Phosphorescence in Mn 4+ -Doped R + / R 2+ Germanates ( R + = Na + or K + , R 2+ = Sr 2+ ).

Single crystals of three new compounds, Na 0.36 Sr 0.82 Ge 4 O 9 ( 1 , proposed composition), Na 2 SrGe 6 O 14 ( 2 ), and K 2 SrGe 8 O 18 ( 3 ), were obtained and characterized using single-crystal X-ray diffraction. Their structures contain three-dimensional (3D) anionic frameworks built from GeO 4 and GeO 6 polyhedra. The presence of octahedral Ge 4+ sites makes the new phases suitable for Mn 4+ substitution to obtain red-emitting phosphors with a potential application for light conversion. Photoluminescence properties of Mn 4+ -substituted Na 2 SrGe 6 O 14 ( 2 ) and K 2 SrGe 8 O 18 ( 3 ) samples were studied over a range of temperatures, and red light photoluminescence associated with the electronic transitions of tetravalent manganese was observed. The Na 2 SrGe 6 O 14 ( 2 ) phase was also substituted with Pr 3+ on the mixed Na-Sr site similar to the previously studied Na 2 CaGe 6 O 14 :Pr 3+ . The red emission peak of the Pr 3+ activator occurs at a shorter wavelength (610 nm) compared to that of Mn 4+ (662-663 nm). Additionally, second harmonic generation (SHG) data were collected for the noncentrosymmetric Na 2 SrGe 6 O 14 ( 2 ) phase, indicating weak SHG activity. Diffuse reflectance spectroscopy and density of states calculations were performed to estimate the band gap values for pristine Na 2 SrGe 6 O 14 ( 2 ) and K 2 SrGe 8 O 18 ( 3 ) phases.