Absorption and Luminescence Spectroscopy of MnO(4)(2)(-)-Doped Crystals of BaSO(4).

Abstract

The first polarized low-temperature absorption and luminescence spectra of manganese-doped crystals of BaSO(4) containing essentially MnO(4)(2)(-) are reported. By using a flux composed of NaCl, KCl, and CsCl we were able to grow BaSO(4):Mn(6+) crystals below 620 degrees C. This prevents the simultaneous presence of MnO(4)(3)(-) besides MnO(4)(2)(-), which was mainly responsible for the erroneous assignments of the absorption spectrum in the literature. In the BaSO(4) host the MnO(4)(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 (2)E --> (2)T(2) ligand-field (LF) transition peak at 11 074, 11 570, and 11 790 cm(-)(1). The lowest-energy component of (2)T(2) 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 &mgr;s, and at 300 K the quantum yield is still 20%. In both the (2)E <--> (2)T(2) (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 (2)E 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 multiply sign in circlee and stronger T(2) multiply sign in circlee Jahn-Teller (JT) effects in the ground and excited LF states, respectively. The linear vibronic coupling constants are f(E)() approximately 180 cm(-)(1) and f(T)() approximately -730 cm(-)(1) and the corresponding JT stabilization energies E(JT)((2)E) approximately 50 cm(-)(1) and E(JT)((2)T(2)) approximately 780 cm(-)(1), respectively.