Study On The Preparation, Structural And Magnetic Properties Of Electron Doped Na_1+xV₂O₅ System

The spin-Peierls transition was discovered in some low dimensional correlated electron system materials. At low temperatureα′-NaV₂O₅ undergoes a spin-Peierls transition whose ground state is spin-singlet with a spin-gap. The research on the crystal structure, electronic structure and magnetic property ofα′-NaV₂O₅ will be important for the development of the spintronics devices, which would make the quantum computing possible.In this thesis, the electron-doped material Na_1+xV₂O₅ (x=0.04, 0.10) were synthesized by solid state reaction. And the crystal structure, chemical composition, short range ordering and magnetic properties were studied through XRD, SEM, ICP, XPS, Raman spectra and SQUID measurements. Furthermore the influence of the different amount of the doped electron on the structure and magnetic properties were also discussed. And some interesting results are obtained.(1) The single phase powder samples of Na_1+xV₂O₅ (x=0.04, 0.10) were prepared by solid state reaction with appropriate ratio of NaVO3, V2O3 and V₂O₅ which sintered at 650℃for 10h under the atmosphere of N2. XRD results indicate that it is beneficial for the synthesis of single phase samples to add excess NaVO3 into the raw materials, but the excess ratio should be less than 100%. The traditional sintering process must be carried out under the vacuum environment to prevent V2O3 and NaV₂O₅ from being oxidized by O2. We use pure N2 to take the place of the vacuum environment which makes the sintering process easier. The SEM photos show that the shape of the grain is cuboid with average grain size of 15μm and dispersed evenly.(2) The ICP results showed that the sample prepared with 25% excess NaVO3 should be Na_1.10V₂O₅ while the sample prepared with 50% excess NaVO3 should be Na_1.04V₂O₅, whose average valence of V would be +4.45 and +4.48 respectively. The XPS results also indicated that the amount of V4+ ions in Na_1.10V₂O₅ is smaller than that of in Na_1.04V₂O₅. And the average valence of V in Na_1.10V₂O₅ and Na_1.04V₂O₅ would be +4.44 and +4.47, respectively, which well matches the ICP results. Under the effect of crystal field, the 3d orbits of V ion would split and the lowest energy orbit would be dxy orbit. The doped electrons would enter the dxy orbit and make V5+ deoxidized to V4+ which decreases the average valence of V.(3) There are three crystallography sites of oxygen ions named O1, O2 and O3. The Coulomb interaction between Na ion and three crystallography oxygen would be changed with the increasing of the amount of doped electrons, which would affect the short range ordering of the structure and change the length and angle of the chemical bond. So the Raman spectra should be different among Na_1.04V₂O₅, Na_1.10V₂O₅ and NaV₂O₅.The Raman spectra showed that with the amount of the doped electrons increased from 4% to 10%, for the Na+‖c Raman vibration mode the effective mass of the oscillator increases, which results in the red shift of the vibration frequency from 174cm-1 to 171cm-1. For the V-O3-V Raman vibration mode, because of the interaction between Na and O3 the angle of V-O3-V bond would alter which decreases the force parameter of the oscillator. So the vibration frequency shifts from 448cm-1 to 446cm-1. For the three O1-V- O2 Raman vibration modes, because of the interaction among the doped Na, O1 and O2 the force parameter of the oscillator would alter and so the three vibration frequencies have a average 2cm-1 of redshift.(4) In the susceptibility curve of Na1.10V₂O₅. There are no sign of spin-Peierls transition but a large"Curie tail"at low temperature range. However spin-Peierls transition is observed in Na_1.04V₂O₅ and a relatively small"Curie tail"whose critical temperature is 34.5K and spin gap is 155K. All these phenomena result from the magnetic V4+ ions which are introduced by the doped electron. By the analysis of the low temperature susceptibility with Curie-Weiss law, the concentration of the V⁴⁺ ions should be 1.6×10²¹/mol in Na_1.10V₂O₅ and 4.5×10²⁰/mol in Na_1.04V₂O₅, which well explains the difference in the susceptibility curve of both electron doped samples.