Structural and spectral characteristics of La-=SUB=-0.99-x-=/SUB=-Tb-=SUB=-x-=/SUB=-Eu-=SUB=-0.01-=/SUB=-BO-=SUB=-3-=/SUB=- orthoborates and energy transfer from Tb-=SUP=-3+-=/SUP=- to Eu-=SUP=-3+-=/SUP=-

Abstract

The structure, infrared (IR) absorption spectra, luminescence spectra (SL), and luminescence excitation spectra (LES) of La0.99-xTbxEu0.01BO3 orthoborates synthesized at 970oC at 0≤ x≤ 0.99 were studied. An increase in x leads to the successive emergence of three structural states of these compounds. At 0≤ x≤0.2, orthoborates have an aragonite structure; then, at 0.2<x <0.89, they become two-phase and contain the aragonite and vaterite phases. At 0.89≤ x≤ 0.99, the compounds have a vaterite structure. A correspondence between the structure and spectral characteristics of these compounds was established. It is shown that in La0.99-xTbxEu0.01BO3 orthoborates, as well as in La0.99-xYxEu0.01BO3, the band with λex=369 nm (7F0->5D2) in the LES and the band in the wavelength range of 577-582 nm (5D0->7F0) in the SL of these compounds can serve as indicators of the structural state of the sample. In the SL of the samples containing the aragonite and vaterite phases, two bands corresponding to these structures were simultaneously observed for the first time. It was established that the luminescence of Eu3+ ions in La0.99-xTbxEu0.01BO3 orthoborates, which occurs when the sample is excited by light in the absorption bands of Tb3+ ions, is due to the transfer of the electron excitation energy from Tb3+ ions to Eu3+ ions. The efficiency of this process in La0.9Tb0.09Eu0.01BO3 samples with an aragonite structure is 86%. Keywords: rare earth orthoborates, crystal structure, X-ray diffraction analysis, IR spectroscopy, luminescence spectra, phosphors for LEDs.

Structural and spectral characteristics of La-=SUB=-0.99-x-=/SUB=-Y-=SUB=-x-=/SUB=-Eu-=SUB=-0.01-=/SUB=-BO-=SUB=-3-=/SUB=- orthoborates

“…The sequence of structural states in La 0.98−x Lu x Eu 0.02 BO 3 compounds with an increase of Lu 3+ concentration can be schematically represented as follows: aragonite (at 0 ≤ x < 0.15) → aragonite + vaterite (0.15 ≤ x ≤ 0.8) → vaterite (0.8 < x < 0.88) → vaterite + calcite (0.88 ≤ x < 0.93) → calcite (0.93 ≤ x ≤ 0.98). The same sequence of structural state change is observed in Pr 0.99−x Lu x Eu 0.01 BO 3 [27] orthoborates synthesized at T = 970 • C (the temperature of presence of the PrBO 3 aragonite structure [22][23][24]). It should be noted that singlephase vaterite in Pr 0.99−x Lu x Eu 0.01 BO 3 orthoborates forms in a wider range of Lu 3+ concentrations (0.6 < x ≤ 0.8) than in La 0.98−x Lu x Eu 0.02 BO 3 (0.8 < x < 0.88).…”

“…Thus, microcrystals sized 1−3 µm are observed in the samples of La 0.99−x Y x Eu 0.01 BO 3 having the aragonite structure, like in the La 0.98−x Re x Eu 0.02 BO 3 compounds (Re = Lu, Pr) [9,27]. Microcrystals sized 1−6 µm, which have discontinuities in the form of cracks, are observed in the Y 0.99 Eu 0.01 BO 3 compounds with the vaterite struc-…”