Mn²⁺ Luminescence in Ca₉Zn_1-MnNa(PO₄)₇ Solid Solution, 0 ≤ x ≤ 1

The solid solution Ca₉Zn_1-MnNa(PO₄)₇ (0 ≤ x ≤ 1.0) was obtained by solid-phase reactions under the control of a reducing atmosphere. It was demonstrated that Mn²⁺-doped phosphors can be obtained using activated carbon in a closed chamber, which is a simple and robust method. The crystal structure of Ca₉Zn_1-MnNa(PO₄)₇ corresponds to the non-centrosymmetric β-Ca₃(PO₄)₂ type (space group R3c), 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 ⁴T₁ → ⁶A₁ electron transition of Mn²⁺ ions in the β-Ca₃(PO₄)₂-type host. The absence of transitions corresponding to Mn⁴⁺ ions confirms the success of the reduction synthesis. The intensity of the Mn²⁺ emission band in Ca₉Zn_1-MnNa(PO₄)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 Mn²⁺ and Zn²⁺ ions replace calcium in the M5 (octahedral) sites in the β-Ca₃(PO₄)₂ crystal structure. According to Rietveld refinement, Mn²⁺ and Zn²⁺ ions jointly occupy the M5 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 Mn²⁺ ions in the neighboring M5 sites are responsible for the lack of concentration quenching of luminescence below x = 0.5.