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Hydrothermal Preparation YPO4: Eu And The Structure Of Host

Posted on:2009-05-07Degree:MasterType:Thesis
Country:ChinaCandidate:H S YuFull Text:PDF
GTID:2121360242980952Subject:Condensed matter physics
Abstract/Summary:PDF Full Text Request
Hydrothermal method can be described in this way,water as a medium,the reagents have a reaction with each other at a given temperature in a closed system,and obtain the new phase or new substance.The reason of Hydrothermal method can be work is that,it make use of the properties of water in the supercritical state and reagents in the conditions of high temperature and high pressure.YPO4 and Eu doped YPO4 nanocrystals were prepared via a simple hydrothermal method.The phase purity,microstructure,surface absorbed water and luminescence properties were characterized by X-ray diffraction (XRD),Transmission electron microscopy(TEM),fluorescence spectrum, infrared spectroscopy,Raman spectroscopy and thermogravimetry analysis(TGA).According to the XRD patterns of the YPO4 nanocrystals obtained at different temperature,all of the diffraction peaks could be readily indexed to the tetragonal phase(space group I41/amd)of YPO4.In the figure of the relationship between particle size and temperature, it can be seen that the particle size reduced with decreasing of the reaction temperature.As the reaction temperature decreased from 220℃to 80℃, the particle size reduced from 51 nm to 16 nm.The lattice parameters were calculated using Retica Rieveld program with peak positions that are calibrated by internal standard of silicon powder(99%pure),the lattice parameters reacted at 80℃were a=b=6.9073(?),c=6.046(?).In the figure of the relationship between the particle size and the lattice volume,with the reduction of particle size,YPO4 nanocrystals exhibited a lattice expansion.As illustrated in the figure of the variation of c/a,with the particle size decreased,the lattice parameters c/a became larger and all of them were smaller than 1,which indicated that the unit cell symmetry of YPO4 enhanced.Yttrium orthophosphate crystallizes with the zircon structure can be described as chains parallel to the c axis of comer-sharing structural units built of(YO8)dodecahedron and a(PO4)tetrahedron linked together by an edge.According to the Inorganic Crystallographic Database(ICSD),(YO8) dodecahedron and(PO4)tetrahedron are distorted,so the positive centers and negative centers of(YO8)dodecahedron and(PO4)tetrahedron are not superposition.It can be described as a electronic dipole when they have a interaction with another substance.When two dipoles close it up,their electronic dipole moment enhances due to the Coulomb interaction.If we can put all the unite cells together and form the full crystal one by one,one unite cell is composed of six(PO4)tetrahedrons and six(YO8) dodecahedrons so that the(PO4)tetrahedron and(YO8)dodecahedron can be considered as electronic dipole.The interaction among these electronic dipoles will make the electronic dipole moment strong which means that the distance between equivalent positive centers and equivalent negative centers will be far more.Consequently,with the grain size increasing,the symmetry of unit cell get lower and lower.The surface absorbed water doesn't have much more influence to c/a,the reason is in what follows: concerning the sample with sphere shape,the surface absorbed water distribution has a higher symmetry,so the interaction force between the surface absorbed water and the electronic dipole are all approximate the same in each directions.Although the sample is not perfect sphere symmetrical,but the surface absorbed water also has a less influence on lattice symmetry.Acoording to the HRTEM images of T100 and T220,it can be seen that both of them could be well crystallized,the fringe spacings are 0.347nm and 0.345nm,corresponding to the fringe spacing of the(020) lattice planes of T100 and T220,respectively,which also give evidence that the lattice expansion with the grain size decreased.Acoording to the IR spectroscopy,the band at 1630cm-1was due to the adsorbed water H-O-H bending mode.The broad band at 3400cm-1 was attributed to the OH vibration of water.The bands around 643cm-1 and 1021cm-1were attributed to the symmetric stretching mode(v4)and asymmetric stretching mode(v3)of PO4 tetrahedron,respectively.In addition,the absorption band of v4 vibration mode moved to higher wavenumber as the grain size decreased.According to the analysis of c/a above,with the grain size decreased,the symmetry of the unit cell enhanced and the crystal field also enhance simultaneity.The stronger crystal field,the larger the energy spacing between the ground state and excited-state.The increased energy spacing led to the corresponding absorption band blue shift as the grain size decreased,which agreed with the blue shift in IR spectra.Therefore,base on the IR spectra,it can be concluded that the symmetry of the lattice enhance with the grain size decreased.These bands around 1021cm-1appear three distinct peaks due to the presence of different P—O distances in the crystal.The higher wavenumber value is due to the shortest P—O band length,and as the grain size decreased,the absorption bands located at the higher wavenumber are weaken and move to the lower wavenumber direction.It means that the P—O band length became longer and the P—O bonds were towards to identity,which further proved the symmetric enhancement of YPO4 crystal unit with the grain size reduction.Acoording to the Raman spectroscopy,it can be seen that the band located at around 487 cm-1showed a blue shift with the grain size decreasing.The bands located at the range from 950cm-1to 1100cm-1were attributed to asymmetric stretching mode(v3)of PO4 tetrahedron.The band located around 487cm-1the symmetric stretching mode(v4),the band has a blue shift which is agree with the result of IR spectroscopy.The conclusions:A single phase of tetragonal(I41/amd)zircon structureYPO4 was prepared by a hydrothermal method.YPO4 nanocrystals showed a size reduction with the reaction temperature decreased.Considering the XRD,IR and Raman results,a lattice expansion and the unit cell symmetric enhancement were observed with the particle size decreased.A dipole to dipole interaction model was applied to investigate the relationship of the variation of microstructure and the particle size.The emission spectrum of Eu doped YPO4 nanocrystals was also given and the emission bands were clearly ascribed to the f to f transitions of Eu3+.
Keywords/Search Tags:hydrothermal method, YPO4:Eu, lattice expansion, blue shift
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