Structural and spectral characteristics of La$_{0.99-x}$Tb$_x$Eu$_{0.01}$BO$_3$ orthoborates and energy transfer from Tb$^{3+}$ to Eu$^{3+}$

The structure, infrared (IR) absorption spectra, luminescence spectra (SL), and luminescence excitation spectra (LES) of La$_{0.99-x}$Tb$_x$Eu$_{0.01}$BO$_3$ orthoborates synthesized at 970$^\circ$C at 0 $\le x\le$ 0.99 were studied. An increase in $x$ leads to the successive emergence of three structural states of these compounds. At 0 $\le x\le$ 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 $\le x\le$ 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 La$_{0.99-x}$Tb$_x$Eu$_{0.01}$BO$_3$ orthoborates, as well as in La$_{0.99-x}$Tb$_x$Eu$_{0.01}$BO$_3$, the band with $\lambda_{\mathrm{ex}}$ = 369 nm ($^7$F$_0$ $\to$ $^5$D$_2$) in the LES and the band in the wavelength range of 577–582 nm ($^5$D$_0$ $\to$ $^7$F$_0$) 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 Eu$^{3+}$ ions in La$_{0.99-x}$Tb$_x$Eu$_{0.01}$BO$_3$ orthoborates, which occurs when the sample is excited by light in the absorption bands of Tb$^{3+}$ ions, is due to the transfer of the electron excitation energy from Tb$^{3+}$ ions to Eu$^{3+}$ ions. The efficiency of this process in La$_{0.99-x}$Tb$_x$Eu$_{0.01}$BO$_3$ samples with an aragonite structure is 86%.