Novel Findings on the Optical Properties of Mn4+-Doped (Rb,K)2Ge4O9 Phosphors

Herein, Mn4+-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 KxRb2-xGe4O9. 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 Mn4+ 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, K2Ge4O9 and Rb2Ge4O9. 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 Mn4+ ions via an excitation of (4)A(2g)-T-4(1g) or T-4(2g).