High Pressure Physics Property Studies Of Selenium And Tellurium

The main contents of high pressure physics behavior studies are the equation of state of materials, all kinds of phase transitions under high pressure and the transformation rules of high pressure physics prperties, etc. High pressure synchrotron X-ray diffraction is one of the most direct and effective methods to investigate the structural phase transitions and states. High pressure Raman spectroscopy is also an important method to study the stability of materials under high pressure. Combining in situ high pressure X-ray diffraction and Raman spectroscopy, we will further understand the changes of materials under high pressure.High pressure studies of chalcohen elements Se and Te are always the research focus of many investigators. Se and Te have the same structure at ambient condition, however, Se has an additional high pressure phase, SeII, which doesn't exist in the high pressure polymorphous of Te. In addition, there are still many controversies in determing the crystal structure of SeII. Therefore high pressure studies of Se and Te is necessary to further understand its complex high pressure polymorphous. The investigation and understanding of high pressure Raman spectra of Se is still a challenging work, at present there is still no detail analysis for high pressure Raman spectra of Se was reported. So we perform high pressure Raman spectra studies on Se to further understand the phase transition dynamics of Se.Resonance Raman scattering (RRS) is a powerful technique to investigate the electronic band structure of semiconductors and to obtain information about the electron-phonon interactions. Generally, the RRS experiments are carried out by varying the incident (scattered) photon energy in order to pass through the interband transition energy. Another, pressure offers an alternate route to ambient wavelength-tuned RRS studies. Hydrostatic pressure can cause changes on vibrational and electronic energy band structures, so it is possible to bring the interband transitions of a semiconductor in resonance with fixed incident (scattered) light by the application of hydrostatic pressure.High pressure Raman spectroscopy studies have been carried out for a-Se at room temperature in the diamond anvil cell with 830 nm exciting line. Raman evidence for the pressure induced crystallization of a-Se and the coexistence of the unknown high pressure phase with the hexagonal phase is presented for the first time. Further experimental proof of high pressure angle dispersive X-ray diffraction studies for amorphous Se indicates that the unknown high pressure phase is also a mixture phase of the tetragonal I41/acd and SeIV structure. Otherwise, the high pressure phase of amorphous Se at 22.7GPa is determined to be body centered triclinic SeIII according to our Rietveld refinement results.Amorphous Se, as an elemental semiconductor, has many defect levels in the band gap. However, the understanding of all these defects is still confused to us according to present limited investigations. Since resonance Raman scattering method has been extensively used to obtain information about electronic phonon interaction and the relevant intermediate electronic states, we utilize it to study the defect levels of amorphous Se. Here, we present the pressure-tuned resonance Raman scattering in amorphous Se at room temperature. Two strong resonances are observed, at 0.86 and 1.02 GPa, with 830 nm excitation laser (ωi = 1.497eV), due to pressure tuning the interband transition energy through the scattering light. The resonance profiles can be explained well by a simple phenomenological model. The resonance energy of ~1.64 eV and its pressure coefficient of -0.19 eV/GPa are obtained according to the positions of the two resonances. We assign this new resonance state to a shallow defect level which situates ~0.3 eV below the conduction band.High pressure synchrotron radiation angle-diapersive X-ray diffraction studies have been performed to study the high pressure behaviors of hexagonal Se. Three phase transitions are observed, at 17GPa, 23GPa and 28GPa, which are attributed to the phase transitions from SeI to SeII, SeII to SeIII, SeIII to SeIV, respectively. The coexistence of SeI and SeII between 17 and 19 GPa has been observed, which indicates that the phase transition from SeI to SeII is of first order. We suggest that SeII should have a more complex cystal structure than previously reported monoclinic C2/m structure. The structure similarities between SeII, SeIII and SeIV are discussed.Three phase transitions are observed, at ~19GPa, 23GPa and 30GPa by high pressure Raman studies of hexagonal Se up to 68.5GPa, which are consistent with our X-ray diffraction results. After the first transition at 19GPa, the abnormal varieties of the pressure dependence of the Raman frequency are found at 23GPa, 30GPa and 43GPa, respectively. The first two abnormal behaviors are assigned to the transitions from SeII to SeIII, SeIII to SeIV, respectively. The changes of the pressure dependence of the Raman intensity which are observed at 23GPa and 30 GPa. The abnormal changes of the Raman intensity are also strong evidences to favor these phase transitions. While for the discontinuity at about 43GPa, no structure change has been found around this pressure, we think that it may be caused by the variety of the pressure dependence of the modulated vector q in the incommensurate SeIV phase. The in situ energy dispersive X-ray diffraction (EDXRD) experiments on hexagonal tellurium have been carried out using a synchrotron X-ray source with laser heated diamond anvil cell (LHDAC) technique under high pressure and high temperature. We got the noncrystalline phase of tellurium under high pressure by quenching the sample at pressure of 10.6GPa and temperature of 1610K, which confirms that the noncrystalline phase of tellurium can be stable under high pressure. We also found the pressure-induced crystallization phenomenon and a phase transition from rhombohedral phase to a new monoclinic phase at the subsequent process of increasing pressure.We studied the isothermal equation of state (EOS) of tellurium by using diamond anvil cell (DAC) and energy dispersive synchrotron radiation X-ray diffraction (EDXRD) technique at room temperature under pressures up to 90.9 GPa. We confirmed the phase transition of tellurium from rhombohedral (Te-Ⅳ) to a body-centered-cubic (Te-Ⅴ) phase at about 27±3 GPa, and found that body-centered-cubic Te-Ⅴto be a stable phase up to 90.9 GPa. The data were fitted by the Murnaghan equation of state to yield the bulk modulus and its first pressure derivative at different reference pressures. By comparising the equation of state with the previous results, we find that body-centered-cubic Te-Ⅴis a softer material than previously thought.