Novel Findings on the Optical Properties of Mn4+‐Doped (Rb, K)2Ge4O9 Phosphors. Issue 9 (7th March 2022) - Oka, Ryohei
Abstract : Herein, Mn 4+ ‐doped alkaline‐germanate phosphors are synthesized by using starting chemicals of alkaline (K, Rb) carbonates, GeO2 and MnO2 . The compounds exhibit bright red photoluminescence (PL) under ultraviolet and blue excitation. X‐ray diffraction (XRD) and Raman investigation show the formation of solid solutions of trigonal K x Rb2− x Ge4 O9 . The potassium content dependence of the red PL shows the intensity maximized at x = 0.5 and 1.5. This behavior is well matched with the absorption of Mn 4+ ions. PL quantum efficiencies (QEs) are also estimated and compared with a previous result for germanates synthesized with Mn carbonates as a starting material. The radiative and nonradiative transition probabilities are also estimated to understand the reason for the observed PL properties, revealing the higher nonradiative transition probability resulting from concentration quenching due to the starting chemical of MnO2 . Ab initio calculations are also performed on the electronic structure of the host crystals of the end members of the obtained compounds, K2 Ge4 O9 and Rb2 Ge4 O9 . The optical bandgap calculated varies between K and Rb, while the observed ones are not sensitive to the K content x . Nevertheless, they are high enough for the optimal red emission of Mn 4+ ions via an excitation of 4 A2g – 4 T1g or 4 T2g . Abstract : Herein, Mn 4+ ‐doped alkaline‐germanate phosphors, K x Rb2− x Ge4 O9 :0.02%Mn 4+, are synthesized. The photoluminescence (PL) quantum efficiencies are quite different from those of a previous phosphor, which shall be discussed in terms of a possible concentration quenching of Mn 4+ PL due to the starting chemical MnO2 . Ab initio calculations are also performed to the host crystals of the end members of the obtained compounds, K2 Ge4 O9 and Rb2 Ge4 O9 .