| This paper applies dual space method (reciprocal and real space), powder X-ray diffraction (PXRD) and partial distribution function (PDF). The relationship between structure and luminescent property has been studied by the dual space method in rare-earth tungstate inorganic luminescent materials, like Y2WO6:Eu3+, LiEu(WO4)2, NaEu(WO4)2 and KEu(WO4)2.PXRD is used to study the crystal structure of crystalline materials, through following steps:indexing and space group determination, structure factor extraction, structure solving by the Direct Mehtod or charge flipping, Rietveld refinement to solve the new crystal structure. We use solid state reaction by adding mineralizer to synthesize two inorganic rare-earth materials, Y2WO6 and Y2WO6:Eu3+. The crystal structure Y2WO6 and the atomic occupation of Eu3+ have been determined by PXRD. The structure of Y2WO6 is characterized by cubic coordination [YO8], trigonal prism [WO6] and [YO6] connecting with each other. The ion Y3+ has two crystalline occupations, namely 2a and 2c (Wyckoff position). Because of the weak bond strength, which is caused by longer Y-0 distance in cubic [YO8] than that in [YO6], the ion Eu3+ partially occupies the 2a. This material can be effectively excited under ultraviolet and blue zone to emit characteristic light of Eu3+. This phenomenon is attributed to cubic coordination setting [YO8] occupied by ion Eu3+PDF is widely used in liquids, glassed, other amorphous materials and recently in crystalline materials with some disorder. Conventional crystallography just takes account of intensity and position and can not discover the diffuse scattering containing local structure information. So, the PDF is the powerful tool for researching the effects of dopants on the local setting of rare earth ion. We also take the method of solid state reaction to synthesize three inorganic rare-earth luminescent materials, LiEu(WO4)2, NaEu(WO4)2 and KEu(WO4)2.The structure of LiEu(WO4)2, NaEu(WO4)2 crystallize in tetragonal class, KEu(WO4)2 in monoclinic class confirmed by Rietveld refinement. LiEu(WO4)2 and NaEu(WO4)2 have the same luminescent properties i.e. the same band gap and the efficient energy transfer. Taking the crystal structure into account, the dopant ion Eu3+ has no influence on the local surrounding. However, the excited efficience of KEu(WO4)2 in ultraviolet zone is better than the other two materials. Considering the crystal structure, the truth will be the angle of Eu-O-W of KEu(WO4)2 close to 180 degree, but the angle of LiEu(WO4)2 and NaEu(WO4)2140 degree. Obviously, the energy transfer efficience of KEu(WO4)2 is stronger than that of LiEu(WO4)2 and NaEu(WO4)2. In addition, the bandwidth of KEu(WO4)2 at the hypersensitive position is larger than that of LiEu(WO4)2 and NaEu(WO4)2.Furthermore, the Stock shift of KEu(WO4)2 is also 1 nm larger than that of LiEu(WO4)2, NaEu(WO4)2. According to the PDF results, the bond length of Eu-O of KEu(WO4)2 is shorter than that of LiEu(WO4)2 and NaEu(WO4)2 which is explained by the bond valence theory. The LiEu(WO4)2 and NaEu(WO4)2 have the same bond length Eu-O, resulting in the same coordination number, eventually forming the same local surrounding of Eu+.Crystallography presents that crystal symmetry is perfectly periodic. However, owing to thermal Brownian movement, the assumption of perfect periodicity will break down even at T=0 K. Therefore the PDF obtained by powder diffraction is simply a Fourier-transform of the structure function, S(Q) (Q= 4πsinθ/λ), and thus the PDF carries no less information than the powder diffraction pattern, compared to the powder diffraction pattern.This paper, which dealing with the relationship in structure periodicity, local structure and luminescent properties by dual space method, has a guide role to design and improve the function of rare-earth luminescent materials. In sum, we can conclude that the cubic local setting is helpful for property of the long wavelength excitation. The alkali metal ions doped have no effect on the luminescent property. We can predict the effective energy transfer and long wavelength excitation by choosing the matrix with stronger superexchange and with local structure of the cubic setting. |
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