The effect of MgO insertion into the interstitial spaces of $BaY_2O_4$ on the upconversion response of $Er^{3+}:Yb^{3+}$ at illumination with 976 nm excitation light

Abstract

The efficiencies and form of the upconversion spectra of rare earth ions embedded in crystalline matrices strongly depend upon the phonon dispersion of these, since the inter-level energy transition probabilities are influenced by the phonon energies. The aim of this study is to show a novel method of influencing the phonon energy values of crystalline $BaY_2O_4$ and, by this, to finely tune the spectra of any rare earth ion as a dopant of it. We used $Er^{3+}$ sensitized by $Yb^{3+}$ as dopants, but the same method can be generalized for other kind of rare earth ions. As such, we synthesized $BaY_2O_4$ and $BaY_2O_4:MgO (BaY_2MgO_5)$ ceramics having as activator-sensitizer pairs the $Er^{3+}$ 1% and $Yb^{3+}$ 2% relative to the substituted $Y^{3+}$. Firstly, we synthesized the mix of precursor metallic oxides by the citrate-gel method, pressed these into pellets, and sintered them to obtain doped crystalline ceramic samples. The ceramic samples were illuminated with 976 nm from a laser diode, and the resulting upconversion spectra of $Er^{3+}$ were measured. $BaY_2O_4$ and $BaY_2O_4:MgO (BaY_2MgO_5)$ have almost the same XRD signature, showing that the crystal unit cells have the same parameters but they differ in the phononic energies due to the presence of the additional MgO. Comparing the results of these two extreme cases $BaY_2O_4$ vs. $BaY_2O_4:MgO (BaY_2MgO_5)$ reveals that the presence of the MgO modifies the upconversion response. So, this study fulfills two goals: First, it reveals the structure of the undocumented $BaY_2O_4:MgO (BaY_2MgO_5)$ and shows how it is based on the crystal structure of $BaY_2O_4$ with $Mg^{2+}$ ions occupying the unit cell interstitions. Secondly, it suggests a method of tuning the phononic energies of $BaY_2O_4$ and, as a consequence, the upconversion response of $Er^{3+}$.