The x-ray structure factor of molten TiO 2 has been measured for the first time, enabled by the use of aerodynamic levitation and laser beam heating, to a temperature of T = 2250(30) K. Ti-O coordination number in the melt is close to n TiO = 5.0(2), with modal Ti-O bond length r TiO = 1.881(5) Å, both values being significantly smaller than for the high temperature stable Rutile crystal structure (n TiO = 6.0, r TiO = 1.959 Å). The structural differences between melt and crystal are qualitatively similar to those for alumina, which is rationalized in terms of the similar field strengths of Ti 4+ and Al 3+. The diffraction data are used to generate physically and chemically reasonable structural models, which are then compared to the predictions based on various classical molecular dynamics (MD) potentials. New interatomic potentials, suitable for modelling molten TiO 2 , are introduced, given the inability of existing MD models to reproduce the diffraction data. These new potentials have the additional great advantage of being able to predict the density and thermal expansion of the melt, as well as solid amorphous TiO 2 , in agreement with published results. This is of critical importance given the strong correlation between density and structural parameters such as n TiO . The large thermal expansion of the melt is associated with weakly temperature dependent structural changes, whereby simulations show that n TiO = 5.85(2) -(3.0(1) x 10 -4 )T (K, 2.75 Å cut-off). The TiO 2 liquid is structurally analogous to the geophysically relevant high pressure liquid silica system at around 27 GPa. We argue that the predominance of 5-fold polyhedra in the melt implies the existence of as yet undiscovered TiO 2 polymorphs, based on lowerthan-octahedral coordination numbers, which are likely to be metastable under ambient conditions. Given the industrial importance of titanium oxides, experimental and computational searches for such polymorphs are well warranted.

Temperature-dependent measurements of the X-ray structure factor of molten Na2B4O7 reveal a continuous structural transition. We demonstrate that the thermodynamic model of ideal associated solutions is capable of predicting this evolution of melt structure, between a low density, depolymerized melt at ≳300 K above the liquidus, toward a dense, polymerized melt close to the glass transition. This temperature-dependent nature of melt structure is predicted to be strongly composition-dependent, with the B–O coordination number depending on temperature only in the range 20–50 mol % Na2O, which appears to be manifest in the broad maximum observed in the glass-transition temperatures. We discuss the ramifications of these findings for the application of topological constraint theory, with relevance to industrial glass design and manufacture, crystal growth from melts of nonlinear optical materials, geochemistry, and the understanding of melt fragility and the glass transition.

diffusion remains below 10 −4 nm 2 s −1. In the liquid state at 3270 K, the rates of U and O diffusion are of similar magnitude (3.7 versus 9.3 nm 2 s −1). Manara et al. measured a melting slope (dT m /dP) that is a factor 1.5 to 2 steeper than expected from the recommended volume change on melting and enthalpy of fusion values (11, 16). The rapid O exchange in these MD models results in a relatively low enthalpy of fusion, consistent with a relatively steep melting slope. Using pressure , an 8-coordinated UO 2 melt can be simulated at a number density of 0.074 Å −3. The 8-coordinated melt has U and O diffusion rates slower by a factor of 3 (1.2 and 3.8 nm 2 s −1) than the low-coordinated melt, demonstrating the strong effect of local structure on the physical properties of this melt. The Andrade theory, for example, is often used to predict melt viscosity but assumes that the melt structure closely resembles that of the solid (17, 18). Portions of the hot solid and liquid UO 2 MD simulations illustrate the large oxygen disorder above the lambda transition and the different UO 6,7 coordination species that predominate in the melt (Fig. 3C). The structure and optimized interatomic potentials for UO 2 allow for accurate atomistic multiscale modeling. The x-ray data are important as an end-member benchmark for models of multicomponent systems, including corium melts and high-level waste glasses (11). (6212), 987-991. 346 Science , this issue p. 987 Science mice could prove valuable for preliminary screens of candidate therapeutics and vaccines. virus was associated with distinct genetic profiles in inflammation, blood coagulation, and vascular function. This panel of series of painstaking experiments performed under stringent biosafety conditions. Resistance and susceptibility to Ebola tested the effects of Ebola virus in mice with defined genetic backgrounds in a et al. as those of humans. Rasmussen Apart from monkeys, there are no animal models available that show the same symptoms of Ebola virus infection Variety of Ebola symptoms in mice ARTICLE TOOLS

Liquid Al 2 O 3 has been supercooled more than 500 K below its melting point (T m = 2,327 K) using aerodynamic levitation and laser heating techniques. High energy synchrotron x-ray measurements were performed over a temperature range of 1,817 ≤ T (K) ≤ 2,700 and stroboscopic neutron diffraction at 1,984 and 2,587 K. The diffraction patterns have been fitted with Empirical Potential Structure Refinement (EPSR) models and compared to classical Molecular Dynamics (MD) simulation results. Both sets of models show similar trends, indicating the presence of high populations of AlO 4 and AlO 5 polyhedral units predominantly linked by triply shared oxygen atoms. EPSR reveals that the mean Al-O coordination number changes linearly with temperature with n AlO = 4.41-[1.25 × 10 −4 ] (T-T m), with a 2.5 Å cutoff. Both EPSR and MD simulations reveal a direction of the temperature dependence of the aluminate network structure which moves further away from the glass forming ideal (n AlO = 3) during supercooling. Furthermore, we provide new experimental data and models for amorphous alumina grown by sequential infiltration synthesis of a polymer template. The amorphous solid form likely has a larger Al-O coordination number than the liquid, consistent with expectations for the hypothetical glass.

Using high energy x-ray diffraction, the structure factors of glassy and molten B2O3 were measured with high signal-to-noise, up to a temperature of T  =  1710(20) K. The observed systematic changes with T are shown to be consistent with the dissolution of hexagonal [B3O6] boroxol rings, which are abundant in the glass, whilst the high-T (>~1500 K) liquid can be more closely described as a random network structure based on [BO3] triangular building blocks. We therefore argue that diffraction data are in fact qualitatively sensitive to the presence of small rings, and support the existence of a continuous structural transition in molten B2O3, for which the temperature evolution of the 808 cm−1 Raman scattering band (boroxol breathing mode) has long stood as the most emphatic evidence. Our conclusions are supported by both first-principles and polarizable ion model molecular dynamics simulations which are capable of giving good account of the experimental data, so long as steps are taken to ensure a ring fraction similar to that expected from Raman spectroscopy. The mean thermal expansion of the B-O bond has been measured directly to be αBO  =  3.7(2)  ×  10−6 K−1, which accounts for a few percent of the bulk expansion just above the glass transition temperature, but accounts for greater than one third of the bulk expansion at temperatures in excess of 1673 K.