Preparation, Characterization And Ln-situ High Pressure Studies Of Te-based Double Perovskite Oxides

The perovskite oxides are one kind of typical strongly correlated electron system,and have been studied widely due to the appearances of the superconductivity, giantmagnetoresistance, large thermoelectric effect, anomalous Hall effect and other exoticphysical properties. In recent years, because of the discovery of manganese-basedperovskite oxides (AMnO3) with colossal magnetoresistance and the copper oxide(Sr2YRu1-xCuxO6) with high-temperature superconductivity, the studies of doubleperovskite oxides (general formula A2B′B″O6) have attracted many attentions due totheir unique physical and chemical properties, which can be widely used as catalysts,sensors, fuel cells, and magneto-optical materials.First, a series of Te-based double perovskite oxides have been synthesized bysolid-state reaction method. Then we have investigated their crystal structures, Ramanspectra and magnetic properties systemically. Second, we have also probed into thehigh pressure behaviors of A2B′TeO6(A=Sr, Ba; B′=Zn, Cu, Co, Ni, Mg) usingangle-dispersive synchrotron X-ray powder diffraction and Raman spectrameasurements at room temperature. Last, the structural and magnetic properties ofSr2B′TeO6(B′=Cu, Co, Ni) with B site substitution of nonmagnetic atom Zn havebeen investigated. The main results of my work are shown as following.1. The Raman spectra of Sr2CuTeO6with tetragonal structure, Sr2NiTeO6withmonoclinic structure, and Ba2CoTeO6with hexagonal structure were investigated forthe first time, and Raman modes were assigned using the method of factor groupanalysis. For Sr2CuTeO6, the peaks of752cm-1,592cm-1,552cm-1,413cm-1wereassigned to the symmetric oxygen stretch vibration of the TeO6octahedra as υ1mode, an oxygen asymmetric stretching vibration of the TeO6octahedra as2υ2mode and anoxygen bending motion in the TeO6octahedra as3υ5mode. For Sr2NiTeO6,775cm-1,605cm-1,436cm-1,425cm-1,417cm-1were assigned to the symmetric oxygen stretchvibration of the TeO6octahedra as υ1mode, an oxygen asymmetric stretchingvibration of the TeO6octahedra as υ2mode and an oxygen bending motion in the TeO6octahedra as3υ5mode. For Ba2CoTeO6,747cm-1,736cm-1，645cm-1and545cm-1were assigned to the2υ1(A1g), υ2(Eg) and υ5(A1g+Eg) mode of TeO6(1) octahedra(sharing apexes with CoO6octahedra);674cm-1,593cm-1，402cm-1,392cm-1and377cm-1were assigned to the υ1(A1g), υ2(Eg) and3υ5(A1g+Eg) mode of TeO6(2)octahedra (sharing faces with CoO6octahedra).2. When the BIIcation varied, the Raman vibration frequencies of Ba2BIITeO6(B=Zn, Ni) will also change, and the change was larger for the TeO6(1) octahedra thanthe TeO6(2) octahedra. As the ionic radius of the cation decreased, the frequency ofthe υ1mode increased. The Raman modes of Ba2BIITeO6were decided not only byTeO6octahedra, but also by the nature of BIIcation.3. The structural and magnetic properties of Sr2B′TeO6(B′=Cu, Co, Ni) with B sitesubstitution of non-magnetic atom Zn have been investigated. For Zn dopedSr2Cu1-xZnxTeO6, it remains the tetragonal structure within the whole doping rangewhile the tetragonal distortion (c/a) decreases with increasing zincum molar fraction.This is due to the release of Jahn-Teller distortion from a d9ion (Cu2+) to a d10ion(Zn2+). The variation of saturation magnetization Ms with different zincumcomposition can be attributed to the introduction nonmagnetic atoms Zn. For Zndoped Sr2Co1-xZnxTeO6and Sr2Ni1-xZnxTeO6, the structures remain the monoclinicphase within the whole doping range, and the magnetic interaction is weakened withincreasing zincum molar fraction.4. The in-situ high-pressure behaviors of double perovskite A2BTeO6(A=Sr, Ba;B=Zn, Cu, Co, Ni, Mg) had been investigated using angle-dispersive synchrotronX-ray powder diffraction and Raman spectroscopy in a diamond anvil cell (DAC) atroom temperature. Although Sr2ZnTeO6and Ba2BTeO6(B=Zn, Co, Ni) remainedstable up to the highest pressure, the different pressure coefficients of the normalized axial compressibility were obtained. Sr2MgTeO6underwent a phase transition at9.7GPa from a cubic (Fm-3m) structure to a tetragonal (I4/m) structure accompanied bythe octahedral rotation along c-axis.5. The results of high pressure Raman spectra of tetragonal Sr2BTeO6(B=Zn, Cu)and monoclinic Sr2BTeO6(B=Co, Ni) indicated that the crystal structure was stable upto the reached pressures. But the symmetric oxygen stretch vibration of the TeO6octahedra, known as υ1mode, has the largest change with increasing pressures. Andthe TeO6octahedra are more sensitive to pressure than the BIIO6octahedra. Althoughthe Ba2BTeO6(B=Zn, Co, Ni) with hexagonal structure remained stable up to thehighest pressure, the influence of pressure was larger for the TeO6(1) octahedra thanthe TeO6(2) octahedra. The results of high pressure Raman spectra in thesecompounds results were consistent with the corresponding high pressure X-raydiffraction data.