Effect of γ-irradiation on optical and electrical properties of Se1-xTex

Energy-dispersive x-ray diffraction (XD), small-angle x-ray scattering (SAXS) and x-ray absorption fine-structure (XAFS) measurements for metallic fluids up to the supercritical region were carried out using synchrotron radiation. We obtained the structure factor S(k) and the pair distribution functions g(r) for expanded fluid Hg from the liquid to the dense vapour region including the metal-non-metal (M-NM) transition region in the density range from 13.6 to 1.9 g cm-3 by means of XD measurements at SPring-8 in Japan. The density variations of the interatomic distance (r1) and coordination number (N1) obtained are discussed in relation to the M-NM transition in fluid Hg. To investigate structural change in the semiconductor-metal (SC-M) transition in expanded fluid Se, XD measurements, at SPring-8, and XAFS measurements, at the European Synchrotron Radiation Facility (ESRF) in France, were carried out at high temperatures and high pressures. It was found that the twofold-coordinated chain structure is preserved and contraction of the covalent bond occurs on the SC-M transition. XAFS measurements for dense Se vapour near the critical point were also carried out to study how dimers in the rarefied vapour condense to the metallic fluid. SAXS measurements were carried out to obtain information on the density fluctuation of fluid Se near the critical point. On the basis of the structural data obtained by XD, XAFS and SAXS measurements for fluid Se, we discuss how the density fluctuation affects the local structure and electronic properties of fluid Se near the critical point.

https://iopscience.iop.org/article/10.1088/0953-8984/13/20/202/pdf

Structural changes and the metal-non-metal transition in supercritical fluids

Influence of composition and heat treatment on the structure of Se-Te films

We report molecular dynamics simulations of liquid and amorphous Se at pressures from 0 to 6 GPa for different quench rates. The atomic volume depends strongly on the quench rate, in contrast to the small variation of the atomic enthalpy. The glass transition temperature Tg, extrapolated to a quench rate of order K/s, agrees within 20% with experiment. Applying a pressure of 1 GPa, Tg is raised by 37 K.

Computer simulation of liquid and amorphous selenium

Using recently developed codes for density-functional total-energy calculations we trace the structural and electronic response of the hexagonal phase of selenium to applied pressure. We find that the anomalous linear expansion coefficient is well reproduced, and the structure reduces its volume by straightening its twofold-coordinated chains and bringing them closer together. The characteristic overbinding of the local-density approximation causes an effect akin to a spurious pressure on the system rather than a straightforward volume rescaling. The model also predicts a band gap closing rapidly with pressure within the same structural space group. This is not the observed metallization pressure, which in practice is induced by a structural phase transition. We further show that the valence bands are correctly associated with covalent bonds, lone pairs and s-type atomic orbitals, with the lone pairs being the least strongly bound.

https://iopscience.iop.org/article/10.1088/0953-8984/5/43/018/pdf

A theoretical study of selenium I under high pressure

The purpose of this work is to give a theoretical explanation of the nonmetal-to-metal (NM-to-M) transition observed in liquid Se when the system is expanded by increasing temperature in the liquid-vapor supercritical region. The NM-to-M transition accompanying the expansion of the system is unexpected since it is in contradiction to the established understanding of NM-to-M transitions. We introduce structural models for expanded Se and calculate the electronic states and the wave functions of these models by the simulated annealing method. From the results of our calculations, we show that, when some of Se–Se bonds are weakened in the process of expanding the system or equivalently in the process of decreasing the density, the splitting between the bonding and anti-bonding level is reduced and consequently the anti-bonding band is relatively lowered, which leads to the disappearance of the band gap between the anti-bonding and lone-pair band, or in other words to the occurrence of the NM-to-M transition.

Structural and electronic properties of expanded selenium in the liquid-vapor supercritical region

By means of theoretical calculations, we elucidate the mechanism for the nonmetal-to-metal (NM-to-M) transition that occurs with density decrease in supercritical Se. We first show from energetical considerations that some of the bonds in Se chains are disrupted when the density is decreased, and secondly we clarify that the bond disruption causes a drastic reduction of the splitting between the bonding and anti-bonding levels. As a consequence, the energy gap separating occupied and unoccupied levels decreases and eventually disappears, thus altering the nature of the system from nonmetallic to metallic. A remarkable point is that, although the bandwidth (W) reduces on decrease of the density, the degree of the reduction in is so marked that the ratio increases. This feature is in contrast with the traditional Wilson transition, in which the decrease of density has no significant influence on the differences between the corresponding energy levels, such as the spacing between the s level and the p level in the case of expanded Hg, while the decrease of the density narrows the bandwidth W and reduces the ratio as well, and accordingly a metal-to-nonmetal transition is induced. Our calculations also show that structural changes such as shortening of the bond lengths take place at the NM-to-M transition.

Theoretical study of expanded selenium in the supercritical region

To study the relation between the interchain and intrachain interactions as well as to compare the chain-like features with the bulk characteristics in chalcogenide alloys, the Te concentration ( x ) dependence of the optical gap E g is theoretically analyzed for Se 1- x Te x of an isolated chain and a trigonal crystal. In the first place, band structures are calculated for an isolated chain and for a trigonal crystal by the tight-binding approximation. The behaviour of the interaction parameters obtained from our calculations indicates that the chain-like nature of a trigonal crystal becomes less distinguished as the increase of x . Secondly, the electronic density of states of Se 1- x Te x is calculated by the coherent potential approximation. The results show that the optical gap E g for a bulk mixture decreases as x increases while E g for a chain does not change very much for x ≥0.2. These results are in good agreement with the experimental data.

A theoretical approach to the optical gaps of 1D- and 3D-Se1-xTex

By theoretical calculations, we describe the mechanism of the non-metal-to-metal transition on the volume expansion of fluid selenium (Se) in the supercritical region. We first show from energetic considerations that some of the bonds in Se chains are weakened when the volume is expanded, and secondly we show that the bond weakening causes a reduction in the splitting, ΔE σ ∗ −σ ≡E σ ∗ −E σ between the bonding level, Eσ and the anti-bonding level, E σ ∗ . In spite of the fact that the band width, W, is decreased on the volume expansion, the degree of the reduction in ΔE σ ∗ −σ is so large that the ratio (W/ΔE σ ∗ −σ ) increases. At some critical volume, the weakened bonds are disrupted and the energy gap separating the occupied and unoccupied bands disappears, thus bringing the system from non-metal to metal. This feature is in contrast with the well-established mechanisms for the Bloch–Wilson transition and the Mott–Hubbard transtion, in which the volume expansion does not affect the characteristic quantity (the level difference ΔE for the former and the electron correlation energy, U, for the latter, respectively), while the band widths are narrowed, so that both (W/ΔE) and (W/U) decrease and the transition is from metal to non-metal. We also discuss the distribution of electrons in the metallic phase.

Electronic and structural properties of fluid selenium at high pressures and temperatures – a new mechanism of non-metal-to-metal transition

Tight-binding molecular dynamics simulations are carried out for liquid germanium with seven different densities. The obtained pair distribution functions are in good agreement with recent experiments. By studying the positions of the first peaks in the pair distribution functions as well as the bond-angle distribution functions, we clarify the change in the local structure of liquid germanium upon compression. We conclude that when the pressure is increased from the low-pressure region to the high-pressure region, the local structure of liquid germanium changes from a ‘disordered’ β-Sn structure to a structure that is closer to the β-Sn structure.

Toshio Yamaguchi

Papers

Structural and electronic properties of expanded selenium in the liquid-vapor supercritical region

Theoretical study of expanded selenium in the supercritical region

A theoretical approach to the optical gaps of 1D- and 3D-Se1-xTex

Electronic and structural properties of fluid selenium at high pressures and temperatures – a new mechanism of non-metal-to-metal transition

Simulational study of liquid germanium under pressure