Luminescence From Self-Trapped Excitons In Bafcl-Bafbr Solid Solutions

Luminescence from BaFCl, BaFBr and their mixtures BaFCl1-xBrx has been investigated under VUV excitation with synchrotron radiation at temperatures 11-200 K. In BaFCl, two emission bands are found to appear at 5.36 eV and 3.37 eV, which presumably originate from self-trapped excitons (STE) situating in the on- and off-center configurations with the Cl-2- core. In mixed crystals, two characteristic bands newly appear in the UV range (e.g., at 5.43 eV and 4.46 eV in the x = 0.1 crystal), indicating preferential self trapping at Br- ion sites. With the increase in x, these bands continuously shift to the lower energy, and finally change into the 5.06 eV and 4.25 eV bands of BaFBr, which arise from STEs with the Br-2- core situating in the on-center configuration and in, presumably, a weakly off-centered configuration, respectively.