First-principles Study Of Structure And Superconductivity For Bi And Ca-Bi

Bismuth is a very important element and many of its compounds present variousproperties. For centuries, pure bismuth and bismuthides have been extensively studied inmany fields, such as thermoelectric materials and superconductors, etc.In this dissertation, by using the evolutionary algorithm implanted in USPEX code, astable orthorhombic Cmcm structure of Bismuth at high pressure has been found. Thecompression behaviors of the new Cmcm structure as well as the A7structure, themonoclinic Bi-II, the complex incommensurate host-guest Bi-III and the bcc Bi-V underdifferent pressure have been studied. Consequently, the equation of states and the phasetransitions among different structures have been obtained. By comparing the Cmcmstructure and bcc structure, it is found that the enthalpy of the former is only10-3eV/atomlower than that of the latter. With the increasing of pressure, such enthalpy differencebecomes to10-2eV/atom. The electron-phonon coupling (EPC) parameter λ of the Cmcmand bcc structure under different pressure are also calculated. We find that the criticaltemperature of superconductivity (Tc) of the Cmcm and bcc structures is5.8K and7.4K,respectively. When the pressure increases to50GPa, the Tcof both structures is decreasingand becomes1.5K and1.8K, respectively.Using a variable-composition ab initio evolutionary algorithm implemented in theUSPEX code, we have performed a systematic search for stable compounds in the Ca-Bisystem at different pressures. In addition to the well-known tI12-Ca2Bi and oS12-CaBi2, afew more structures were found by our calculations, among which phase transitions werealso predicted in Ca2Bi (tI12→oI12→hP6), Ca3Bi2(hP5→mC20→aP5) andCaBi (tI2→tI8), as well as a new phase (Ca3Bi) with a cF4structure. All the newlypredicted structures can be both dynamically and thermodynamically stable withincreasing pressure. The superconductive properties of cF4-CaBi3, tI2-CaBi andcF4-Ca3Bi were studied and the superconducting critical temperature Tccan be as high as5.16,2.27and5.25K, respectively. Different superconductivity behaviors with pressureincreasing have been observed by further investigations.