Optical materials

Crystal structure, luminescence properties and thermometric characteristics of Ba$_2$Gd$_2$Ge$_4$O$_{13}$:Tb$^{3+}$, Tb$^{3+}$/Eu$^{3+}$ germanates

A. V. Chvanova, O. A. Lipina, A. Yu. Chufarov, A. P. Tyutyunnik, L. L. Surat, V. G. Zubkov

Institute of Solid State Chemistry, Urals Branch of the Russian Academy of Sciences, Ekaterinburg

Abstract: In this work, the synthesis of Ba$_2$Gd$_{2-x}$Tb$_x$Ge$_4$O$_{13}$ ($x$ = 0.05–0.2) and Ba$_2$Gd$_{1.85-y}$Tb$_{0.15}$Eu$_y$Ge$_4$O$_{13}$ ($y$ = 0.1–0.6) solid solutions was carried out for the first time and the crystal structure of the synthesized phases was studied. The XRPD study has verified that all the samples are isostructural to Ba$_2$Gd$_2$Ge$_4$O$_{13}$ and crystallize in the monoclinic system, S.G. $C2/c$, $Z$ = 4. The photoluminescence spectra of Ba$_2$Gd$_{2-x}$Tb$_x$Ge$_4$O$_{13}$ germanates consist of a number of lines in the range of 365–650 nm, caused by $^5D_{3,4}\to{}^7F_J$ transitions in Tb$^{3+}$ ions. Based on the results of concentration studies, it has been found that Ba$_2$Gd$_{1.85}$Tb$_{0.15}$Ge$_4$O$_{13}$ germanate exhibits the highest emission intensity. This content of Tb$^{3+}$ ($x$ = 0.15) was fixed and the codoped Ba$_2$Gd$_{1.85-y}$Tb$_{0.15}$Eu$_y$Ge$_4$O$_{13}$ phases were synthesized. Codoping with Eu$^{3+}$ ions has led to the appearance of additional luminescent lines in the 570–720 nm region, which is associated with $^5D_0\to{}^7F_J$ transitions in europium ions. As the Eu$^{3+}$ content increases, the color coordinates gradually change, which allows fine-tuning of the emission color of the phosphors. High-temperature studies ($T$ = 298-498 K) were carried out for Ba$_2$Gd$_{1.75}$Tb$_{0.15}$Eu$_{0.1}$Ge$_4$O$_{13}$ germanate. The temperature dependences of the luminescence of the bands at 530–560 nm ($^5D_4\to{}^7F_5$ transition in Tb$^{3+}$) and 603–640 nm ($^5D_0\to{}^7F_2$ transition in Eu$^{3+}$) as well as LIR = $I_{603-640}$ nm/$I_{530-560}$ nm were plotted. The calculated maximum values of absolute and relative sensitivity were 0.34% $\times$ K$^{-1}$ and 0.15% $\times$ K$^{-1}$.

Keywords: luminescence, germanate, europium, terbium, thermometry.

Received: 15.04.2024
Revised: 27.05.2024
Accepted: 27.05.2024

DOI: 10.61011/OS.2024.06.58644.6268-24

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