The High Pressure Synthesis And Physical Property Researches Of Novel Cuprate Superconductors And Layered Materials

The discovery of high-temperature cuprate superconductors has generated great interest in exploring high temperature superconductivity until today.Research for high T,superconductors with new structures and new systems remain as a central problem.Due to the quasi-two dimension structures feature and novel physical properties,layered materials have drawn many interests since the high T,superconductors were discovered.In this dissertation,a new superconductor of cuprate with unique features and several other new layered materials have been discovered and synthesized initially employing a high temperature and high pressure route.The crystal structures,electronic transport behaviors,magnetic properties and thermodynamic properties have been studied systematically.The main results are shown as follows:?1?The mechanism of superconductivity of cuprates remains one of the biggest challenges in condensed matter physics.High-T,cuprates crystallize into layered perovskite structure typically featured with copper-oxygen froming two-dimention[CuO₂]planes.Due to strong Jahn-Teller effect the octahedron in high T,cuprates is elongated,placing the planar Cu3dx²-y² orbital level at top and Cu3dx²-y² orbital below Fermi level,and the electrons are highly correlated.Research for new cuprate superconductors with new structures and new properties are very important for both material studies and mechanism exploring.A new superconductor Ba₂CuO_3+? with La₂CuO₄ structure and with critical temperature?Tc?exceeding 70 K was synthesized using a 6-8 double stage high pressure apparatus.The x-ray absorption measurement gives evidence that this cuprate is doped with a fairly large amount of holes.However,in contrast to the so-far known hole-doped high-Tc cuprates the new cuprate possesses an extraordinarily compressed octahedron with much shorter apical oxygen distance of 1.861?8?A as well as much expanded in-plane Cu-0 bond of 2.0015?2??.In compressed octahedron the Cu3d3z²-r² orbital level is higher than the Cu3dx²-y² orbital level.This suggests that considerable d3z-r² orbital character might be mixed up in the states near the Fermi level which has been thought to be harmful for high-Tc superconductivity.It also means pairing symmetry of Ba₂CuO_3+? may carry admixtures from more than one gaps,and Ba₂CuO_3+? posses more three-dimensional features.It worth noting that Ba₂CuO_3+? shows a T,above 70K though it has a in-plane Cu-0 bond length as long as 2.0015?2??,a compresses octahedron,and heavily over doped holes.The present work sheds renewed light on comprehensive understanding of necessary ingredients for high T,cuprate superconductors.?2?Research on new layered cuprate materials has received considerable attention as the complex structure features and potential superconductivity.A new layered oxyselenide Ba₂CuO₂Cu₂Se₂ has been synthesized under high-pressure and high-temperature conditions,which provide a new view for synthesis of new layered materials using unique high pressure method.Ba₂CuO₂Cu₂Se₂ was characterized via structural,magnetic,transport measurements.It consists of square lattice of[CuO₂]planes and anti-fluorite-type[Cu₂Se₂]layers,which are alternately stacked along c axis.The lattice parameters are obtained to be a=b=4.0908?9?A,c=19.6785?5?A.The Cu-0 bond length is given by the half of lattice constant a.,i.e.2.O₄43 A.Ba₂CuO₂Cu₂Se₂ is a semiconductor with the resistivity?18 mQ-cm at room temperature.No magnetic transition was found in the measured temperature range both in magnetic susceptibility and neutron diffration.The DFT+Ueff calculation demonstrates that the density of state near the top of the valence band is mainly contributed from the Se 4p electrons,and the exchange interaction is very weak.There is no structure phase transition at high pressure until 63GPa.Ba₂CuO₂Cu₂Se₂ presents a complex conductivity behavior under pressure and seems undergo a superconductivity-like transition at 49GPa.?3?High pressure is a powerful tool for research on new materials,new structures and new physics.New layered structure compounds Ba₂CuO₂Ag₂Se₂ and new double-perovskite PbCro.67Wo.33O₃ were synthesized under high pressure and high temperature conditions.Ba₂CuO₂Ag₂Se₂ is isostructural with Ba₂CuO₂Cu₂Se₂ and consists of alternately stacking square lattice of[CuO₂]planes and anti-fluorite-type[Ag₂Se₂]layers.The lattice parameters are obtained to be a = b =4.25244?4?A and c = 20.1072?2??.Ba₂CuO₂Ag₂Se₂ presents a metallic behavior instead of semiconductivity in Ba₂CuO₂Cu₂Se₂,which may attribute to the Ag vacancy of[Ag₂Se₂]layers.Ba₂CuO₂Ag₂Se₂ possess a lager Cu-0 bond length,which may weaken the hybridizationand betwwen Cu and O.Ba₂CuO₂Ag₂Se₂ present a lower Cu oxidate valence which was clarified by soft X-ray absorption spectroscopy.New metrial PbCr_0.67W_0.33O₃ crystallizes into cubic perovskite structure with a Pm-3m space group.It shows an antiferromagnetic transition at about 30K and competivity between ferromagnetism and antiferromagnetism at lower temperature.X-ray diffraction at low temperature prensts a structural transition near 50K wich may be related with the antimagnetism.This suggests that there may be 2:1 or other order station on B site.PbCr_0.67W_0.33O₃ shows a behavior of relaxor multiferroics,and has a charge distribution of Pb²⁺Cr³⁺_0.67W⁶⁺_0.33O₃.