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d in electrical conductivity data by Hoshino and co-workers.3,4 Figure 1 shows the contours of constant de conductivity plotted on the p r e s s u r e - t e m p é r a t u r e (P-T) plane.5'6 The high-conductivity région appears in the immédiate vicinity of the critical point. Another indication of the SC-M transition was obtained from the measurement of the optical reflectivity by Seyer et al.7 Drude-like energy dependence of the reflectivity spectra was observed at elevated pressures and températures. Récent optical reflectivity measurements by Ikemoto et al.8 confirm the appearance of metallic behavior near the critical point. Measurements of the optical absorption coefficient in the highabsorption range 9 hâve also shown that the région where the optical gap disappears is located near the critical point, as shown by the bold chain line in Figure 1. From the measurements of viscosity, 10 magnetic susceptibility," and nuclear magnetic résonance ( N M R ) ' near the melting point, an average chain length of 104-105 atoms can be estimated. At sufficiently high température and pressure, near the critical point, the average chain molécule contains only about 10 atoms.f' The characteristic features of the SC-M transition in fluid Se are as follows. First, the transition occurs with volume expansion. Second, the transition occurs when the chain length becomes very

short, which seems to correlate with the instability of the chain structure. Arrow 1 shown in Figure 1 dénotes a typical process from the semiconducting liquid to the metallic fluid at high pressure. A distinct behavior accompanying the SC-M transition is also observed in its t h e r m o d y n a m i c properties. The isochores (constant-volume contours) start to bend at températures above 1100°C and pressures above 400 bar, 21213 defining a contour of constant de conductivity of 30 Cl cm" 1 (Figure 1). The bending of isochores suggests the onset of a transition to a densely packed structure. It has been speculated that a large number of structural defects, such as threefold-coordinated centers in the branched chain, appear. 12 On the basis of a model proposed by Cabane and Friedel,14 neutral threefold-coordinated centers hâve been considered to play a crucial rôle in the mechanism of the metallic state in fluid Se. It is obvious that structural data are important for understanding the mechanism of the SC-M transition of fluid Se. Edeling and Freyland' 5 used neutron diffraction to investigate the static structure of liquid Se in the température range of 600-1400°C at pressures up to 265 bar. Their data gave no indication of the existence of a significant concentration of

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Figure 1. The contours of constant de conductivity in liquid and supercritical fluid Se35 plotted in the pressure température phase diagram. The illustration is taken from Endo et al. and Warren and Dupree56 The broken curve is the saturated vaporpressure curve and the circle at the end dénotes the criti