combination with isotopic substitution on the zero-scatterer62Ni4363Cu57shows indications for chemical short-range order in the stable liquid as evidenced by oscillations in the concentration-concentration structure factorSCC(q). This points towards a non-ideal solution behavior of Ni-Cu contrary to common believe but in agreement with measurements of free enthalpy of mixing. The temperature dependence ofSCCat small momentum transfer provides evidence of critical compositional fluctuations in Ni43Cu57melts.

High-Temperature Liquid-Liquid Phase Transition in Glass-Forming Liquid Pd43Ni20Cu27P10

Liquid-liquid phase transition (LLPT) is a transition from one liquid state to another with the same composition but distinct structural change, which provides an opportunity to explore the relationships between structural transformation and thermodynamic/kinetic anomalies. Herein the abnormal endothermic LLPT in Pd₄₃Ni₂₀Cu₂₇P₁₀ glass-forming liquid was verified and studied by flash differential scanning calorimetry (FDSC) and ab initio molecular dynamics (AIMD) simulations. The results show that the change of the atomic local structure of the atoms around the Cu-P bond leads to the change in the number of specific clusters <0 2 8 0> and <1 2 5 3>, which leads to the change in the liquid structure. Our findings reveal the structural mechanisms that induce unusual heat-trapping phenomena in liquids and advance the understanding of LLPT.

Liquid-Liquid Phase Transition in Metallic Droplets

We report theoretical evidence of the substrate-induced liquid-liquid phase transition (LLPT) behaviors in a single Al droplet and Ti-Al droplets. The Al droplet can produce an LLPT induced by substrates in part, forming a special three-layer structure. However, the introduction of a Ti droplet can promote the LLPT in an Al droplet. Al and Ti droplets do not coalesce into a homogeneously mixed droplet but produce the ordered liquid films. The substrate-induced LLPT in the Al droplet is characterized by the transition from the disordered to ordered structure. Results indicate that the substrate and the Ti droplet are the driving forces to promote the LLPT. The LLPT of the Ti-Al droplets in the wedge-shaped substrate is also observed, indicating that the confined Ti-Al droplets are more likely to undergo an LLPT.

Pressure-induced liquid-liquid transition in a family of ionic materials

Liquid−liquid transition (LLT) between two disordered phases of single-component material remains one of the most intriguing physical phenomena. Here, we report a first-order LLT in a series of ionic liquids containing trihexyl(tetradecyl)phosphonium cation [P_666,14]⁺ and anions of different sizes and shapes, providing an insight into the structure-property relationships governing LLT. In addition to calorimetric proof of LLT, we report that ion dynamics exhibit anomalous behavior during the LLT, i.e., the conductivity relaxation times (τ_σ) are dramatically elongated, and their distribution becomes broader. This peculiar behavior is induced by isobaric cooling and isothermal compression, with the τ_σ(T_LL,P_LL) constant for a given system. The latter observation proves that LLT, in analogy to liquid-glass transition, has an isochronal character. Finally, the magnitude of discontinuity in a specific volume at LLT was estimated using the Clausius-Clapeyron equation.

Liquid−liquid transitions (LLTs) have been reported for some molecular systems but are difficult to observe under high pressure conditions. Here the authors report and characterize a first-order LLT in a series of ionic liquids containing the trihexyl(tetradecyl)phosphonium cation and anions of different sizes and shapes, using calorimetric and dielectric measurements.

Liquid structure under extreme conditions: high-pressure x-ray diffraction studies

Under extreme conditions of high pressure and temperature, liquids can undergo substantial structural transformations as their atoms rearrange to minimise energy within a more confined volume. Understanding the structural response of liquids under extreme conditions is important across a variety of disciplines, from fundamental physics and exotic chemistry to materials and planetary science.In situexperiments and atomistic simulations can provide crucial insight into the nature of liquid-liquid phase transitions and the complex phase diagrams and melting relations of high-pressure materials. Structural changes in natural magmas at the high-pressures experienced in deep planetary interiors can have a profound impact on their physical properties, knowledge of which is important to inform geochemical models of magmatic processes. Generating the extreme conditions required to melt samples at high-pressure, whilst simultaneously measuring their liquid structure, is a considerable challenge. The measurement, analysis, and interpretation of structural data is further complicated by the inherent disordered nature of liquids at the atomic-scale. However, recent advances in high-pressure technology mean that liquid diffraction measurements are becoming more routinely feasible at synchrotron facilities around the world. This topical review examines methods for high pressure synchrotron x-ray diffraction of liquids and the wide variety of systems which have been studied by them, from simple liquid metals and their remarkable complex behaviour at high-pressure, to molecular-polymeric liquid-liquid transitions in pnicogen and chalcogen liquids, and density-driven structural transformations in water and silicate melts.

Structure of disordered materials under ambient to extreme conditions revealed by synchrotron x-ray diffraction techniques at SPring-8-recent instrumentation and synergic collaboration with modelling and topological analyses

The structure of disordered materials is still not well understood because of insufficient experimental data. Indeed, diffraction patterns from disordered materials are very broad and can be described only in pairwise correlations because of the absence of translational symmetry. Brilliant hard x-rays from third-generation synchrotron radiation sources enable us to obtain high-quality diffraction data for disordered materials from ambient to high temperature and high pressure, which has significantly improved our grasp of the nature of order in disordered materials. Here, we introduce the progress in the instrumentation for hard x-ray beamlines at SPring-8 over the last 20 years with associated results and advanced data analysis techniques to understand the topology in disordered materials.

Anomalous structure transition in undercooled melt regulates polymorphic selection in barium titanate crystallization

The crystallization processes of titanates are central to the fabrication of optical and electrical crystals and glasses, but their rich polymorphism is not fully understood. Here, we show when and how polymorphic selection occurs during the crystallization of barium titanate (BaTiO₃, BT) using in situ high energy synchrotron X-ray diffraction and ab initio molecular dynamic simulation. An anomalous structure transition is found in molten BT during cooling across the cubic-hexagonal transition temperature, which enables nucleation selection of BT by manipulating the undercooling: a cubic phase is preferred if nucleation is triggered at large undercooling, whereas a hexagonal phase is promoted at small undercooling. We further reveal that the nucleation selection between the cubic and the hexagonal phase is regulated by the intrinsic structure property of the melt, in particular, the degree of polymerization between Ti-O polyhedra. These findings provide an innovative perspective to link the polymorphic crystallization to the non-isomorphic structure transition of the melt beyond the conventional cognition of structural heredity.

Y2O3-Al2O3 microsphere crystallization analyzed by electron backscatter diffraction (EBSD)

The crystallization of glass microspheres in the Y₂O₃-Al₂O₃-system produced from precursor powders of four different nominal compositions via flame synthesis is analyzed in detail by electron microscopy with a focus on electron backscatter diffraction (EBSD). Growth models are formulated for individual microspheres crystallized during flame synthesis as well as after an additional heat treatment step. 16 different types of crystallized bodies are cataloged for future reference. They are presented without regard for their relative occurrence; some are extremely rare but illustrate the possibilities of flame synthesis in the analyzed system. All three phases in the binary Y₂O₃-Al₂O₃-phase diagram (Y₃Al₅O₁₂, YAlO₃ and Y₄Al₂O₉) and α-alumina are located by EBSD. Energy dispersive X-ray spectrometry results obtained from these microspheres show that their chemical composition can deviate from the nominal composition of the precursor powder. The multitude of differing microsphere types showing polygon and dendritic crystal growth as well as phase separation indicate that flame synthesis can lead to a wide variety of parameters during microsphere production, e.g. via irregular flight paths through the flame, contaminants or irregular cooling rates.

Stable Prenucleation Calcium Carbonate Clusters Define Liquid-Liquid Phase Separation

Liquid-liquid phase separation (LLPS) is an intermediate step during the precipitation of calcium carbonate, and is assumed to play a key role in biomineralization processes. Here, we have developed a model where ion association thermodynamics in homogeneous phases determine the liquid-liquid miscibility gap of the aqueous calcium carbonate system, verified experimentally using potentiometric titrations, and kinetic studies based on stopped-flow ATR-FTIR spectroscopy. The proposed mechanism explains the variable solubilities of solid amorphous calcium carbonates, reconciling previously inconsistent literature values. Accounting for liquid-liquid amorphous polymorphism, the model also provides clues to the mechanism of polymorph selection. It is general and should be tested for systems other than calcium carbonate to provide a new perspective on the physical chemistry of LLPS mechanisms based on stable prenucleation clusters rather than un-/metastable fluctuations in biomineralization, and beyond.

From Molten Calcium Aluminates through Phase Transitions to Cement Phases

Crystalline calcium aluminates are a critical setting agent in cement. To date, few have explored the microscopic and dynamic mechanism of the transitions from molten aluminate liquids, through the supercooled state to glassy and crystalline phases, during cement clinker production. Herein, the first in situ measurements of viscosity and density are reported across all the principal molten phases, relevant to their eventual crystalline structures. Bulk atomistic computer simulations confirm that thermophysical properties scale with the evolution of network substructures interpenetrating melts on the nanoscale. It is demonstrated that the glass transition temperature (T g) follows the eutectic profile of the liquidus temperature (T m), coinciding with the melting zone in cement production. The viscosity has been uniquely charted over 14 decades for each calcium-aluminate phase, projecting and justifying the different temperature zones used in cement manufacture. The fragile-strong phase transitions are revealed across all supercooled phases coinciding with heterogeneous nucleation close to 1.2T g, where sintering and quenching occur in industrial-scale cement processing.

Dimeric Molecular Structure of Molten Gallium Trichloride and a Hidden Evolution toward a Possible Liquid-Liquid Transition

Group 13 trihalides MY₃ (M = Al, Ga, and In; Y = Cl, Br, and I) mostly having a dimeric M₂Y₆ molecular structure in the solid state and a mixture of M₂Y₆ dimers and MY₃ monomers in the vapor phase are potential candidates for entropy-driven liquid-liquid transition M₂Y₆ ⇄ 2MY₃ at elevated temperatures. Using pulsed neutron diffraction and high-energy X-ray scattering supported by structural modeling, we show a dimer molecular structure of liquid GaCl₃ above the melting point at 351 K and midway between the boiling point (474 K) and the critical temperature (694 K) with almost hidden characteristic evolution toward a possible liquid-liquid transition. In contrast to edge-sharing (ES) dimers in solid and vapor of D_2h symmetry, the ES Ga₂Cl₆ molecules in the melt have a puckered structure of the central four-membered ring with shorter Cl-Cl (2.90-3.09 Å) and longer Ga-Ga (3.20-3.26 Å) second-neighbor correlations. The elongation of Ga-Ga intramolecular distances with increasing temperature simultaneously with diminished Cl-Cl nearest neighbor contacts destabilizes the ES dimers, indicating the first step toward dimer dissociation.

Long-Range Mass Transport during Structural Transitions in Metallic Glass-Forming Melts

We investigate the structure and dynamics of the bulk metallic glass-forming alloys Zr_{41.2}Ti_{13.8}Cu_{12.5}Ni_{10}Be_{22.5} and Zr_{58.5}Cu_{15.6}Ni_{12.8}Al_{10.3}Nb_{2.8}. Combining in situ synchrotron x-ray diffraction and quasielastic neutron scattering with electrostatic levitation, we directly observe an abrupt change in the temperature dependence of the first structure factor maximum of these melts. We find that the kinetics of this liquid-liquid transition during cooling are on the order of tens of seconds, whereas its onset temperature depends only weakly on the applied cooling rate. Such slow transition kinetics require long-range mass transport, which is incompatible with a transition mechanism involving only local structural changes as in oxides or molecular liquids.

Liquid-liquid transition in supercooled gallium alloys under nanoconfinement

NMR studies of the thermal evolution of the Ga-In-Sn and Ga-In liquid alloys embedded into opal matrices were carried out. Temperature dependences of the gallium lineshape, shift of the resonance frequency (Knight shift), and intensity were obtained upon cooling down to the alloy freezing and subsequent warming. A second high-frequency ⁷¹Ga NMR signal emerged for both alloys upon cooling, the NMR line intensity transferring gradually into this additional signal. The Knight shifts of the signals differed noticeably. The transformations of the gallium line upon warming were continuous and not affected by changes in the alloy compositions induced by melting. ¹¹⁵In NMR measurements were conducted to monitor the alloy compositions at freezing and melting. The findings suggest the occurrence of the liquid-liquid phase transition in the strongly supercooled alloys under nanoconfinement.

Polymorphic transition and nucleation pathway of barium dititanate (BaTi2O5) during crystallization from undercooled liquid

The nucleation pathway plays an important role in vitrification, preparation of glass-ceramic composites and synthesis of metastable materials. In this paper, we studied the nucleation pathway of a novel ferroelectric BaTi₂O₅ (BT2) during crystallization from undercooled liquid by aerodynamic levitation (ADL) containerless processing and structural analysis. An interesting polymorphic transition of BT2 regulated by the undercooling was observed during the crystallization process: the ferroelectric monoclinic phase (γ-BT2) was fabricated at low undercoolings and the paraelectric orthorhombic metastable phase (β-BT2) was obtained from hypercooled liquid. This polymorphic transition phenomenon corresponds to a non-classical nucleation pathway: metastable β-BT2 preferentially nucleates from undercooled melt and γ-BT2 is generated from β phase by solid-state phase transition. The two-step nucleation pathway stems from the structural heredity between the undercooled liquid and crystals. A stronger structural homology exists between the undercooled melt and β-BT2 than γ-BT2 based on diffraction data and atomic configurations analysis. This structural homology coupled with nucleation barrier calculation was used to elucidate the non-classical nucleation pathway of BT2 crystallization: the similarity of the structural unit (Ti-O polyhedra) between the undercooled liquid and the metastable β-BT2 reduces the nucleation barrier and contributes to the preferential precipitation of β-like clusters. This work reveals the formation route of BT2 from cooling melt, which not only benefits the synthesis and application of this novel functional material but also provides a guideline of the crystallization process of titanates from melt at atomic level.

Highly negative Poisson's ratio in a flexible two-dimensional tungsten carbide monolayer

Auxetic materials have numerous promising engineering applications such as fracture resistance and energy storage due to their negative Poisson's ratios (NPRs). However, compared to materials possessing positive Poisson's ratios (PPRs), auxetic materials are rare. In this paper, by employing first principles calculations, we found a high NPR two-dimensional (2D) material, tungsten carbide (W2C), in the transition metal carbides (MXenes). Our results of the relatively moderate Young's modulus and fracture strength as well as the critical strain showed that the 2D monolayer W2C is an extraordinary flexible material. Our DFT results also demonstrated that W2C possesses high NPRs while Hf2C and Ta2C have PPRs. Furthermore, the mechanically induced deformation mechanism and the NPR formation mechanism of W2C have been proposed. Such an intrinsic NPR in W2C is attributed to the strong coupling between the C-p and W-d orbitals in the pyramid structural unit. The mechanically induced deformation mechanism and the PPR formation mechanism of Hf2C have also been determined. The intrinsic NPR for W2C transforms to PPR upon the surface functionalization induced. The behavior occurs due to the W-C interaction weakening. The excellent NPR in the 2D MXene material combined with other outstanding properties such as the metallic state would bring about its promising engineering prospects, ranging from the metal-ion battery, to automobiles and aircraft.

Pressureless glass crystallization of transparent yttrium aluminum garnet-based nanoceramics

Transparent crystalline yttrium aluminum garnet (YAG; Y₃Al₅O₁₂) is a dominant host material used in phosphors, scintillators, and solid state lasers. However, YAG single crystals and transparent ceramics face several technological limitations including complex, time-consuming, and costly synthetic approaches. Here we report facile elaboration of transparent YAG-based ceramics by pressureless nano-crystallization of Y₂O₃–Al₂O₃ bulk glasses. The resulting ceramics present a nanostructuration composed of YAG nanocrystals (77 wt%) separated by small Al₂O₃ crystalline domains (23 wt%). The hardness of these YAG-Al₂O₃ nanoceramics is 10% higher than that of YAG single crystals. When doped by Ce³⁺, the YAG-Al₂O₃ ceramics show a 87.5% quantum efficiency. The combination of these mechanical and optical properties, coupled with their simple, economical, and innovative preparation method, could drive the development of technologically relevant materials with potential applications in wide optical fields such as scintillators, lenses, gem stones, and phosphor converters in high-power white-light LED and laser diode.

Transparent YAG crystals are ubiquitous in phosphors, scintillators and lasers, but are complex and costly to make. Here, the authors use a one-step pressureless crystallization of bulk glass to make a transparent biphasic YAG nanoceramic that can be doped for optical applications.

Solution electrostatic levitator for measuring surface properties and bulk structures of an extremely supersaturated solution drop above metastable zone width limit

We report on the first integrated apparatus for measuring surface and thermophysical properties and bulk structures of a highly supersaturated solution by combining electrostatic levitation with real-time laser/x-ray scattering. Even today, a proper characterization of supersaturated solutions far above their solubility limits is extremely challenging because heterogeneous nucleation sites such as container walls or impurities readily initiate crystallization before the measurements can be performed. In this work, we demonstrate simultaneous measurements of drying kinetics and surface tension of a potassium dihydrogen phosphate (KH₂PO₄) aqueous solution droplet and its bulk structural evolution beyond the metastable zone width limit. Our experimental finding shows that the noticeable changes of the surface properties are accompanied by polymerizations of hydrated monomer clusters. The novel electrostatic levitation apparatus presented here provides an effective means for studying a wide range of highly concentrated solutions and liquids in deep metastable states.

δ-Phosphorene: a two dimensional material with a highly negative Poisson's ratio

As a basic mechanical parameter, Poisson's ratio (ν) measures the mechanical responses of solids against external loads. In rare cases, materials have a negative Poisson's ratio (NPR), and present an interesting auxetic effect. That is, when a material is stretched in one direction, it will expand in the perpendicular direction. To design modern nanoscale electromechanical devices with special functions, two dimensional (2D) auxetic materials are highly desirable. In this work, based on first principles calculations, we rediscover the previously proposed δ-phosphorene (δ-P) nanosheets [Jie Guan, et al., Phys. Rev. Lett., 2014, 113, 046804] which are good auxetic materials with a high NPR. The results show that the Young's modulus and Poisson's ratio of δ-P are all anisotropic. The NPR value along the grooved direction is up to -0.267, which is much higher than the recently reported 2D auxetic materials. The auxetic effect of δ-P originating from its puckered structure is robust and insensitive to the number of layers due to weak interlayer interactions. Moreover, δ-P possesses good flexibility because of its relatively small Young's modulus and high critical crack strain. If δ-P can be synthesized, these extraordinary properties would endow it with great potential in designing low dimensional electromechanical devices.

The role of liquid–liquid transition in glass formation of CuZr alloys

The structure evolution during LLTs is beneficial to the glass forming ability (GFA) of Cu–Zr systems.

Mercury Sulfide Dimorphism in Thioarsenate Glasses

Crystalline mercury sulfide exists in two drastically different polymorphic forms in different domains of the P,T-diagram: red chain-like insulator α-HgS, stable below 344 °C, and black tetrahedral narrow-band semiconductor β-HgS, stable at higher temperatures. Using pulsed neutron and high-energy X-ray diffraction, we show that these two mercury bonding patterns are present simultaneously in mercury thioarsenate glasses HgS-As2S3. The population and interconnectivity of chain-like and tetrahedral dimorphous forms determine both the structural features and fundamental glass properties (thermal, electronic, etc.). DFT simulations of mercury species and RMC modeling of high-resolution diffraction data provide additional details on local Hg environment and connectivity implying the (HgS2/2)m oligomeric chains (1 ≤ m ≤ 6) are acting as a network former while the HgS4/4-related mixed agglomerated units behave as a modifier.

Time-Resolved Light Scattering Study on the Kinetics of the Liquid-Liquid Transition in Triphenyl Phosphite

There is experimental evidence suggesting the existence of a liquid-liquid transition (LLT) in a single-component liquid. However, none of this evidence is free from controversy, including the case of a molecular liquid, triphenyl phosphite, which we study here. Furthermore, the kinetics of LLT has been largely unexplored. Here we study the phase-transition dynamics of triphenyl phosphite in a supercooled liquid state by means of time-resolved polarized and depolarized small-angle light scattering to clarify whether the transition is a liquid-liquid transition (LLT) or merely nanocrystal formation. A part of this study was recently reported in another of our papers [Shimizu, R.; Kobayashi, M.; Tanaka, H. Phys. Rev. Lett. 2014, 112, 125702]. A detailed analysis of our experimental results of light scattering and the comparison with heat evolution during LLT have revealed the following facts. The polarized scattering from domains has a finite (nonzero) intensity in the low-wavenumber limit, and the time evolution of its average intensity is almost proportional to the square of the heat-releasing rate. The depolarized scattering intensity monotonically increases in the process of LLT during isothermal annealing above the spinodal temperature TSD but exhibits a peak below TSD. On the basis of these results, we suggest that the primary process is LLT, whose order parameter is of a nonconserved nature, but accompanies nanocrystal formation. In the NG-type LLT, the sharp interface between liquid II droplets and the liquid I matrix promotes nanocrystal formation there, whereas much less nanocrystal formation is induced in the SD-type LLT due to the lack of such sharp interfaces.

Hybrid glasses from strong and fragile metal-organic framework liquids

Hybrid glasses connect the emerging field of metal-organic frameworks (MOFs) with the glass formation, amorphization and melting processes of these chemically versatile systems. Though inorganic zeolites collapse around the glass transition and melt at higher temperatures, the relationship between amorphization and melting has so far not been investigated. Here we show how heating MOFs of zeolitic topology first results in a low density 'perfect' glass, similar to those formed in ice, silicon and disaccharides. This order-order transition leads to a super-strong liquid of low fragility that dynamically controls collapse, before a subsequent order-disorder transition, which creates a more fragile high-density liquid. After crystallization to a dense phase, which can be remelted, subsequent quenching results in a bulk glass, virtually identical to the high-density phase. We provide evidence that the wide-ranging melting temperatures of zeolitic MOFs are related to their network topologies and opens up the possibility of 'melt-casting' MOF glasses.

Novel experimental design for high pressure-high temperature electrical resistance measurements in a "Paris-Edinburgh" large volume press

We present a novel experimental design for high sensitivity measurements of the electrical resistance of samples at high pressures (0-6 GPa) and high temperatures (300-1000 K) in a "Paris-Edinburgh" type large volume press. Uniquely, the electrical measurements are carried out directly on a small sample, thus greatly increasing the sensitivity of the measurement. The sensitivity to even minor changes in electrical resistance can be used to clearly identify phase transitions in material samples. Electrical resistance measurements are relatively simple and rapid to execute and the efficacy of the present experimental design is demonstrated by measuring the electrical resistance of Pb, Sn, and Bi across a wide domain of temperature-pressure phase space and employing it to identify the loci of phase transitions. Based on these results, the phase diagrams of these elements are reconstructed to high accuracy and found to be in excellent agreement with previous studies. In particular, by mapping the locations of several well-studied reference points in the phase diagram of Sn and Bi, it is demonstrated that a standard calibration exists for the temperature and pressure, thus eliminating the need for direct or indirect temperature and pressure measurements. The present technique will allow simple and accurate mapping of phase diagrams under extreme conditions and may be of particular importance in advancing studies of liquid state anomalies.

An in situ spectroscopic study of the local structure of oxyfluoride melts: NMR insights into the speciation in molten LiF-LaF3-Li2O systems

The local structure of molten LaF3-LiF-Li2O has been investigated by high temperature NMR spectroscopy. The (139)La and (19)F chemical shifts have been measured as a function of temperature and composition. The NMR spectra show that Li2O reacts completely with LaF3 to form a LaOF compound in the solid state below the melting temperature of the sample. LaOF is not completely dissolved in the fluoride melt and solid LaOF is observed in the (19)F spectra for Li2O concentrations above 10 mol%. We discuss the local environment of lanthanum ions in molten LaF3-LiF-Li2O and compare the results to those with the LaF3-LiF-CaO system. The analysis of the temperature and Li2O concentration dependences of the (139)La and (19)F chemical shifts suggests that several kinds of lanthanum oxyfluoride long-lived LaOxFy(3-x-y) units are present in the melt.

Beyond simple small-angle X-ray scattering: developments in online complementary techniques and sample environments

Small- and wide-angle X-ray scattering (SAXS, WAXS) are standard tools in materials research. The simultaneous measurement of SAXS and WAXS data in time-resolved studies has gained popularity due to the complementary information obtained. Furthermore, the combination of these data with non X-ray based techniques, via either simultaneous or independent measurements, has advanced understanding of the driving forces that lead to the structures and morphologies of materials, which in turn give rise to their properties. The simultaneous measurement of different data regimes and types, using either X-rays or neutrons, and the desire to control parameters that initiate and control structural changes have led to greater demands on sample environments. Examples of developments in technique combinations and sample environment design are discussed, together with a brief speculation about promising future developments.

Critical behavior of a water monolayer under hydrophobic confinement

The properties of water can have a strong dependence on the confinement. Here, we consider a water monolayer nanoconfined between hydrophobic parallel walls under conditions that prevent its crystallization. We investigate, by simulations of a many-body coarse-grained water model, how the properties of the liquid are affected by the confinement. We show, by studying the response functions and the correlation length and by performing finite-size scaling of the appropriate order parameter, that at low temperature the monolayer undergoes a liquid-liquid phase transition ending in a critical point in the universality class of the two-dimensional (2D) Ising model. Surprisingly, by reducing the linear size L of the walls, keeping the walls separation h constant, we find a 2D-3D crossover for the universality class of the liquid-liquid critical point for L=h ≃ 50, i.e. for a monolayer thickness that is small compared to its extension. This result is drastically different from what is reported for simple liquids, where the crossover occurs for L=h ≃ 5, and is consistent with experimental results and atomistic simulations. We shed light on these findings showing that they are a consequence of the strong cooperativity and the low coordination number of the hydrogen bond network that characterizes water.

Absolute thermodynamic properties of molten salts using the two-phase thermodynamic (2PT) superpositioning method

We show that the absolute thermodynamic properties of molten salts (mixtures of KCl and LiCl) can be accurately determined from the two-phase thermodynamic (2PT) method that is based on superpositioning of solid-like and gas-like (hard-sphere) vibrational density of states (DoS). The 2PT predictions are in excellent accordance with those from the thermodynamic integration method; the melting point of KCl evaluated from the free energy and the absolute entropy shows close conformity with the experimental/NIST data. The DoS partitioning shows that the Li(+) ions in the eutectic LiCl-KCl molten mixture are largely solid-like, unlike the K(+) and Cl(-) ions, which have a significant gas-like contribution, for temperatures ranging from 773 K to 1300 K. The solid-like states of the Li(+) ions may have practical implications when employed for chemical and nuclear reprocessing applications.

Liquid-liquid transition in a strong bulk metallic glass-forming liquid

Polymorphic phase transitions are common in crystalline solids. Recent studies suggest that phase transitions may also exist between two liquid forms with different entropy and structure. Such a liquid-liquid transition has been investigated in various substances including water, Al2O3-Y2O3 and network glass formers. However, the nature of liquid-liquid transition is debated due to experimental difficulties in avoiding crystallization and/or measuring at high temperatures/pressures. Here we report the thermodynamic and structural evidence of a temperature-induced weak first-order liquid-liquid transition in a bulk metallic glass-forming system Zr(41.2)Ti(13.8)Cu(12.5)Ni10Be(22.5) characterized by non- (or weak) directional bonds. Our experimental results suggest that the local structural changes during the transition induce the drastic viscosity changes without a detectable density anomaly. These changes are correlated with a heat capacity maximum in the liquid. Our findings support the hypothesis that the 'strong' kinetics (low fragility) of a liquid may arise from an underlying lambda transition above its glass transition.

Short range order in liquid pnictides

Liquid pnictides have anomalous physical properties and complex radial distribution functions. The quasi-crystalline model of liquid structure is applied to interpret the three-dimensional structure of liquid pnictides. It is shown that all the column V elements can be characterized by a short range order lattice symmetry similar to that of the underlying solid, the A7 structure, which originates from a Peierls distorted simple cubic lattice. The evolution of the liquid structure down the column as well as its temperature and pressure dependence is interpreted by means of the effect of thermodynamic parameters on the Peierls distortion. Surprisingly, it is found that the Peierls effect increases with temperature and the nearest neighbour distances exhibit negative thermal expansion.

Aerodynamic levitator furnace for measuring thermophysical properties of refractory liquids

The development of novel contactless aerodynamic laser heated levitation techniques is reported that enable thermophysical properties of refractory liquids to be measured in situ in the solid, liquid, and supercooled liquid state and demonstrated here for alumina. Starting with polished crystalline ruby spheres, we show how, by accurately measuring the changing radius, the known density in the solid state can be reproduced from room temperature to the melting point at 2323 K. Once molten, by coupling the floating liquid drop to acoustic oscillations via the levitating gas, the mechanical resonance and damping of the liquid can be measured precisely with high-speed high-resolution shadow cast imaging. The resonance frequency relates to the surface tension, the decay constant to the viscosity, and the ellipsoidal size and shape of the levitating drop to the density. This unique instrumentation enables these related thermophysical properties to be recorded in situ over the entire liquid and supercooled range of alumina, from the boiling point at 3240 K, until spontaneous crystallization occurs around 1860 K, almost 500 below the melting point. We believe that the utility that this unique instrumentation provides will be applicable to studying these important properties in many other high temperature liquids.

Determination of the structure of liquids: an asymptotic approach

Accurate determination of a liquid structure, especially at high temperatures, remains challenging, as reflected in the scatter between different measurements. The experimental challenge is compounded by the process of the numerical transformation from the structure factor to the radial distribution function. The resulting uncertainty is often greater than that required to resolve issues associated with changes in the short-range order of the liquid, such as the existence of liquid–liquid phase transitions or correlations between thermophysical properties and structure. In the present contribution it is demonstrated for liquid bismuth as a model system that the structure factor can be obtained to high accuracy, by comparing several independent measurements in different setups. A simple method is proposed for improving the accuracy of the radial distribution functions, based on the extension of the finite range of momentum transfer,q, in the measured data by analytical asymptotic expressions. A unified mathematical formalism for the asymptotic dependence of the structure factor is developed and the asymptotic form of the Percus–Yevick hard-sphere solution is obtained as a special limiting case. The multiple expressions in the literature are shown to reflect uncertainty in the nature of the repulsive interatomic interaction at short separation distances. Applying this asymptotic method, it is shown that it enables access to details of the fine structure of the liquid and its temperature dependence.

Topological, geometric, and chemical order in materials: insights from solid-state NMR

Unlike the long-range order of ideal crystalline structures, local order is an intrinsic characteristic of real materials and often serves as the key to the tuning of their properties and their final applications. Although researchers can easily assess local ordering using two-dimensional imaging techniques with resolution that approaches the atomic level, the diagnosis, description, and qualification of local order in three dimensions is much more challenging. Solid-state nuclear magnetic resonance (NMR) and its panel of continually developing instruments and methods enable the local, atom-selective characterization of structures and assemblies ranging from the atomic to the nanometer length scales. By making use of the indirect J-coupling that distinguishes chemical bonds, researchers can use solid-state NMR to characterize a variety of materials, ranging from crystalline compounds to amorphous or glassy materials. In crystalline compounds showing some disorder, we describe and distinguish the contributions of topology, geometry, and local chemistry in ways that are consistent with X-ray diffraction and computational approaches. We give examples of materials featuring either chemical disorder in a topological order or topological disorder with local chemical order. For glasses, we show that we can separate geometric and chemical contributions to the local order by identifying structural motifs with a viewpoint that extends from the atomic scale up to the nanoscale. As identified by solid state NMR, the local structure of amorphous materials or glasses consists of well-identified structural entities up to at least the nanometer scale. Instead of speaking of disorder, we propose a new description for these structures as a continuous assembly of locally defined structures, an idea that draws on the concept of locally favored structures (LFS) introduced by Tanaka and coworkers. This idea provides a comprehensive picture of amorphous structures based on fluctuations of chemical composition and structure over different length scales. We hope that these local or molecular insights will allow researchers to consider key questions related to nucleation and crystallization, as well as chemically (spinodal decomposition) or density-driven (polyamorphism) phase separation, which could lead to future applications in a variety of materials.

Native amorphous nanoheterogeneity in gallium germanosilicates as a tool for driving Ga2O3 nanocrystal formation in glass for optical devices

Nanoparticles in amorphous oxides are a powerful tool for embedding a wide range of functions in optical glasses, which are still the best solutions in several applications in the ever growing field of photonics. However, the control of the nanoparticle size inside the host material is often a challenging task, even more challenging when detrimental effects on light transmittance have to be avoided. Here we show how the process of phase separation and subsequent nanocrystallization of a Ga-oxide phase can be controlled in germanosilicates - prototypal systems in optical telecommunications - starting from a Ga-modified glass composition designed to favour uniform liquid-liquid phase separation in the melt. Small angle neutron scattering data demonstrate that nanosized structuring occurs in the amorphous as-quenched glass and gives rise to initially smaller nanoparticles, by heating, as in a secondary phase separation. By further heating, the nanophase evolves with an increase of nanoparticle gyration radius, from a few nm to a saturation value of about 10 nm, through an initial growing process followed by an Ostwald ripening mechanism. Nanoparticles finally crystallize, as indicated by transmission electron microscopy and X-ray diffraction, as γ-Ga(2)O(3)- a metastable gallium oxide polymorph. Infrared reflectance and photoluminescence, together with the optical absorption of Ni ions used as a probe, give an indication of the underlying interrelated processes of the structural change in the glass and in the segregated phase. As a result, our data give for the first time a rationale for designing Ga-modified germanosilicates at the nanoscale, with the perspective of a detailed nanostructuring control.

Mechanisms of network collapse in GeO2 glass: high-pressure neutron diffraction with isotope substitution as arbitrator of competing models

The structure of the network forming glass GeO(2) is investigated by making the first application of the method of in situ neutron diffraction with isotope substitution at pressures increasing from ambient to 8 GPa. Of the various models, the experimental results are in quantitative agreement only with molecular dynamics simulations made using interaction potentials that include dipole-polarization effects. When the reduced density ρ/ρ(0) > or approximately equal to 1.16, where ρ(0) is the value at ambient pressure, network collapse proceeds via an interplay between the predominance of distorted square pyramidal GeO(5) units versus octahedral GeO(6) units as they replace tetrahedral GeO(4) units. This replacement necessitates the formation of threefold coordinated oxygen atoms and leads to an increase with density in the number of small rings, where a preference is shown for sixfold rings when ρ/ρ(0) = 1 and fourfold rings when ρ/ρ(0) = 1.64.