New Solid Solutions Based on Barium Borates Incorporating Bismuth and Rare-Earth Elements: Thermal Expansion, Crystal Structures, and Photoluminescence

Seven new series of solid solutions (42 compositions total), doped with REE³⁺ ions, were synthesized based on two barium borate matrices: BaBi₂B₂O₇ and Ba₃REE₂(BO₃)₄ (REE = Y, Eu). Nine crystal structures were refined using single-crystal X-ray diffraction. Additional characterization included Raman spectroscopy, IR spectroscopy, thermal analysis , and high-temperature powder X-ray diffraction. Photoluminescence spectra were recorded for all series, and the temperature dependence of photoluminescence intensity was examined.

For the BaBi₂B₂O₇ matrix, solid solutions doped with REE³⁺ were obtained. The compositional ranges of continuous solid solutions were determined. Co-doping of the BaBi₂B₂O₇ lattice was found to expand the miscibility regions of the solid solutions. Eight crystal structures were refined from single-crystal data for BaBi_2−xEu_xB₂O₇ (х = 0.1, 0.2, 0.4), BaBi_2−xSm_xB₂O₇ (х = 0.05, 0.3), BaBi_2−xTb_xB₂O₇ (х = 0.1, 0.3, 0.4). All compounds crystallize in the BaBi₂B₂O₇structure type (hexagonal system, space group P6₃). The structure contains three independent crystallographic sites for large cations, each split into Ba and Bi subsites. REE³⁺ ions occupy the Bi subsites. Upon REE³⁺activation, the larger Sm and Eu ions preferentially occupy the M1 and M2 sites (largest coordination polyhedra volumes), whereas Tb³⁺, with the smallest ionic radius, occupies the M3 site (smallest polyhedral volume).

A new solid-solution series, Ba₃Y_2−xEr_x(BO₃)₄ (х = 0.01—0.3), along with end-member borates, Ba₃Y₂(BO₃)₄, Ba₃Eu₂(BO₃), was synthesized via melt crystallization. Thermal expansion was studied for Ba₃Eu₂(BO₃) and Ba₃Y₂(BO₃)₄, and the crystal structure of Ba₃Y₂(BO₃)₄ was refined across a wide temperature range (40 data points). Photoluminescence spectra and their temperature dependence were investigated for the Ba₃Y_2−xEr_x(BO₃)₄ series. The optimal activator concentration was determined to be x = 0.1.

This work was supported by the RSF (No. 22-13-00317-P) and utilized equipment from the "RDMI" and "OLMIV" of the Scientific Park of SPbSU.