Time-resolved luminescence spectroscopy of ultrafast emissions in BaGeF6

Phase-pure crystalline micropowder samples of BaGeF6 were prepared and studied under excitation by tuneable synchrotron radiation and 10 keV electron beam. Time-resolved photoluminescence emission and excitation spectra and a set of single emission decay curves were recorded at 7 K for the exciting photon energy region of 4.3-45 eV. Several intrinsic emissions were revealed in BaGeF6 and their origin investigated. A single broad emission band peaking at 455 nm is assigned to be of excitonic origin due to its long decay time in the mu s range and due to the presence of an intense excitation peak at 10.1 eV right in the region of the host absorption onset. The energy gap width of BaGeF6 was determined experimentally from the photoluminescence excitation spectra of the 455 nm emission to be 10.9 eV. Several emission bands, including distinct peaks at 270 and 455 nm, with the main decay component of similar to 180 ps were revealed across the wavelength range of 200-500 nm. The revealed ultrafast emissions were studied by means of time-resolved photoluminescence spectroscopy and their origin was assigned to cross-luminescence resulting from radiative transitions between the Ba 5p core level and sub-bands of the valence band (Ge 4s, Ge 4p and F 2p hybridized states) and to intraband luminescence between the valence band sub-bands. Photoluminescence excitation spectra of the ultrafast emissions revealed a gently sloping onset at 17 eV, related to transitions from the Ge 4s states. It is followed by a distinct peak at 19.4 eV, which corresponds to the ionization of the Ba 5p cation states and is related to the excitation threshold of cross-luminescence.