Thomas C. Brunold and Hans U. Güdel
Absorption and Luminescence Spectroscopy of MnO₄^2--Doped Crystals of BaSO₄
Inorg. Chem. 36, 1946-1954 (1997)                 

Abstract: The first polarized low-temperature absorption and luminescence spectra of manganese-doped crystals of BaSO₄ containing essentially MnO₄^2- 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 MnO₄^3- besides MnO₄^2-, which was mainly responsible for the erroneous assignments of the absorption spectrum in the literature. In the BaSO₄ host the MnO₄^2- ion occupies a site of C_s 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 T_d 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 ms, 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 Ee and stronger T₂e Jahn-Teller (JT) effects in the ground and excited LF states, respectively. The linear vibronic coupling constants are f_E 180 cm^–1 and f_T -730 cm^–1 and the corresponding JT stabilization energies E_JT(²E) 50 cm^–1 and E_JT(²T₂) 780 cm¹, respectively.