Liquids, diffraction supercooled

Electron Diffraction Study of the Structure of Supercooled Liquid Bismuth. J. Phys. Soc. Japan 11, 396—404 (1956). 

The X-ray diffraction measurements were performed with an X-ray diffractometer (Bruker D8 Discover) in a standard Bragg-Brentano reflection configuration. In a typical experiment the temperature of the cell containing the emulsified sample was decreased from ambient to 173 K at a rate of 10 K mm In order to measure the temperature at which the supercooled liquid droplets homogenously froze to ice, the diffraction angle of a... 

Wasse JC, Salmon PS (1999) Stmcture of molten lanthanum and cerium tri-halides by the method of isomorphic substitution in neutron diffraction. J Phys Cond Matter 11 1381-1396 Iwadate, Suzuki, K, Onda, N et al (2006) Structural changes on supercooling liquid silicon. J AUoys Compd 408 12 248-252... 

Many experiments have been performed to test the various hypotheses discussed in the previous section, but there is as yet no widespread agreement on which physical picture, if any, is correct. The connection between liquid and the two amorphous forms predicted by the LLPT hypothesis is difficult to prove experimentally because supercooled water freezes spontaneously below the nucleation temperature Tw, and amorphous ice crystallizes above the crystallization temperature Tx [32,33]. Crystallization makes experimentation on the supercooled liquid state between Th and Tx almost impossible. However, comparing experimental data on amorphous ice at low temperatures with those of liquid water at higher temperatures allows an indirect discussion of the relationship between the liquid and amorphous states. It is found from neutron diffraction studies [10] and simulations that the structure of liquid water changes toward the LDA structure when the liquid is cooled at low pressures and changes toward the HDA structure when cooled at high pressures, which is consistent with the LLPT hypothesis. Because their entropies are small, the two amorphous states are presently considered to be smoothly connected thermodynamically to the liquid state [34]. 

If the temperature of a supercooled liquid can be reduced far enough below the normal freezing point without crystallization occurring, the specific heat and thermal expansivity may be observed to decrease abrupdy over a narrow range of temperature. Below this temperature range the material is in a vitreous or glassy state and possesses many of the physical properties of solids while retaining the amorphous X-ray diffraction pattern of a liquid. Vitrification has been observed in all types of liquids and is an important aspect of the thermal behavior of polymers. 

In 2000 Katayama et al. [35] conducted an in situ X-ray dillraction experiment on liquid phosphorus which strongly suggested a first order liquid-liquid phase transition between a molecular liquid and a polymeric liquid. This was particularly notable because not only did it occur in the stable liquid, as opposed to the more conunon metastable supercooled liquid, it also demonstrated coexistence of the two phases. This was clearly visible from a weighted sum of the diffraction patterns from either side of the transition compared to the diffraction pattern at the transition (Fig. 2.9). The coexistence was also apparent from in situ X-ray radiography [36] and had been suggested by ab initio MD simulations [54], However a further, more extensive, in situ X-ray diffraction investigation by Monaco et al. [53] established that the transition was in fact between a polymeric liquid and a molecular fluid the latter being the fluid form associated with the metastable white phosphorus crystal, which takes a molecular tetrahedral structure. Therefore while undoubtedly a first order liquid-fluid phase transition, phosphorus did not provide the first evidence for a first order liquid-liquid phase transition. 

The atomic sizes of the constituent elements in the ternary R-Al-M amorphous alloys differ significantly. Therefore, the interpretation of the total radial distribution function (RDf) obtained by the ordinary X-ray diffraction method is complicated, and it is extremely hard to obtain structural parameters for each independent pair of elements. By using the anomalous X-ray scattering (AXS) method with which the structural environment around a particular constituent element can be determined, it is expected that this difference is observed and the structural environment around Ni in the amorphous La55Al25Ni2o alloy is estimated in as-quenched, annealed (in the supercooled liquid region) and crystallized states. From these systematic AXS measurements, the structural changes due to crystallization were discussed. 

The structural properties of a second, apparently amorphous phase (all) of the molecular glass former triphenyl phosphite were studied by means of multidimensional solid-state NMR spectroscopy and X-ray diffraction. Phase all was prepared by annealing the supercooled liquid in the temperature range 210 K < T < 230 K. In addition to ID H and P spectra and Ti data, P radio-frequency-driven spin-diffusion exchange spectroscopy were used to analyse the arrangement of neighboring TPP molecules on both a local and intermediate scale. ... 

At lower temperatures, a number of liquid-crystalline transitions may occur which again can be recorded by DSC/DTA as exothermic first-order transitions (Fig. 10.22). Hot-stage microscopy and X-ray diffraction are used to determine the nature of these transitions. At lower temperatures, solid crystals may be formed. The latter are revealed by DSC/DTA as an exothermic first-order transition, by TMA as an increase in sample stiffness and by X-ray diffraction as sharp Bragg reflections. Some liquid-crystalline polymers, e.g. copolyesters, are supercooled to a glassy state without crystallizing (Fig. 10.22). 

By introducing an anisotropic contribution to the intermolecular interaction potential, the isotropic liquid gets enriched in structures abundant in bond orientational order (BOO) at temperatures below the melting point. Unfortunately, direct observation of BOO in supercooled liquids by classic diffraction techniques, like X-rays, electron or neutron diffraction, has so far failed. 

Despite the interest raised by medium ordered structures in the last decades, direct experimental evidence of their presence in supercooled liquids has not been achieved so far by means of diffraction techniques like X-rays, electron or neutron diffraction. In 2010, Zeng and co-workers obtained indirect evidence of large ordered crystalline structures in metallic glasses, [11] whose presence was deduced by the pressure induced crystallization of the entire system. However the detection of the incipient, local cluster was not possible. This was explained by the fact, proved by classical molecular simulations [11], that the structure factor of the glass with or without crystalline structures is virtually undistinguishable. Such result would also explain the previous failures. 

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