The structure, IR absorption, luminescence and luminescence excitation spectra of Ce3+ and Tb3+ ions have been studied in the crystals of Lu1−x−yCexTbyBO3 solid solutions at 0 ≤ x ≤ 0.01, 0.07 ≤ y ≤ 0.16. It has been shown that Lu1−yTbyBO3 solid solution synthesized at Т = 970°С has the structure of a calcite at 0  y ≤ 0.07, the structure of a vaterite at y ≥ 0.16, and is two-phase in a range of 0.07  y < 0.16. It has been determined that additional alloying of two-phase Lu1-yTbyBO3 samples with the vaterite phase fraction of not more than ~ 20 % with 0.5 – 1 at.% of Ce3+ leads to a significant decrease in the amount of the vaterite phase. It has been demonstrated that Ce3+ ions can be used as structure-sensitive and optically active marks when analyzing the structural state of rare earth orthoborates.

The structure, IR absorption and luminescence spectra of Pr0.99xLuxEu0.01BO3 orthoborates synthesized at 970°C were studied at 0 ≤ x ≤ 0.99. An increase in the concentration of lutetium leads to a sequential change of the structural state of the orthoborates. At first, the orthoborates are single-phase and have an aragonite structure (0 ≤ x ≤ 0.1). Then, they become two-phase and contain the aragonite and vaterite phases (0.1 < x < 0.6). With a further increase in х (0.6 < x ≤ 0.8), the compounds are single-phase with a vaterite structure, then they contain the vaterite and calcite phases (0.8 < x ≤ 0.95), and, finally, they become single-phase with a calcite structure (0.95 < x ≤ 0.99). An unambiguous correspondence between the structural modification and IR spectra of these compounds was established. It is shown that the emission of Eu3 + ions is observed in samples where the concentration of europium exceeds that of praseodymium.

The structure, IR absorption, luminescence, and luminescence excitation spectra of La0.99xYxEu0.01BO3 orthoborates (0 ≤ x ≤ 0.25) synthesized at 970°С were studied at 0 ≤ x ≤ 0.99. An increase in х leads to a sequential change of the structural state of these compounds. At 0 ≤ x ≤ 0.1, the samples are single-phase and have the aragonite structure. Within the range of 0.1 < x ≤ 0.8, the samples are two-phase: the vaterite phase is observed along with the aragonite structure. At 0.8 < x ≤ 0.99, the samples are single-phase and have the vaterite structure. Correspondence between the structure and spectral characteristics of these compounds was established. It is demonstrated that with an increase in the Y3+ concentration, the vaterite phase is formed first in the bulk of microcrystals having the aragonite structure and then in the entire sample. It is shown for the first time that a band with the maximum of 469 nm is observed in the luminescence excitation spectrum (LES) of samples having the vaterite structure and is absent in samples having the aragonite structure. It is revealed that a band in the luminescence spectrum, corresponding to the 5D0→7F0 electron transition, as well as 469-nm-band in the LES, can be an indicator of the structural state of the sample.

The structure, IR absorption, luminescence, and luminescence excitation spectra of La0.98xLuxEu0.02 BO3 orthoborates synthesized at 970°C were studied at 0 ≤ x ≤ 0.98. An increase in х leads to a sequential change of the structural state of the orthoborates. At first, the compound has the aragonite structure. Then, it becomes two-phase and contains the aragonite and vaterite phases. With a further increase in х, the compounds have the vaterite structure, then the vaterite and calcite structure, and, finally, the calcite structure. Correspondence between the structure and spectral characteristics of these compounds was established. Luminescence spectra were investigated at different wavelengths of exciting light. This allowed obtaining information on the structure of a near-surface layer and the bulk of microcrystals of the investigated samples. It is shown that the vaterite phase arises in the bulk of microcrystals of samples that have the aragonite structure.

Investigated are the structure, luminescence, and IR absorption spectra of (Lu1−xEux)2(WO4)3 solid solutions in a wide range of Eu concentrations (0 ≤ x ≤ 1). A sequential replacement of two types of crystalline phases occurs with an increase in Eu concentration. The orthorhombic phase (Pbcn space group) of tungstate solid solutions is observed at 0 ≤ x < 0.5. A monoclinic phase (C2/c space group) arises along with the orthorhombic one in the range of 0.5 ≤ x ≤ 0.8, while at x > 0.8, the solid solution has a monoclinic structure. The correspondence between the structure and spectral characteristics of these compounds is established. A change in the structural state results in a change in luminescence spectra, as well as in the spectra of luminescence excitation of tungstates. The isotherm of water adsorption by the tungstate samples and its effect on their spectral and structural characteristics are studied in view of the wetting ability of (Lu1−xEux)2(WO4)3 solid solutions. It is established that the maximum luminescence under the resonant excitation of Eu3+ ions is observed in the samples with a monoclinic C2/c structure at x ~0.9.