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

为了阐明流体Se在液-气临界点（Se的临界数据; T_c= 1590 ℃，P_c= 380 bar，d_c= 1.85 g/cm ^3）附近发生的超导-金属转变的机理，我们对流体Se在1500 ℃和815 bar的温度和压力范围内进行了X射线衍射测量。我们开发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样品池。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关系。从对分布函数的详细分析中，我们得出结论，在光学带隙消失的区域，在很大程度上保留了双重配位结构，电导率的逐步增加变得明显。这与传统的理论形成对比，传统的理论认为，从两重到三重配位结构的结构变化引起的电子-金属转变。在此基础上，我们推测存在的小分子链与平面锯齿形构象，这可能会导致过渡到金属状态的流体Se。此外，我们已经应用我们的实验技术的测量膨胀流体Hg上没有结构的研究已经取得了迄今为止。我们测量了密度在13.55 ~ 6.8 g/cm ^3之间的流体汞的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.