Orthorhombic (NH₄,Li)Eu(MoO₄)₂ Solid Solution: Crystal Structure and Red-Emitting Photoluminescence

A novel orthorhombic polymorph of (NH₄,Li)Eu(MoO₄)₂ enabled by partial substitution of NH₄ with Li was synthesized via a one-pot low-temperature hydrothermal route using a Li/NH₄ precursor solution. Synchrotron X-ray powder diffraction and Rietveld refinement confirmed that this phase crystallizes in an orthorhombic lattice, representing a new structure type distinct from the known triclinic and tetragonal phases (<i>P</i>1 and <i>I</i>4₁/<i>a</i> space groups). Under UV excitation, the orthorhombic ((NH₄)_1-<i>x</i>Li_<i>x</i>)Eu(MoO₄)₂ (<i>x</i> = 0.074-0.172) with <i>Pbcn</i> space group exhibits intense red photoluminescence. The emission spectrum shows multiple sharp lines from Eu³⁺ (⁵D₀ → ⁷F_<i>J</i>) transitions, with a dominant peak at ∼615 nm (⁵D₀ → ⁷F₂) indicative of a noncentrosymmetric Eu³⁺ site environment. The pronounced crystal-field splitting of these emission lines further corroborates the Eu³⁺ site symmetry deduced from the structure. This facile hydrothermal approach demonstrates an effective strategy to stabilize previously unreported polymorphs at mild conditions, underscoring its advantage in accessing new structure-property regimes for rare-earth molybdate phosphors.