Phase Separation and Solidification of Fluid Phosphorus
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0987-PP02-04
Phase Separation and Solidification of Fluid Phosphorus Yoshinori Katayama1, Yasuhiro Inamura2, Hiroyuki Saitoh1, and Wataru Utsumi1 1 Synchrotron Radiation Research Center, Quantum Beam Science Directorate, Japan Atomic Energy Agency, Sayo, Hyogo, 679-5148, Japan 2 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195, Japan
ABSTRACT Fluid phosphorus exhibits a macroscopic phase separation during a pressure-induced first-order phase transition between low-density fluid and high-density fluid. Solidification process of the phase-separated fluid sample was monitored by x-ray radiography and the obtained solid sample was investigated by optical and scanning electron microscopy. A solid mixture of black parts and red parts was obtained from the phase-separated sample. Three different morphologies were observed in the red part of the sample. INTRODUCTION It is well known that phosphorus has many allotropes in the solid state. White phosphorus (white P) is a molecular solid which consists of P4 tetrahedral molecules. Black phosphorus (black P) has a layer structure. Red phosphorus (red P) is usually amorphous in which 3-fold coordinated P atoms form a three-dimensional network structure. Black P is most stable at ambient condition while white P is metastable. White P melts at 44 °C and the molten white P is a molecular liquid. It transforms to red P when it is heated at several hundreds °C. Red P and Black P melt around 600 °C. We carried out in-situ high-pressure high-temperature x-ray diffraction experiments on molten black P in GPa region and revealed that there are two distinct fluid phases [1]. The lowpressure, low-density fluid (LDF), which was observed below 1 GPa, was assigned as a molecular fluid that consists of P4 molecules because structure factor, S(Q), resembled that of molten white P[2]. The structure factor of the high-pressure, high-density fluid (HDF) was similar to those of red P [3] and liquid arsenic [4]. The similarity indicates that P4 molecules dissociate and form a polymeric fluid in HDF. These assignments were also based on the features of radial distribution functions and results of a first-principles molecular dynamics simulation on polymerization transition of liquid phosphorus [5]. Later studies suggested that the network structure is not rigid and that HDF has metallic character [6-8]. Thus it may be better to describe HDF as a Peierls-distorted fluid which is characterized by a low coordination number [4]. The difference in density between the two phases was first suggested by a discontinuous change of slope of the melting curve as a function of pressure [9, 10] and then verified by a direct measurement of density by means of x-ray absorption [6]. The difference in density is about 40 % of the density of HDF. A change in pressure induced a sharp and reversible transition between the two fluid phases [1]. The scenario of the transition, transition between the molecular fluid and the
polymeric fluid, is supported by several first-principles m
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