Study of Mn ion charge state in Zn₂TiO₄ and its impact on the photoluminescence and optical absorption spectra

The Zn₂TiO₄ ceramics doped with manganese of nominal content from 0.0016 to 1.6 mol% were produced by a solid-state reaction and investigated by the X-ray diffraction, photoluminescence (PL), diffuse reflectance spectroscopy and electron paramagnetic resonance (EPR) methods. The influence of sintering conditions (annealing temperature in the range of 800–1200 °C and atmosphere, Mn concentration) on Mn charge state and red PL has been studied. All samples contained a spinel Zn₂TiO₄ crystal phase with a minor amount of ZnO. The ceramics showed a PL band peaked at 680 nm, which was ascribed to ²E_g → ⁴A_2g transition of Mn⁴⁺_Ti in the Zn₂TiO₄. The PL intensity increased with annealing temperature and Mn concentration. The highest PL intensity was found in the samples with 0.1 mol% Mn sintered at 1100 °C. In the EPR spectra, the signals ascribed to residual Cr³⁺ in the Zn₂TiO₄, Mn²⁺ in the ZnO (g = 2.0014, A = 74.1 × 10⁻⁴ cm⁻¹, b₂⁰ = 235 × 10⁻⁴ сm⁻¹) and Mn²⁺ in the Zn₂TiO₄ (g = 1.987, A = 74.510 × 10⁻⁴ сm⁻¹, b₂⁰ = 45 × 10⁻⁴ сm⁻¹) were detected. The intensity of EPR signal due to Mn²⁺ in the Zn₂TiO₄ increased noticeably in the samples with 1.0 mol% Mn and in those sintered at 1200 °C. This was accompanied by the decrease of Mn⁴⁺ PL intensity. It is concluded that manganese dopes the Zn₂TiO₄ as both Mn⁴⁺ and Mn²⁺, the Mn²⁺ incorporation is strongly promoted by high annealing temperature (> 1100 °C) and high Mn concentration (~ 1 mol%). It is proposed that optical absorption of Mn-doped Zn₂TiO ceramics in the visible spectral range is determined by spin allowed transition of Mn⁴⁺_Ti and Mn^2+/3+ donor-type photoionization transition in the Zn₂TiO₄.