The series of luminescent NaYF₄:Sm³⁺ nano- and microcrystalline materials co-doped by La³⁺, Gd³⁺, and Lu³⁺ ions were synthesized by hydrothermal method using rare earth chlorides as the precursors and citric acid as a stabilizing agent. The phase composition of synthesized compounds was studied by PXRD. All synthesized materials except ones with high La³⁺ content (where LaF₃ is formed) have a β-NaYF₄ crystalline phase. SEM images demonstrate that all particles have shape of hexagonal prisms. The type and content of doping REE significantly effect on the particle size. Upon 400 nm excitation, phosphors exhibit distinct emission peaks in visible part of the spectrum attributed to ⁴G_5/2→⁶H_J transitions (J = 5/2–11/2) of Sm³⁺ ion. Increasing the samarium (III) content results in concentration quenching by dipole–dipole interactions, the optimum Sm³⁺concentration is found to be of 2%. Co-doping by non-luminescent La³⁺, Gd³⁺ and Lu³⁺ ions leads to an increase in emission intensity. This effect was explained from the Sm³⁺ local symmetry point of view.

Crystalline lanthanide terephthalates, Ln₂bdc₃‧nH₂O (Ln = La–Lu, excluding Pm), were synthesized using a surfactant-free, ultrasound-assisted method. This approach yielded microcrystals with diverse shapes and sizes ranging from 2 to 10 μm. Notably, under these conditions, lutetium terephthalate uniquely crystallized as Lu₂(1,4-bdc)₃·2.5H₂O, while the remaining lanthanides formed tetrahydrate terephthalates, Ln₂bdc₃‧4H₂O (Ln = La–Nd, Sm–Yb).

The complexation behavior of lanthanide(III) ions with terephthalic acid (1,4-benzene-dicarboxylic acid) in 0.01 M KNO₃ aqueous solutions was studied across a broad pH range and at two metal-to-ligand ratios using potentiometric titration combined with photoluminescence spectroscopy. Chemometric analysis of titration curves enabled the determination of relative molar fractions, stability constants, and probable stoichiometry of the formed complexes. In solutions with a 1:2 metal-to-ligand ratio, bis-complexes (two terephthalate ligands per lanthanide ion) predominated, while ligand-rich conditions favored the formation of tetra-complexes (four ligands per metal ion). In alkaline media, bis-complexes transform into mixed hydroxy-terephthalate species. Meanwhile, for the tetra-complexes, the addition of NaOH results in the formation of lanthanide ion hydroxo complexes without organic ligands. The structural diversity of these complexes, driven by the terephthalate ligand’s tendency to maximize denticity, suggested dimeric or oligomeric configurations. The stability constants and structural features of complexes in solution were found to align with those of known solid-state lanthanide–terephthalate polymers, highlighting their potential as models for polymeric structures.

Stability constants and fractional distribution of europium(III) ion complexes with aminotrismethylenephosphonic acid in aqueous solutions at 25°C were determined potentiometrically in numerous systems with various metal-to-ligand ratio namely 1 : 1, 1 : 2 and 1 : 4. This approach allowed to accurately describe equilibrium process by set of complexes including previously unreported Eu₂(Н₂L)₂, Eu₂Н₃L₂, Eu₂(НL)₂, Eu₂НL₂ forms. The dimerization is thermodynamically favorable for complexes of all protonation degrees. Apparently the dimerization is a way to achieve the most advantageous arrangement of positively charged europium ions and negatively charged phosphonic groups. Eu : ATMP = 1 : 2 complexes are formed in excess of the ligand as a result of a thermodynamically favorable process, but the formation reactions of EuL₂ и Eu(НL)₂ species seem to have kinetic restrictions. The luminescence spectroscopy method was applied for testing proposed complexation model. We demonstrated that obvious differences in the fine structure of spectra are observed between the spectra of solutions containing predominantly dimeric forms Eu₂H_xL₂ and the spectra of solutions containing predominantly bis-complexes EuH_xL₂. Thus, the results of the luminescence spectroscopy are in good agreement with the results of potentiometrictitration in the europium(III)–ATMP system.