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

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The polarized low-temperature absorption spectra of the 3d1 ion MnO2−4 in the Cs2SO4 host consist of a very weak, highly-structured band in the near-infrared (NIR) region corresponding to the 2E→2T2(d→d) transition and a series of intense ligand-to-metal charge transfer (LMCT) excitations above 16 000 cm−1. As a result of the low-symmetry crystal-field (CF) potential in Cs2SO4 the 2T2 ligand-field (LF) state of MnO2−4 is split into its three orbital components at 10 557, 10 848, and 10 858 cm−1 above the ground state. The lowest-energy component serves as initial state for broadband luminescence to the 2E ground state, exhibiting unusually well-resolved fine structure at 15 K. The orbital splitting of 2E is 969 cm−1 and thus larger by more than 1 order of magnitude and of opposite sign compared to the result of a ligand-field calculation within the angular-overlap model (AOM). This discrepancy is explained with the large contribution of the second-nearest neighbor Cs+ ions to the CF potential of MnO2−4 in the Cs2SO4 host lattice. The vibrational progressions in the 2E↔2T2 absorption and luminescence spectra are dominated by O-Mn-O bending modes. This is the result of a weak E⊗e and a stronger T2⊗e Jahn–Teller (JT) effect in the ground and excited LF states, respectively. The observed vibronic levels in the luminescence spectrum are fitted with a single-mode E⊗e JT Hamiltonian with an additional term representing the noncubic CF potential in Cs2SO4. The JT effect in the 2T2 LF state causes a large displacement of the emitting level along the two coordinates of the e mode and thus substantially affects the intensity distribution in the luminescence spectrum. The fitted linear and quadratic vibronic constants for the 2E ground state are 91 and 12 cm−1, respectively, and for the 2T2 excited state the linear coupling constant is −790 cm−1. The corresponding JT stabilization energies are 14 and 925 cm−1 for 2E and 2T2, respectively.