Quantum Properties Of New Perovskite-related Functional Materials

Quantum functional materials present novel and abundant physical phenomena due to the competition and coupling of charge,orbit,spin and other degrees of freedom,such as high-temperature superconductors,diluted magnetic semiconductor,topological insulators and magnetoresistance materials.Exploring new quantum functional materials with new quantum states and studying the physical properties and mechanism of these new quantum functional materials are very important frontier directions in condensed matter physics.In this dissertation,a series of halooxocuprate superconductors and their parent single crystals were synthesized using high temperature and high pressure conditions and solid state reaction method,and the doping effect on T_c was also investigated.Besides,several A-site ordered perovskite type compounds were designed and fabricated for the first time under high temperature and high pressure conditions,and the crystal structures and basic physical properties were systematically studied.Meanwhile,related physical mechanisms are discussed based on first pricinple calculations.The main results are shown as follows:?1?High quality and large size Ca₃Cu₂O₄Cl₂ single crystal were grown by flux method for the first time.Ca₃Cu₂O₄Cl₂ is an antiferromagnetic Mott insulator with Neel temperature about 230 K.The fitted band gap?_g is evaluated to be 0.146 eV according to the thermal activation model of semiconductor.The polarization dependent x-ray absorption spectroscopy results show that the upper Hubbard band of Ca₃Cu₂O₄Cl₂ are predominantly from in-plane O2p_x,2p_y and Cu 3d²²_x-y orbitals.Compared with Ca₂CuO₂Cl₂,Ca₃Cu₂O₄Cl₂ shows a much stronger two-dimensional property than Ca₂CuO₂Cl₂ due to larger distance between layers.This single crystal provides a good material basis for studying the mechanism of high T_c copper oxide superconductor.A series of monolayer oxychloride cuprate samples with nominal composition(Sr_1-zCa_z)_1.6Na_0.4CuO₂Cl₂ were successfully synthesized under high pressure and high temperature conditions.From Ca₂CuO₂Cl₂ to Sr₂CuO₂Cl₂,theexperiment results indicate that superconducting transition temperature decreases with the decreasing of the Ca concentration and finally superconductivity disappears completely accompanied by the monotonously decrease of the actual Na cocentration y_s.The skimpy holes carrier is the main reason for the difficulty in making Sr₂CuO₂Cl₂ to be superconductor.Furthermore,we also synthesized a series of double layer nominal composition(Ca_1-zSr_z)_2.5Na_0.5Cu₂O₄Cl₂ compounds.We found that from Ca₃Cu₂O₄Cl₂ to Ca₂SrCu₂O₄Cl₂,superconducting transition temperature increases with decreasing of the Ca concentration,which is very different from monolayer system behaviour.The mechanism of this phenomenon needs to be further investigated.?2?The new A-site-ordered perovskites AHg₃Ti₄O₁₂?A=Pb,Sr?with A?-site occupied by Hg²⁺were synthesized for the first time under high pressure and high temperature.This study not only expanded the A?-site ions from 3d(Cu²⁺,Mn³⁺,Co²⁺etc.)to 4d(Pd²⁺)to 5d(Hg²⁺),but also established a series of new A-site-ordered perovskites AHg₃B₄O₁₂ materials.Structure analysis indicated that AHg₃Ti₄O₁₂ has larger lattice parameter and less titling of corner shared TiO₆ octahedrons compared with other iso-structural compounds with A?-site occupied by Cu²⁺and Mn³⁺.The main reason is that the A?-site of AHg₃Ti₄O₁₂ was fully occupied by Hg²⁺ions,which is a main group element with large ionic radius.In PbHg₃Ti₄O₁₂,there is a structure transformation from centrosymmetric Im-3 to non-centrosymmetric Imm2accompanied a paraelectric to ferroelectric transition around 250 K.This is the first report on the symmetric broken phase transition in A-site-ordered perovskite and provides a new direction to search ferroelectric materials.Phonon spectrum results show that PbHg₃Ti₄O₁₂ possesses imaginary frequency modes at room temperature and the ferroelectric distortion was dominated by Ti-O mode anomaly at critical temperature.SrHg₃Ti₄O₁₂ is a nonmagnetic insulator with narrow band gap.Dielectric physical characterization indicated there is a peak around 256 K for dielectric constant accompanied a simultaneous phase transition.The band gap energy of SrHg₃Ti₄O₁₂ is about 2.41 eV as compared with 3.2 eV observed in SrTiO₃,the smaller band gap expands the light response range and absorption edges exhibited the red shift for absorbing solar energy efficiently.Therefore,SrHg₃Ti₄O₁₂ is a candidate material for photocatalysis and further study on catalytic capacity is in process.?3?We synthesized the new series A-site-ordered perovskite ACu₃Ti₄O₁₂?A=Pb,Hg?under high pressure and high temperature for the first time.Both two compounds possess cubic lattice with space group Im-3 at room temperature,which are iso-structural to that CaCu₃Ti₄O₁₂.Magnetic characterizations indicated both compounds showing antiferromagnetic order with transition temperature are 27 K and31 K respectively.The fitting band gap are 0.91 eV for PbCu₃Ti₄O₁₂ and 0.64 eV for HgCu₃Ti₄O₁₂ according to the UV-Vis-NIR results.The valence states of Cu and Ti determined by XAS are+2?3d⁹?and+4?3d⁰?.The dielectric constant of ACu₃Ti₄O₁₂are around several hundreds,which are greatly smaller than the CaCu₃Ti₄O₁₂.The dielectric constant of both PbCu₃Ti₄O₁₂ and HgCu₃Ti₄O₁₂ show anomalous at low temperature,which is highly related to the antiferromagnetic transformation.Those results imply the possible magnetodielectric effect in those compounds.