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In situ synchrotron X-ray diffraction and absorption studies elucidated the existence of two liquid states of SnI4 with different structures and densities under pressure. Here, in situ XAFS measurements help shed additional light on a difference in physicochemical features between the two liquid states on a molecular length scale. [ABSTRACT FROM AUTHOR]

SnI4 and GeI4 have been confirmed to have another liquid state appearing on compression. To identify the microscopic pathway from the low- to high-pressure liquid states, the structure of these liquids in the appropriate thermodynamic regions was analyzed using a reverse Monte Carlo method. The occurrence of pressure-induced symmetry lowering of molecules, from regular tetrahedral to ammonia-like pyramidal symmetry, was then recognizable in these systems. This symmetry lowering is reflected in the change in shape of the molecular form factor. The latter change occurs abruptly near the expected transition pressure in liquid SnI4, whereas it proceeds gradually in GeI4. This is consistent with our observation that SnI4 seems to undergo a first-order liquid–liquid transition, whereas the transition seems to end up with a crossover in liquid GeI4. Interestingly, when the molecular density becomes high, it is possible for the two-body intermolecular interaction to have a double-minimum character, which offers two characteristic length scales corresponding to two liquid states with different densities. However, quantum chemical calculations show that molecular deformation for this type of symmetry lowering results in an increase in electronic energy, which leaves the problem of the physical origin for this anisotropic deformation. We speculate that this symmetry lowering occurs as a precursor to the whole change in the liquid structure. [ABSTRACT FROM AUTHOR]

SnI4 and GeI4 have been confirmed to have another liquid state appearing on compression. To identify the microscopic pathway from the low- to high-pressure liquid states, the structure of these liquids in the appropriate thermodynamic regions was analyzed using a reverse Monte Carlo method. The occurrence of pressure-induced symmetry lowering of molecules, from regular tetrahedral to ammonia-like pyramidal symmetry, was then recognizable in these systems. This symmetry lowering is reflected in the change in shape of the molecular form factor. The latter change occurs abruptly near the expected transition pressure in liquid SnI4, whereas it proceeds gradually in GeI4. This is consistent with our observation that SnI4 seems to undergo a first-order liquid–liquid transition, whereas the transition seems to end up with a crossover in liquid GeI4. Interestingly, when the molecular density becomes high, it is possible for the two-body intermolecular interaction to have a double-minimum character, which offers two characteristic length scales corresponding to two liquid states with different densities. However, quantum chemical calculations show that molecular deformation for this type of symmetry lowering results in an increase in electronic energy, which leaves the problem of the physical origin for this anisotropic deformation. We speculate that this symmetry lowering occurs as a precursor to the whole change in the liquid structure. [ABSTRACT FROM AUTHOR]

The low-pressure crystalline phase of a molecular crystal, SnI4, has a rising melting curve that breaks abruptly at around 1.5 GPa, beyond which it becomes almost flat, with a slight maximum at about 3 GPa. Although the overall aspect of this melting curve can be captured by the Kumari-Dass-Kechin equation, the values for the parameters involved in the equation were definitely different from those predicted on the basis of the Clapeyron-Clausius relationship. On the other hand, the accuracy of our experimental data prevented us from judging whether the parameters are derivable from the Lindemann melting law, as shown independently by Kumari and Dass, and by Kechin. The Kraut-Kennedy and Magalinskii-Zubov relationships seem to be valid in the low-pressure region where the melting curve is rising. The breakdown of these relationships suggests a qualitative change in the intermolecular interaction upon compression, thereby making the melting behavior unusual. [ABSTRACT FROM AUTHOR]

We have shown from in situ synchrotron x-ray diffraction measurements that there are two thermodynamically stable liquid forms of SnI4, depending on the pressure. Based on the liquid-liquid critical point scenario, our recent measurements suggest that the second critical point, if it exists, may be located in a region close to the point at which the melting curve of the crystalline phase abruptly breaks. This region is, unlike that of water, experimentally accessible with relative ease. [ABSTRACT FROM AUTHOR]

The discovery of a first-order phase transition in fluid phosphorus aroused renewed interest in polyamorphism in liquids with a locally tetrahedral molecular structure. We have performed in situ synchrotron x-ray diffraction measurements on tin tetraiodide, which consists of SnI4 tetrahedral molecules at ambient pressure, and established that the liquid forms existing above and below 1.5 GPa, where the slope of the melting curve of the solid phase changes abruptly, have different structures. This discovery offers evidence of thermodynamically stable polyamorphism in general compounds as well as in elements. A possible phase diagram that includes the two amorphous states already found is proposed based on the pseudobinary regular solution model. The vertex-to-face orientation between the nearest molecules plays a key role in the transition from the low-pressure to the high-pressure liquid phase. [ABSTRACT FROM AUTHOR]

Synchrotron x-ray diffraction measurements were performed on liquid SnI4 up to a scattering vector of 25 A-1, utilizing a horizontal two-axis diffractometer installed at the SPring-8 bending magnet beam line BL04B2 in Japan. An effective method based on the maximum entropy method was devised to transform the measured total structure factor to the reduced radial distribution function. The reliability of the density estimation is discussed. [ABSTRACT FROM AUTHOR]

Synchrotron x-ray diffraction measurements were performed on liquid SnI4 up to a scattering vector of 25 A-1, utilizing a horizontal two-axis diffractometer installed at the SPring-8 bending magnet beam line BL04B2 in Japan. An effective method based on the maximum entropy method was devised to transform the measured total structure factor to the reduced radial distribution function. The reliability of the density estimation is discussed. [ABSTRACT FROM AUTHOR]

The location of the liquidus in the low-pressure crystalline phase of SnI4 was determined utilizing in situ x-ray diffraction measurements under pressures up to approximately 3.5 GPa. The liquidus is not well fitted to a monotonically increasing curve such as Simon's equation, but breaks near 1.5 GPa and then becomes almost flat. The results are compared to those from molecular dynamics simulations. Ways to improve the model potential adopted in the simulations are discussed. [ABSTRACT FROM AUTHOR]

The location of the liquidus in the low-pressure crystalline phase of SnI4 was determined utilizing in situ x-ray diffraction measurements under pressures up to approximately 3.5 GPa. The liquidus is not well fitted to a monotonically increasing curve such as Simon's equation, but breaks near 1.5 GPa and then becomes almost flat. The results are compared to those from molecular dynamics simulations. Ways to improve the model potential adopted in the simulations are discussed. [ABSTRACT FROM AUTHOR]