Electrical And Optical Properties Of HgX (X=S, Se And Te) Under High Pressure

With the development of science and technology, more and more in situ experiments can be performed in diamond anvil cell (DAC), such as synchrotron X-ray diffraction, neutron scattering, Mossbauer spectrum, laser Raman scattering, photoluminescence, optical absorption, Hall effect and so on. These increasingly matured techniques have led to a great improvement in the research of high pressure physics. Meanwhile, the technique fabricated a microcircuit on a DAC has rapidly been developing. In this thesis, using the microcircuit fabricated on a DAC, we have performed in-situ electrical measurements under high pressure on the samples of HgX (X=S, Se and Te).For the electrical resistance or conductivity measurements in a DAC, there are two ways of the probe arrangements: manual arrangement and integration method by using thin film sputtering and photolithographying techniques. The way of probe arrangement adopted in our electrical measurement is the second method. It exhibits some outstanding advantages as following: (1) the sputtered probes can be designed with various regular shapes by photolithographying and exactly placed at a desirable place on the diamond culet; (2) the probes can remain unchanged under high pressure. These advantages make it possible for the accurate measurement of conductivity in DAC.van der Pauw method is adopted in our experiment. This arrangement can eliminate most of contact resistance between the sample and the probes. In our microcircuit we chose molybdenum (Mo) for the conductor and alumina (Al2O3) for the insulator and protective materials in the resistivity measurement.The conductivity is an important property of materials under high pressure, which may imply the phase transitions or some clues of the changes in the electronic structures. According to the order of magnitude of conductivity, one can judge the type of conduction of electricity, and whether the metallization happens or not. A abrupt change in conductivity often corresponds to a structure transition and a change in the distribution of electrons. In addition, the temperature dependence of conductivity can be severed as a tool in the measurement of energy gap of a semiconductor.For the mercury chalcogenides, despite their structures are identical or similar to that of the zinc chalcogenides under ambient pressure, they have different properties. Firstly, the mercury chalcogenides are semimetals, however, the zinc chalcogenides are semiconductors with the wider energy gap. Secondly, the sequence of the pressure-induced phase transition is different. From the view of structure, 5d electrons play an important role in the properties of the mercury chalcogenides.α-HgS is a semiconductor with the energy gap of 2.1eV, and its structure consists of atom in parallel helical chains. Above 8GPa, the conductivity increases rapidly with pressure, and the conductivity also increases with temperature, indicating a transport characteric of a semiconductor. Above 29GPa, the conductivity does almost not increase with pressure, and the conductivity decreases with temperature, indicating a transport characteric of a metal. It is confirmed that the metallization pressure is about 29GPa.At ambient conditions,β-HgS is a semimetal. It was observed that the black sample changes into the red sample at 5.0GPa, indicating thatβ-HgS transform toα-HgS. Our experimental result displays that the conductivity drops abruptly by several orders of magnitude from ambient to 5GPa; and that the conductivity increases rapidly with pressure above 5.0GPa. The order of magnitude of conductivity and the temperature dependence of conductivity show that the sample exhibits a transport perporty of a metal above 27GPa. So we ascertain that the metallization pressure is 27GPa by the method of measuring conductivity.HgSe is a semimetal with the energy band overlap at ambient conditions. The sequence of the pressure-induced phase transition is from zincblende-structure to cinnabar-structure to rocksalt-structure to orthorhombic-structure, corresponding to the transport perporty change from semimetal to semiconductor to metal at 1.0GPa and 16GPa, respectively. Our in-situ conductivity measurement shows that the electrical perporty of the sample transports from a semimetal to a semiconductor to a metal, corresponding to the structure transition.Our experimental result shows that the change of conductivity of HgTe with pressure is similar to that of HgSe. The conductivity drops abruptly by 4~5 orders of magnitude from 1.0 to 2.0GPa, corresponding to the transition from ZB- to the cinnabar structure. Within the pressure between 2.0 and 8.0GPa, the conductivity increases smoothly with pressure, indicating a transport property of a semiconductor. And the conductivity increases rapidly by 5 orders of magnitude from 8.0 to 10.0GPa. So we ascertained that the metallization pressure of HgTe is about 10.0GPa.Our first-principle calculations for the electronic property are performed with the CASTEP code in MS modeling based on the plane wave basis set, the ultrasoft pseudopotential for electron-ion interaction, and the exchange correlation potential of Perdew et al. in the generalized gradient approximation (GGA) for electron-electron interaction.The experimental results indicate that the structural transition sequence and the structure of the energy bands for the three samples shows similarity. The sequence of the structural transition under high pressure is from ZB-structure to the cinnabar structure to RS-structure, and then to the orthorhombic structure. The electrical transport property exhibits the transition from a semimetal to a semiconductor to a metal, corresponding to the structural transition. Our calculated result indicates that the fourth phase of HgS is a orthorhombic structure with space group Cmcm, and that the transition pressure is about 55GPa. We can not compare the calculated results because of no available experimental data. For HgSe, our calculated result shows that the fourth phase is a orthorhombic structure with the space group Cmcm, and that the transition pressure is about 40.1GPa. In addition, we calculated the band gap as a function of pressure for the three samples with the cinnabar structure. The calculated results are in agreement with the experimental data.Our first-principle calculations for the optical property are performed with the CASTEP code in MS modeling based on the plane wave basis set, the norm-conserving pseudopotentials for electron-ion interaction, and the exchange correlation potential of Perdew et al. in the generalized gradient approximation (GGA) for electron-electron interaction. We obtained the character peaks and the so-called plasma frequencies of the ambient and high pressure structures for the three samples. It was found that dielectrical functionε1(ω) andε2(ω) and the energy loss function L(ω) shift to the high energy( so-called blue-shift) with the increasing pressure.In summary, we carried out in-situ conductivity measurement on HgX (X=S, Se and Te) under high pressure using a microcircuit fabricated on a DAC. The pressure dependence of conductivity was obtained in the three cmpounds with metastable phases. In addition, an investigation on the structural stabilities and optical properties of three componds under high pressure was conducted using first-principle based on density functional theory. Our calculated result is helpful for the future experimental work.