Study On The Electrical Property Of AgI And ZnS 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, neytron 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 AgI and ZnS.For the electrical resistance or conductivity (resistivity) 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(resistivity) 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.Superionic conductors are cmpounds that exhibit exceptionally high values of ionic conductivity within the solid state. Their conductivities often reach values of the order of 1?-1cm-1, which are comparable to those observed in the molten state. At 420K, AgI undergoes an abrupt increase in conductivity of over three orders of magnitude associated with a solid to solid phase transition, and its conductivity reaches a value of 2?-1cm-1.β/γ-AgI transforms to rs-AgI at about 1.1GPa and ambient temperature. rs-AgI exhibits a high value of ionic conductivity also. So it is proven that the high-pressure can be used as a useful tool for probing the superionic conduction.The property of the electron transport is sensitive to the variation of lattice. Especially, the change of crystal structure may lead to the variation of the transport property of electrons. By means of the relation between the resistivity and pressure, the potential phase transition (the electronic phase transition and structural phase transition) can be detected. For nano-ZnS sample, the experimental result indicates that the resistivity decreases rapidly with six orders of magnitude within the pressure from 21 to 25GPa. The abrupt change in resistivity corresponds to the phase transition of the sample from zinc-blende(ZB) to rocksalt(RS) structure. On decompression, the magnitude of resistivity returns the original value almostly, indicating the phase transition from ZB to RS structure is reversible. Compared with the compression, the pressure hysteresis in the phase transition from RS to ZB is observed, which is in relation to the difference of the activation energy between the two structures during the phase transition.For block ZnS, the abrupt decrease in the resistivity is also observed at 15GPa. However, the pressure, at which the resistivity changes abruptly, is lower than that of nano-ZnS. The reason for this is that the surface energy of nano-ZnS is high than that of block ZnS. So the more energy is needed to rebuild the new phase.In conclusion, electrical measurement is an important way to investigate the electron transport behavior in materials. This thesis improves the in-situ high-pressure electrical measurements in diamond anvil cell.