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 (P1 and I4₁/a space groups). Under UV excitation, the orthorhombic ((NH₄)_1-xLi_x)Eu(MoO₄)₂ (x = 0.074-0.172) with Pbcn space group exhibits intense red photoluminescence. The emission spectrum shows multiple sharp lines from Eu³⁺ (⁵D₀ → ⁷F_J) 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.