The Structural And Electrical Transport Properties Of The Fe₃O₄Nano-powders, Fese And α-FeOOH Powders Under High Pressure

In this paper, the structure and electrical properties of Fe₃O₄nano-powders, FeSeand α-FeOOH powders under high pressure have been studied by using a variety ofin-situ electrical measurement technologies under high pressure based on diamondanvil cell. The results are shown as follows.1. The Fe₃O₄nano-powdersHigh-pressure X-ray diffraction experiments shows that Fe₃O₄nano-particles(40-60nm) maintain the inverse spine structure till42.3GPa. This result is differentfrom the bulk, which changes from the inverse spine structure to the high-pressurephase at about25GPa. And the result indicates that Fe₃O₄nano-particles are morestable than the bulk.High-pressure Hall effects measurements shows that the resistivity of the Fe₃O₄nano-powders decreases with the pressure increasing, but there is a change in theslope. Below6.0GPa, the resistivity decreases fast; above6.0GPa, the resistivitydecreases slowly. In the range of0-10GPa, the decrease of the resistivity is mainlydue to carrier concentration increase. Above14GPa, the decease of the resistivity ismainly caused by the mobility increase with the pressure increasing. Pressure reducesthe electron-phonon coupling constants, and the effect is particularly evident at higherpressure. Thus the mobility improves significantly above14GPa.The magnetoresistivity measurement of the Fe₃O₄nano-powders shows that thereis a MR transition from positive to negative at about6.0GPa. The quasi-linearpositive MR effect below6.0GPa is unexpected, and we attribute it to the confinementeffect caused by nano-scale heterogeneity at the contact surface between Mo andFe₃O₄sample, and the high negative spin polarization of magnetite. The MR transitionat about6.0GPa comes from a phase transition from half-metal to metal induced by pressure. As a metal with magnetic moment, the magnetic field supports theferromagnetic order and reduces the magnetic scattering by the suppression of themagnetic fluctuations, resulting in the resistivity decrease.2. The FeSe powdersHigh-pressure Raman scattering measurement of the FeSe powders shows thatthere are two new peaks appearance at around4GPa, indicating that a structure phasetransition occurs. According to the predecessors' reports, we speculate that the samplechanges from the tetragonal structure to the monoclinic structure. When pressureexceeds12GPa, a second new peak appears, we think that the sample changes fromthe monoclinic structure to hexagonal structure. The two structure phase transitionsare a fully reversible process.High-pressure Hall effects measurement of FeSe powders shows that theresistivity, the carrier concentration and the mobility all take place a discontinuouschange at about4GPa and10GPa, respectively, which are just the new peaksappearance in Raman spectrum. To further study the pressure-induced structural phasetransition, we calculate the enthalpy of the α-FeSe and the β-FeSe with generalizedgradient approximation method. It is found that the calculated phase transitionpressure from the tetragonal PbO-type to the hexagonal NiAs-type phase is about12GPa. From the charge densities of the Fe and Se atoms, we can see that the structuraltransition changes the bonding character between the Fe and Se atoms. The chemicalbonds between the Fe and Se atoms of α-FeSe are more covalent. And the bondingbetween the Fe and Se atoms of β-FeSe is mainly electrovalent.3. The α-FeOOH powdersThe grain the grain boundary resistance of α-FeOOH powders are separated byAC impedance spectrum measurement, and it is deduced that the grain boundary isdominant in the electrical transport process. The discontinuous change of the grainboundary resistance and the relaxation frequency occurs at the controversy pressure(10GPa) at which if there is a structure phase transition. Our high-pressure Ramanscattering measurement shows that there is no structure phase transition until22.3GPa. We attribute the discontinuous change to the grain boundary Rearrangement induced by pressure. Rearrangement of the grain boundary makes the energy barrierdecrease with the pressure increasing in the relaxation process, the carrierconcentration increase with the pressure increasing and the frequency responsecharacteristics of the grain boundary changes from a capacitor to an inductor. Inaddition, the space charge layer model is used to analysis the influence of the pressureon the grain boundary. It is deduced that the grain boundary core is positive, and therearrangement induced by pressure changed the created energy of the cation, whichchanges from decrease to increase with the pressure increasing at around10GPa. Theconclusion that the space charge layer resistance is dominant in grain boundaryresistance is obtained by analyzing the space charge potential, and the discontinuouschange of the grain boundary resistance results from the space charge layer resistancechange.