Interplay between Crystal Structure and Photoluminescence Properties of β-Ca3SiO4Cl2:Eu2+

The crystal structure of beta-Ca3SiO4Cl2 was determined by the ab initio structure determination method based on the synchrotron powder XRD data for the first time, and the luminescence properties of a Eu2+-doped beta-Ca3SiO4Cl2 phosphor were characterized. beta-Ca3SiO4Cl2 was found to be monoclinic (space group P2(1)/c) with the lattice parameters, a = 5.91234(1) angstrom, b = 10.20128(1) angstrom, c = 10.98866(1) angstrom, beta = 90.3423(1)degrees. This structure can be considered as an intergrowth structure built up from alternating stacks of two layered sublattices, (2)(infinity)[Ca2SiO4] and (2)(infinity)[CaCl2], along the [100] direction. In this structure, the Ca atoms occupy three crystallographically distinct sites: CalO(4)Cl(3), Ca2O(5)Cl(2), and Ca3O(3)Cl(4). The photoluminescence of the Eu2+-doped Ca3SiO4Cl2 phosphor excited at 450 nm blue light shows the 150 nm wide-band emission peaked at 635 nm with about 70% quantum efficiency. The photoluminescence properties, such as centroid shifts, crystal-field splitting, and Stokes shifts, were correlated with the crystal structure through the calculation of shared-electron populations reflecting the bond covalency between Ca and O/Cl. Effects of doping concentrations on the luminescence spectra and temperature stability were also discussed based on the inhomogeneous energy transfer property determined by the structural geometric factor.