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–xLix)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 → 7FJ) 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.