Impurity centers and host microstructure in weakly doped SrTiO3:Mn crystals: new findings

Enhancing Luminescence and Controlling the Mn Valence State of Gd3Ga5-x-δAlx-y+δO12:yMn Phosphors by the Design of the Garnet Structure

Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn solid solutions with improving luminescence properties were prepared via cation substitution and a controllable Mn valence state. The abnormal autoreduction from Mn⁴⁺ to Mn²⁺ ions was observed during the formation of Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn. The doped manganese ions occupy octahedral Ga³⁺(1) and Al³⁺(1) sites to form the Mn²⁺ luminescent center with red emission at 630 nm and Mn⁴⁺ luminescent centers with deep red light emission at 698 nm, respectively, matching well with the red light absorption of phytochrome (P_R) and the far-red light absorption of phytochrome (P_FR). With the design of the concentration of Al³⁺ and doped manganese ions, the photoluminescence (PL) of Mn⁴⁺/Mn²⁺ (corresponding to P_FR/P_R) can be tuned, which is very useful for controlling the plant growth. Moreover, the PL intensity of Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn can be increased by 6.8 times by substituting Al³⁺ for Ga³⁺. The thermal stability is also enhanced significantly. Finally, a series of warm white-light-emitting diodes (WLEDs) with good performance were fabricated using the as-prepared Gd₃Ga_5-x-δAl_x-0.012+δO₁₂:0.012Mn phosphor. The results show that the designed Gd₃Ga_5-x-δAl_x-y+δO₁₂:yMn phosphors have potential practical values in plant-growth light-emitting diodes (LEDs) and high-performance WLEDs. Moreover, our strategy not only provides a unique inspiration for tuning the valence states of Mn but also designs new advanced luminescent materials.

(Sr,Mn)TiO3: a magnetoelectric multiglass

By close analogy with multiferroic materials with coexisting long-range electric and magnetic orders a "multiglass" scenario of two different glassy states is observed in Sr(0.98)Mn(0.02)TiO(3) ceramics. Sr-site substituted Mn2+ ions are at the origin of both a polar and a spin glass with glass temperatures T(g) approximately equal to 38 K and < or =34 K, respectively. The structural freezing triggers that of the spins, and both glassy systems show individual memory effects. Thanks to strong spin-phonon interaction within the incipient ferroelectric host crystal SrTiO3, large higher order magnetoelectric coupling occurs between both glass systems.