Mn 2+ Luminescence in Ca 9 Zn 1- x Mn x Na(PO 4 ) 7 Solid Solution, 0 ≤ x ≤ 1.

The solid solution Ca 9 Zn 1- x Mn x Na(PO 4 ) 7 (0 ≤ x ≤ 1.0) was obtained by solid-phase reactions under the control of a reducing atmosphere. It was demonstrated that Mn 2+ -doped phosphors can be obtained using activated carbon in a closed chamber, which is a simple and robust method. The crystal structure of Ca 9 Zn 1- x Mn x Na(PO 4 ) 7 corresponds to the non-centrosymmetric β-Ca 3 (PO 4 ) 2 type (space group R 3 c ), as confirmed by powder X-ray diffraction (PXRD) and optical second-harmonic generation methods. The luminescence spectra in visible area consist of a broad red emission peak centered at 650 nm under 406 nm of excitation. This band is attributed to the 4 T 1 → 6 A 1 electron transition of Mn 2+ ions in the β-Ca 3 (PO 4 ) 2 -type host. The absence of transitions corresponding to Mn 4+ ions confirms the success of the reduction synthesis. The intensity of the Mn 2+ emission band in Ca 9 Zn 1- x Mn x Na(PO 4 ) 7 rising linearly with increasing of x at 0.05 ≤ x ≤ 0.5. However, a negative deviation of the luminescence intensity was observed at x = 0.7. This trend is associated with the beginning of a concentration quenching. At higher x values, the intensity of luminescence continues to increase but at a slower rate. PXRD analysis of the samples with x = 0.2 and x = 0.5 showed that Mn 2+ and Zn 2+ ions replace calcium in the M 5 (octahedral) sites in the β-Ca 3 (PO 4 ) 2 crystal structure. According to Rietveld refinement, Mn 2+ and Zn 2+ ions jointly occupy the M 5 site, which remains the only one for all manganese atoms within the range of 0.05 ≤ x ≤ 0.5. The deviation of the mean interatomic distance (∆ l ) was calculated and the strongest bond length asymmetry, ∆ l = 0.393 Å, corresponds to x = 1.0. The large average interatomic distances between Mn 2+ ions in the neighboring M 5 sites are responsible for the lack of concentration quenching of luminescence below x = 0.5.