The semiconductor-metal transition and the structural change in fluid selenium near the liquid-gas critical point.

为了阐明液体Se在气液临界点附近发生半导体-金属转变的机制(Se的临界点T_c=1590ﾟC，P_c=380bar，d_c=1.85g/cm~3)，我们在1500ﾟC和815bar的温度和压力范围内对液体Se进行了X射线衍射测量。我们研制了一种新型的高压容器和一种由我们自己设计的单蓝宝石制成的新型样品盒。对于高温高压下的X射线衍射测量，我们采用了能量色散方法，得到了结构因子和对分布函数随温度和压力的变化。通过对对分布函数的详细分析，我们得出结论：在光学带隙消失和电导率逐渐增加的区域，双配位结构基本保持不变。这与传统的认为半导体-金属转变是由二配位结构转变为三配位结构引起的理论不同。在这些发现的基础上，我们推测了具有平面之字形构象的小分子的存在，它可能导致流体Se向金属状态的转变。此外，我们还将我们的实验技术应用于迄今尚未进行结构研究的膨胀流体Hg的测量。我们测量了密度在13.55-6.8g/cm^3范围内的流体汞的X射线衍射。

In order to clarify the mechanism of the semiconductor-metal transition in fluid Se which occurs near the liquid-gas critical point (critical data of Se ; T_c= 1590ﾟC, P_c= 380 bar, d_c= 1.85 g/cm^3) , we have performed X-ray diffraction measurements for fluid Se in the temperature and pressure range up to 1500ﾟC and 815 bar. We have developed a new type of high-pressure vessel and a new sample cell made of single-sapphire of our own design. For the X-ray diffraction measurements at high temperatures and pressures we have adopted an energy-dispersive method.We have obtained the structure factor and the pair distribution function as functions of temperature and pressure. From the detailed analysis of the pair distribution function we conclude that the twofold coordinated structure is largely preserved in the region where the optical gap disappears and the progressive increase of conductivity becomes evident. This contrasts with the conventional theory that the semiconductor-metal transition is induced by the structural change from the twofold to threefold coordinated structure. On the basis of these findings we speculate on the existence of the small chain molecule with planer zigzag conformation which may cause the transition to the metallic state in fluid Se.In addition, we have applied our experimental technique to the measurements for expanded fluid Hg on which no structural studies have been made so far. We have measured X-ray diffraction for fluid Hg in the density range from 13.55 to 6.8 g/cm^3.