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Crystal structure, photoluminescence and cathodoluminescence of Sr1-xCaxAl2O4 doped with Eu2+

The synthesis, crystal structure, photoluminescence (PL) and cathodoluminescence (CL) spectra of Ba1-xCaxAl2O4 doped with 3 mol% Eu2+ between x = 0 and x = 1 are described. The molar fractions of the alkaline earth elements were varied in steps of 0.1. The materials have been synthesized by all solid state reactions at 1300°C in mixed gas (H2/N2). The identification of the crystal phases in the samples was based on analyses of the X-ray diffraction patterns. The Ba1-xCaxAl2O4 system contains one dominant monoclinic phase, one dominant hexagonal phase and two different cubic phases that were present in low concentrations. The main characteristic of the PL spectra was that the intensity of the Eu2+ photoluminescence decreased upon adding a second alkaline earth ion in the aluminate lattice. The hexagonal and monoclinic phases in the Ba1-xCaxAl2O4 samples showed an unexpected behaviour, namely increasing their unit cell volumes upon decreasing the mole fraction of Ba2+. For the hexagonal phase this behaviour has been explained qualitatively in terms of enhanced spontaneous polarization of the uncompensated anti-ferroelectric state.