ИСТИНА

Mn2+-doped willemite Zn2SiO4 is a well known green emitting phosphor widely used in cathode ray tubes, fluorescent lamps, and plasma display panels thanks to its high luminescent efficiency and chemical stability. It is generally assumed that the observed green emission originates from 4T1 -> 6A1 spin-flip transition within the crystal-field split 3d5-shell of Mn2+ ions, giving rise to an unstructured broad band at room temperature. The emission band observed at 4.2 K, however, shows considerable fine structure with two sharp lines at 2.455 eV (504.6 nm) and 2.404 eV (515.3 nm), which were interpreted as 4T1  6A1 zero phonon transitions within Mn2+ ions occupying two slightly non-equivalent tetrahedral Zn sites in the willemite host material, accompanied by vibronic side bands [1]. In this work, we present a sequential, fully ab initio study of the Mn2+ emission bands in this material. Periodic DFT calculations using the Quantum Espresso package [2] are used to find the equilibrium ground and lowest quartet excited state structures of Mn2+-doped material, and cluster ab initio calculations of the 4T1  6A1 vertical emission energies are performed using high-level electronic structure methods (CASSCF plus quasi-degenerate 2nd order perturbation theory) implemented in the Firefly QC package [3]. The vibronic structure of the emission bands is simulated using a multi-dimensional harmonic model with a linear electron-phonon coupling, with the parameters taken from the above first-principle calculations. We observe fairly good agreement in the emission band positions, widths, and relative intensities according to the Mn2+ distribution over the two Zn sites, also providing some theoretical evidence and further insights for the experimentally established features of the Mn2+ emission in this material. The work was partially supported by the Russian Foundation for Basic Research (project 10-03-00665-a). References 1. A.L.N. Stevels and A.T. Vink, J. Lumin. 8 (1974) 443–451 2. P. Giannozzi et al, J.Phys.:Condens.Matter, 21 (2009) 395502 1–19 3. A. Granovsky, Firefly version 7.1.G, http://classic.chem.msu.su/gran/firefly/index.html