Pure and RE 3+ -Doped La7O6 (VO4) 3 (RE = Eu, Sm): Polymorphism Stability and Luminescence Properties of a New Oxyvanadate Matrix
Résumé
Two polytypes of the new oxyvanadate matrix La 7 O 6 (VO 4) 3 were identified and deeply characterized. The crystal structure of the α-polytype was solved using a combination of precession electron diffraction and X-ray powder diffraction techniques. It crystallizes in a monoclinic unit cell with space group P2 1 , a= 13.0148(3) Å, b= 19.1566(5) Å, c= 7.0764(17) Å and β= 99.87(1)°. Its structure is built upon [La 7 O 6 ] +9 polycationic units at the origin of a porous 3D networks evidencing rectangular channels filled by isolated (VO 4) tetrahedra. An in situ High Temperature XRD study highlights a number of complex phase transitions assorted with the existence of a β-polytype also refined in a monoclinic unit cell, space group P2 1 /n, a= 13.0713(4) Å, b= 18.1835(6) Å, c= 7.1382(2) Å and β= 97.31(1)°. Thus, during the transitions, while the polycationic network is almost identical, the vanadate's geometry is largely modified. The use of Eu 3+ and Sm 3+ at different concentrations into the host lattice is possible using solid-state techniques. The photoluminescence (PL), PL excitation (PLE) spectra and luminescence decay times were recorded and discussed. The phosphors present an emission light being bright and reddish-orange after excitation under UV. This is mainly due to the V-O band and f-f transitions. Whatever the studied polytype, the final luminescence properties are retained during the heating/cooling process.
Domaines
Chimie
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