A first-order liquid-liquid phase transition in phosphorus

First-order structural phase transitions are common in crystalline solids, whereas first-order liquid-liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances. But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water and liquid carbon. Here we report an in situ X-ray diffraction observation of a liquid-liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms. In addition to a known form of liquid phosphorus--a molecular liquid comprising tetrahedral P4 molecules--we have found a polymeric form at pressures above 1 GPa. Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form. The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 0.02 GPa. During the transformation, the two forms of liquid coexist. These features are strongly suggestive of a first-order liquid-liquid phase transition.

The Phase Boundary Between agr- and beta-Mg2SiO4 Determined by in Situ X-ray Observation.

The stability of Mg(2)SiO(4), a major constituent in the Earth's mantle, has been investigated experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800 degrees to 1600 degrees C. The phase stability of alpha-Mg(2)SiO(4) and beta-Mg(2)SiO(4) was determined by taking account of the kinetic behavior of transition. The phase boundary between alpha-Mg(2)SiO(4) and beta-Mg(2)SiO(4) is approximated by the linear expression P = (9.3 +/- 0.1) + (0.0036 +/- 0.0002)T where P is pressure in gigapascals and T is temperature in degrees Celsius.

In situ XAFS and XRD studies of pressure-induced local structural change in liquid AgI.

In order to examine the pressure-induced structural change of liquid silver iodide (AgI), high-pressure and high-temperature in situ x-ray absorption fine structure (XAFS) and x-ray diffraction (XRD) studies have been carried out up to 1200 K and 6 GPa. The modifications in the x-ray absorption near edge structure (XANES) spectra and x-ray structure factors, S(Q), with increasing pressure provide evidence for changes in the short-range order of liquid AgI. The I-Ag bond length in liquid AgI increases by compression up to 2 GPa, which proposes that components with higher coordination than fourfold are introduced. The I-Ag bond length decreases monotonically with compression above 2 GPa, indicating that the structural change involving a coordination-number change is completed below 2 GPa and then a high-pressure form of liquid AgI is stabilized. Comparing the I-Ag bond lengths of liquid AgI with those of crystalline phases, we conclude that the high-pressure form of liquid AgI has a rocksalt-like structure with large vacancies.

Molecular-network-ionic structure transitions in liquid AlCl(3) and ZnCl(2) halogenides under pressure.

We present the in situ high-pressure-high-temperature x-ray diffraction study of the liquid AlCl(3) and ZnCl(2) halogenides having a quasi-molecular network structure in liquid state at normal pressure. These liquids are intermediate between pure covalent and ionic melts. Structural study of these liquid halogenides is indicative of a rapid and strong breakdown of an intermediate-range order in a tetrahedral network of melts for the initial pressure range, 0-2.5 GPa for AlCl(3) and 0-1.8 GPa for ZnCl(2), and points to rather sharp transitions in liquids with the formation of a short-range order structure similar to ionic melt structures around 4 GPa for AlCl(3) and 3 GPa for ZnCl(2). Thus, pseudo-covalent liquid halogenides like AlCl(3) and ZnCl(2) provide testimony to two phenomena under high pressures, namely, a gradual decay of structural correlations in the tetrahedral network of the melt and a sharp transition from molecular-network to ionic structure in liquid on further compression. Such a two-stage structural transformation under pressure is the general feature for a wide class of simple melts, including most of the pseudo-covalent halogenides.

A cubic-anvil high-pressure device for pulsed neutron powder diffraction.

A compact cubic-anvil high-pressure device was developed for in situ neutron powder diffraction studies. In this device, a cubic shaped pressure medium is compressed by six anvils, and neutron beams pass through gaps between the anvils. The first high-pressure experiment using this device was conducted at J-PARC and clearly showed the neutron diffraction patterns of Pb. Combining the cubic-anvil high-pressure device with a pulsed neutron source will prove to be a useful tool for neutron diffraction experiments.

Transformation in intermediate-range structure of vitreous silica under high pressure and temperature.

The pressure dependence of the structure factor for vitreous silica was measured up to 8.3 GPa at 500 °C by means of an in situ x-ray diffraction method. The position of the first sharp diffraction peak (FSDP) moved to a higher momentum transfer as the pressure increased. It moved rapidly between 3.3 and 5.8 GPa, and then the slope of the FSDP position as a function of pressure decreased. The pressure dependence of the position is the same as that for a sample heated to 500 °C after room-temperature compression. On decompression at 500 °C, the position of the FSDP showed hysteresis. The pressure dependence of the FSDP position suggests that the permanent densification of vitreous silica is realized due to preservation of the intermediate-range structure stabilized at high pressure and temperature.

Structural changes of quartz-type crystalline and vitreous GeO2 under pressure.

Using a large volume high pressure apparatus, quartz-type crystalline GeO2 and Li2O-4GeO2 glass have been compressed up to 14 GPa at room temperature and their local structural changes have been investigated by an in-situ XAFS method. In quartz-type crystalline GeO2, the change of the coordination number from 4 to 6 begins above 8 GPa and finishes below 12 GPa. On decompression, reversal transition begins below 8 GPa and there is a large hysteresis. Almost no sixfold coordination of Ge is preserved after releasing pressure. Change of coordination number in vitreous Li2O-4GeO2 begins above 6 GPa and is completed below 10 GPa. Reversal transition begins below 10 GPa and the hysteresis is smaller than that of quartz-type GeO2. Change of coordination number in vitreous Li2O-4GeO2 is also reversal.

The postspinel phase boundary in Mg2SiO4 determined by in situ X-ray diffraction

The phase boundary between spinel (gamma phase) and MgSiO3 perovskite + MgO periclase in Mg2SiO4 was determined by in situ x-ray measurements by a combination of the synchrotron radiation source (SPring-8) and a large multianvil high-pressure apparatus. The boundary was determined at temperatures between 1400 degrees to 1800 degreesC, demonstrating that the postspinel phase boundary has a negative Clapeyron slope as estimated by quench experiments and thermodynamic analyses. The boundary was located at 21.1 (+/-0.2) gigapascals, at 1600 degreesC, which is approximately 2 gigapascals lower than earlier estimates based on other high-pressure studies.