Yttrium(III) oxomolybdates(VI) as potential host materials for luminescence applications: an investigation of Eu~(3+)-doped Y2[MoO4]3 and Y2[MoO4]2[Mo2O7]

摘要

Two ternary yttrium(III) oxomolybdates(VI) are investigated, both structurally and spectroscopically. The crystal structure of Y2[MoO4]3 was solved at room temperature in the orthorhombic space group Pba2 (a = 1030.21(3), b = 1032.41(3), c = 1057.25(3) pm, Z = 4). In the unit cell, three discrete ortho-oxomolybdate(VI) units [MoO4]~(2-) and two Y~(3+) cations, both with CN = 7 featuring a monocapped trigonal-prismatic oxygen environment, can be distinguished. Y2[MoO4]2[Mo2O7] crystallizes monoclinically in the space group P2_1/c (a = 681.85(2), b = 959.13(3), c = 1052.99(3) pm, β = 105.586(2)°) with two formula units per unit cell. In this compound the anionic environment of the crystallographically unique Y~(3+) cations also comprises seven oxygen atoms forming a monocapped trigonal prism. Furthermore, the crystal structure features both tetrahedrai [MoO4]~(2-) and pyroanionic [Mo2O7]~(4-) entities, the latter in staggered conformation, with the bridging oxygen atom between the two vertex-shared [MoO4]~(2-) tetrahedra residing on an inversion centre. Besides self-activated emission, resulting from the oxomolybdate(VI) units (maximum at around 600 nm), both compounds have the potential to be used as luminescence host materials, shown by spectroscopic studies involving Eu~(3+) as a sensitive probe. The emission spectra of Y2[MoO4]3:Eu~(3+) and Y2[MoO4]2[lvlo2O7]:Eu~(3+) are dominated by the Eu~(3+)-typical ~5D0→ ~7F2 transition at 614 nm. In the excitation spectra, aside from 4f-interconfigurational Eu~(3+) transitions at lower energies, broad charge-transfer (CT) bands due to O~(2-) → Eu~(3+) or O~(2-) → Mo~(6+) transitions dominate at higher energies. Comparing the diffuse reflectance spectra (DRS) of the undoped with the Eu~(3+)-doped materials, the O~(2-) → Mo~(6+) LMCT process proves to be crucial for the position of the broad CT band in the excitation spectra of both yttrium oxomolybdates(VI).