A series of new anionic lanthanide(III) complexes with the general formula NEt 4 [LnL 4 ] (1-Ln; HL = dimethyl[(4-methylphenyl)sulfonyl]amidophosphate; Ln = La, Nd, Eu), were synthesized and characterized by IR, UV− vis, and NMR spectroscopies, the differential scanning calorimetry method, thermogravimetric and X-ray analysis, and photoluminescence measurements. Single-crystal structures of NEt 4 [EuL 4 ] (1-Eu) were determined at 293 and 100 K and evidenced the single-crystal-to-single-crystal phase transition. Both phases are in the monoclinic crystal system in centrosymmetric groups of the same Laue class. The room temperature structure is in C2/c (No. 15), while lowtemperature structure is in the P2 1 /c (No. 14) space groups. The coordination environment geometry around the central europium(III) ion is a distorted square antiprism in both polymorphs, while the peripheral methoxy and tolyl groups show different orientations. This phenomenon indicates the occurrence of a thermally driven second-order phase transition during the cooling−heating process. The europium(III) complex exhibits an unusual emission spectrum, clearly dominated by the 5 D 0 − 7 F 4 bands, and an emission decay time equaling 3.5 ms, being among the highest values known for europium coordination compounds.