Absorption and Luminescence Spectroscopy of MnO₄^2--Doped Crystals of BaSO₄

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.

Low-temperature flux growth of sulfates, molybdates, and tungstates of Ca, Sr, and Ba and investigation of doping with Mn 6+

“…5b. Their characteristics are similar to the absorption spectra published earlier by Brunold and Güdel [19]. According to that publication, the absorption bands are assigned as 2 E → 2 T 2 between 700 and 900 nm and a ligand-to-metal charge-transfer band between 460 and 600 nm.…”

“…However, Mn 6+ tends to be reduced to Mn 5+ at temperatures above ∼ 600 • C [18]. Therefore, satisfactory results on the incorporation of manganese in its hexavalent oxidation state have been obtained only with lowtemperature flux-growth techniques [13,19].…”

Jahn–Teller effect in the ground and excited states of MnO2−4 doped into Cs2SO4

“…In tetrahedral complexes the metal d orbitals are split into a antibonding set of e molecular orbitals, transforming as ͉z 2 ͘ and ͉xy͘ for the axes orientation given in Fig. 1 15 where the absorption spectra are strongly polarized and thus allow a clear identification of the excited states. The relatively low energies of the LMCT states are explained with a substantial covalency of the Mn-O bond.…”

Electronic and phonon processes of MnO 4 – molecular impurities in alkali halide crystals

“…The inhomogeneous broadening of the vibronic bands was observed for MnO 4 2-doped crystals. The NIRL spectrum induced by the MnO 4 2-ions in different matrixes has been attributed to d-d ( 2 T 2 -2 E) ligand-field transition of 3d 1 MnO 4 2-ion [1][2][3][4]. The theoretical and experimental analysis of the luminescence intensity in the interval from 8 to 300 K shows up the different thermoactivation character of its temperature behavior for the MnO 4 -and MnO 4 2-doped hosts.…”

“…1 Introduction Interest in the study of MnO 4 -doped crystals as potential candidates for possible laser applications came from the first observation by Guedel et al [1,2] the NIR-luminescence spectra of MnO 4 2-in non-cubic crystals, such as BaSO 4, Cs 2 SO 4 . We have recently reported the NIRluminescence spectra in alkali-iodides doped with the MnO 4 -and MnO 4 2-ions that can be considered as model systems offering a high (O h ) site symmetry for the substitutional defect of T d symmetry [3,4].…”