Study of Chemical Doping and Crystal Phase Transition in FeSe Superconductor System

• Main Authors: Chia-MingYang, 楊佳明
• Other Authors: In-Gann Chen
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/84347390057085597528

博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 101 === We focused on (1) chemical dopping effect on physics property of FeSe superconductor system. (2) Phase transition and microstructure analysis of FeSe crystal. To discusss the chemical doping effect, substitution of Te and insert of Li were used to change the atomic structure, valance state, and phonon vibration mode of FeSe1-xTex, which may affect superconductivite critical temperature. To discuss the phase transformation, the high prefer orientation FeSe crystal is fabricated by Bridgman process. It is possible to control the microstructure by realizeing the phase transform routes, orienation distribution and second phase precipitation. The low atomic number Li was used to dope into FeSe1-xTex with different concentration. Two factors are controlled. First, the different Li doping concentrration in specific x value (Se/Te ratio is a constant). Second, the Li doping effect in different Se/Te ratio. It is showed that the superconductivite critical temperature increased about 1.5K, and at least two possible doping mechanisms. In the region of x=0.5~0.7, the inserted Li ions occupy the interstitial sites between the Fe-Se layers, expands the unit cell in the c-axis direction. In the region of x=0.7~0.9, due to the disordered coordination of Se and Te, the interstitial sites in the c-axis may exhibit a greater degree of crystal distortion as compared to that of x=0.5~0.6. It exhibited the different responses of valence states for Te and Se with Li doping, and it is the possible reason cause different doping mechansims. However, both mechanisms affect the vibration of iron, and the well fitting curves are displayed, which shows the relationship between Tc and Raman active modes. It implicated the specific vibration frequency may lilmit the superconductivity, and Tc will increase when the vibration frequency change. The high preferred orientation celluar like crystal is fabricated by the high undercooling Bridgman process. FeSe tetragonal phase, FeSe hexagonal phase, and two Fe phase was defined by the x-ray diffraction and energy dispersive spectrum. Orientation is investigated by two dimension x-ray diffraction and electron backscatteered diffraction, which showed six possible preferred orientations in the FeSe crystal. Two types of the crystal symetry are observed: First, due to the crystal transform from hexagonal symetry to tetragonal symetry, the difference symmetry model caused the rotation symetry in final microstructure. Another reflection symetry structure is caused by the two direction of phase transformation for every plane. Thermal differential analysis suggested the diffusion less phase transform induce the periodic symetry structure.