Yttrium(III) oxomolybdates(VI) as potential host materials for luminescence applications: an investigation of Eu3+-doped Y-2[MoO4](3) and Y-2[MoO4](2)[Mo2O7]

Two ternary yttrium(III) oxomolybdates(VI) are investigated, both structurally and spectroscopically. The crystal structure of Y-2[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 Y3+ cations, both with CN = 7 featuring a monocapped trigonal-prismatic oxygen environment, can be distinguished. Y-2[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, beta = 105.586(2)degrees) with two formula units per unit cell. In this compound the anionic environment of the crystallographically unique Y3+ cations also comprises seven oxygen atoms forming a monocapped trigonal prism. Furthermore, the crystal structure features both tetrahedral [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 Eu3+ as a sensitive probe. The emission spectra of Y-2[MoO4](3):Eu3+ and Y-2[MoO4](2)[Mo2O7]:Eu3+ are dominated by the Eu3+-typical D-5(0) -> F-7(2) transition at 614 nm. In the excitation spectra, aside from 4f-interconfigurational Eu3+ transitions at lower energies, broad charge-transfer (CT) bands due to O2- -> Eu3+ or O2- -> Mo6+ transitions dominate at higher energies. Comparing the diffuse reflectance spectra (DRS) of the undoped with the Eu3+-doped materials, the O2- -> Mo6+ LMCT process proves to be crucial for the position of the broad CT band in the excitation spectra of both yttrium oxomolybdates(VI).