High Pressure Synthesis And Physical Property Tuning Of High-ordered Quadruple Perovskites

Perovskite materials have flexible crystal structures as well as various combinations of ions and valence states.As a result,they exhibit a wide variety of physical properties,making them to be one of the most interesting system in morden condensed matter physics and material sciences.In this doctoral dissertation,by means of the unique advantages of high pressure and high temperature conditions,a series of new perovskite-structure oxides have been prepared.The crystal structures and comprehensive physical properties have been studied in detail.The main conclusions are shown as follows:(1)PbCoO₃ was synthesized under high pressure and high temperature(12 GPa,1323 K).Although this material has a simple ABO₃ perovskite formula,Pb²⁺and Pb⁴⁺form a 1:3 charge order at the A site,and the high-spin Co²⁺and low-spin Co³⁺form a second charge order at the B site with a 1:1 mole ratio.Therefore,after considering the special charge ordering distribution,Pb Co O₃possesses a peculiar A-and B-site ordered charge distribution Pb²⁺Pb₃⁴⁺Co₂²⁺Co₂³⁺O₁₂ with space group Pn-3.In-situ high pressure electrical transport,synchrotron X-ray powder diffraction,X-ray absorption spectroscopy,and neutron powder diffraction measurements show that,with increasing pressure,the Co²⁺gradually transforms from high-spin state into low-spin state in 0-15GPa.Consequently,the Co²⁺-O bond length decreases sharply,and the resistance of Pb Co O₃ also unusually increases with pressure.In 15-30 GPa,charge transfer occurs between Pb⁴⁺and Co²⁺.Part of Pb⁴⁺ions changes into Pb²⁺ions,while the low-spin Co²⁺ions gradually turn into low-spin Co³⁺ions.The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of?20 GPa near room temperature.On further compression over 30 GPa,the charge transfer completes,giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.The present Pb Co O₃ provides the first example where spin state,charge state,crystal structure,and metal-insulator transitions take place collectively in the same material system,opening up a new avenue for study spin state transition and intermetallic charge transfer.(2)An A-and B-site ordered quadruple perovskite oxide Ca Cu₃Co₂Re₂O₁₂ with space group Pn-3 was synthesized by high-pressure annealing(7 GPa,1323 K)method for the first time.The charge combination is determined to be Ca Cu₃²⁺Co₂²⁺Re₂⁶⁺O₁₂by bond valence sum analysis and X-ray absorption spectroscopy.In contrast to other isostructural ACu₃B₂B'₂O₁₂ compounds with a single magnetic transition,a long-range antiferromagnetic phase transition originating from the A'-site Cu²⁺sublattice is found to occur at T_N?28 K.On further cooling to?20 K,the spin coupling between the B-site Co²⁺and B'-site Re⁶⁺ions contributes to a ferrimagnetic transition.Strong electrical insulating behavior is identified by optical measurement with an energy gap about 3.75e V.(3)A highly A-and B-site ordered quadruple perovskite oxide La Cu₃Co₂Re₂O₁₂was prepared at 9 GPa and 1323 K with space group Pn-3.The charge distribution was determined to be La Cu₃²⁺Co₂²⁺Re₂^5.5+O₁₂by bond valence sum analysis and X-ray absorption spectroscopy methods.Magnetization and X-ray magnetic circular dichroism spectra results indicate a Cu²⁺(?)-Co²⁺(?)-Re^5.5+(?)ferrimagnetic coupling,in agreement with density functional theory calculations.Electrical transport measurements and theoretical calculations confirm that the Fi M ground state is half-metallic with minority-spin bands crossing the Fermi level and a large majority-spin band gap about 1.9 e V being opened.The present work provides a rare B-site nearly100%ordered perovskite system with enhanced half-metallic performance.(4)A cubic quadruple perovskite La Cu₃Fe₂Re₂O₁₂ with high degree of order at both A-and B-site was synthesized by high pressure and high temperature(8 GPa,1323K)annealing method for the first time.The charge distribution was determined to be La Cu₃²⁺Fe₂³⁺Re₂^4.5+O₁₂by bond valence sum analysis and X-ray absorption spectroscopy measurements.A high ferrimagnetic Curie temperature T_C?720 K was observed in this material.The large saturated moment near 8.0m_B/f.u.at 2 K together with the x-ray magnetic circular dichroism spectra results indicate the ferrimagnetic spin alignment Cu²⁺(?)-Fe³⁺(?)-Re^4.5+(?)in La Cu₃Fe₂Re₂O₁₂.Magnetoresistence and theoretical calculations show that the ferrimagnetic ground state is half-metallic with spin-up bands crossing the Fermi level but the down-spin channel is insulating with a wide band gap about 2.27 e V.Benefitting from the high ordering between Fe and Re,the low-field magnetoresistance is enhanced a lot compared with other isostructural materials.The combination of high T_C,large saturated moment,wide spin-up energy gap,and enhanced low-field magnetoresistance in La Cu₃Fe₂Re₂O₁₂ provides an excellent example of half metal with high performance.