Absorption and Luminescence Spectroscopy of MnO42--Doped Crystals of BaSO4

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Abstract
The first polarized low-temperature absorption and luminescence spectra of manganese-doped crystals of BaSO4 containing essentially MnO42- are reported. By using a flux composed of NaCl, KCl, and CsCl we were able to grow BaSO4:Mn6+ crystals below 620 °C. This prevents the simultaneous presence of MnO43- besides MnO42-, which was mainly responsible for the erroneous assignments of the absorption spectrum in the literature. In the BaSO4 host the MnO42- ion occupies a site of Cs symmetry, and the orbital degeneracies of the E and T states are thus lifted. Above 16 000 cm-1 the absorption spectra consist of a series of intense ligand-to-metal charge transfer (LMCT) excitations. Their marked polarization dependence allows an unambiguous band assignment in the parent Td symmetry. The three origins of the 2E → 2T2 ligand-field (LF) transition peak at 11 074, 11 570, and 11 790 cm-1. The lowest-energy component of 2T2 serves as the initial state for broadband luminescence in the near-infrared (near-IR) region with a maximum at 9300 cm-1. Below 100 K the quantum yield is unity and the radiative lifetime is 2.75 μs, and at 300 K the quantum yield is still 20%. In both the 2E ↔ 2T2 (d → d) absorption and luminescence spectra the vibrational structure is dominated by progressions in O−Mn−O bending modes whereas coupling to the totally symmetric Mn−O stretching mode is less pronounced. The luminescence band shapes for the transitions to the two orbital components of 2E are strikingly different; the Huang−Rhys parameters for the bending-mode progressions obtained from fits of simulated band shapes to the experimental spectra are 1.3 and 3.7, respectively. This is due to weak E⊗e and stronger T2⊗e Jahn−Teller (JT) effects in the ground and excited LF states, respectively. The linear vibronic coupling constants are fE ≈ 180 cm-1 and fT ≈ −730 cm-1 and the corresponding JT stabilization energies EJT(2E) ≈ 50 cm-1 and EJT(2T2) ≈ 780 cm-1, respectively.