Yttrium(iii) oxomolybdates(vi) as potential host materials for luminescence applications: an investigation of Eu3+-doped Y2[MoO4]3 and Y2[MoO4]2[Mo2O7]

Abstract

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 Y³⁺ 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 P2₁/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 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 Y₂[MoO₄]₃:Eu³⁺ and Y₂[MoO₄]₂[Mo₂O₇]:Eu³⁺ are dominated by the Eu³⁺-typical ⁵D₀ → ⁷F₂ 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).