Anomalous spontaneous-reduction of Mn7+/Mn4+ to Mn2+ and luminescence properties in Zn2GeO4:Mn

Abstract

The reduction-atmosphere of CO, H₂/N₂ or H₂ is commonly used for the synthesis of divalent manganese ion (Mn²⁺) activated luminescent materials. Herein, by using KMnO₄ or MnO₂ as the manganese source, the spontaneous-reduction of Mn⁷⁺ or Mn⁴⁺ to Mn²⁺ in air with intense Mn²⁺-emission peaking at 536 nm has been first demonstrated in Zn₂GeO₄ synthesized by a high temperature solid state reaction method. The effects of synthetic conditions including Mn sources (MnCO₃, MnO₂ or KMnO₄) and doping concentration on the crystal structure, spontaneous-reduction behaviors, photoluminescence and long-persistent luminescence properties have been investigated and discussed in detail. Based on the static and dynamic spectral measurements, electron paramagnetic resonance (EPR), XPS and crystal structure characterization, a charge compensation mechanism has been proposed to interpret the spontaneous-reduction phenomenon of Mn⁴⁺(Mn⁷⁺) to Mn²⁺ in Zn₂GeO₄, which is extended to some other germanates A₂BGeO₄ (A = Li, Na; B = Zn, Mg) as well. These findings would be beneficial not only for developing a low-cost and safe strategy to produce Mn²⁺ activated luminescent materials but also for providing new insights into improving the photoluminescence and long-persistent luminescence properties of Mn²⁺.