Study On Electrical Property And Structure Transformation Of Iron Under High Pressure

The Earth has a liquid outer and solid inner core. The outer coreis predominantly composed of Fe, alloyed with small amounts oflight elements, such as S, O and Si, while the solid inner-core is afew percent less dense than crystalline Fe. Achieving the extremepressures (up to 360 GPa) and temperatures (~6000 K) found in thecore provide a major experimental challenge, Thus the structuralproperties and phase diagram of iron at high pressure are significantfor understanding the earth-core research.We present high pressure electrical resistance measurementsand X-ray diffraction studies of iron. A method of sample assemblyfor quasi-four probe electrical resistance measurement on iron usinga diamond anvil cell is presented. With the experimental system ofthe Au wires welded to sample wire at the surface of diamond anvil,we have studied the electronic character of Fe(75um diameter) under20 GPa. The results indicate that the sample occurs a transition of αto ε phase at about 14GPa.The electrical resistance decreases before14.3GPa,then increases at 14.3 GPa~17.3 GPa pressure range;theresistance changes slightly at 17.3GPa~19GPa and decreases againsince 19GPa.We think the phase transition has ended at pressure of18GPa,the α-Fe has become ε-Fe fully. It agrees well with the X-raydiffraction results discussed behind. The rate of maximum andminimum electrical resistance is 1.365.The difference betweenelectrical resistance of α-Fe and ε-Fe may be due to the change inelectron-phonon scatter and electrical transport property influenceby disappearance of ferromagnetic.The electrical conductivity can be calculated by the resistancedata and the sizes of sample. We can measure the length of samplewire,the another factor(the area: S) may be calculated by the volumeof sample.In-situ X-ray diffraction(XRD) on iron powder(10um) hasbeen carried out by using a diamond anvil cell(DAC),and thepressure is up to 25GPa.High pressure X-ray diffraction is one ofeffective methods to study lattice structure and equation of state.TheX-ray diffraction instrument we adopt here is SMART APEX, thelamp-house adopted is Mo kα which the wave length is 0.7093 ?.Theexperiment results indicate that the pressure of sample structurephase transition is 13.7GPa.The experimental data under differentpressures is calculated by using Materials Studio (MS).Thecalculated results approve the experiments results. We alsocalculated equation of state and the relation of lattice parameter topressure of α-Fe and ε-Fe. The bulk modulus and pressure derivativeof iron powder are derived by fitting the equation of Murnaghan. Thebulk modulus of α-Fe and ε-Fe are (177.2 ± 18.6) GPa and (240.1 ±55.5) GPa, bigger than other experiment values and smaller thantheoretical calculation results. We gain the c/a value of hcp structurehas the relation with pressure: c/a=1.6209(1)-0.0005×P (GPa).In a summary,our investigations of structure phase transition andelectronic character under high pressure, are the foundation to studythe phase transition rule and characters of ferromagnetic material.