Crystal structure, photoluminescence and cathodoluminescence of Ba_1-xCa_xAl₂O₄ doped with Eu²⁺

Abstract

The synthesis, crystal structure, photoluminescence (PL) and cathodoluminescence (CL) spectra of Ba 1−x Ca x Al 2 O 4 doped with 3 mol% Eu 2+ 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 (H 2 /N 2). The identification of the crystal phases in the samples was based on analyses of the X-ray diffraction patterns. The Ba 1−x Ca x Al 2 O 4 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 Eu 2+ photoluminescence decreased upon adding a second alkaline earth ion in the aluminate lattice. The hexagonal and monoclinic phases in the Ba 1−x Ca x Al 2 O 4 samples showed an unexpected behaviour, namely increasing their unit cell volumes upon decreasing the mole fraction of Ba 2+. For the hexagonal phase this behaviour has been explained qualitatively in terms of enhanced spontaneous polarization of the uncompensated anti-ferroelectric state.

Photoluminescence and cathodoluminescence of BaAl2O4:Eu2+ and undoped BaAl2O4: evidence for F-centres

“…The non-reduced BaAl 2 O 4 did not show luminescence, while the samples reduced in H 2 at rather low temperatures of 700°C and 900°C produced a broad luminescence band at 750 nm. The sample that was doped with Eu 2+ yielded also a broad emission band at 750 nm, which deviated strongly from our results [5,6] and those of other researchers indicated in our previous work. In contrast to the work of Zhang et al, both Pandey and Chithambo [34] and Benourdja et al [35] reported a broad emission band at about 405 nm in the spectrum of undoped, non-reduced BaAl 2 O 4 .…”

“…Recently we have published the crystal structures of the phases found in the Eu 2+ [4][5][6]. These studies were supported by analyses of the photoluminescence (PL) of these phosphor series and the cathodoluminescence spectra (CL) of the parent compounds CaAl 2 O 4 :Eu 2+ , SrAl 2 O 4 :Eu 2+ and BaAl 2 O 4 :Eu 2+ .…”

“…From these measurements the Eu concentration in the other samples has been checked. From the XRD patterns recorded at room temperature it was concluded that all samples presented in Table 2 consisted of one phase, namely the hexagonal P6 3 structure (ferroelectric) [5,6]. Figure 2 presents the CL spectra of undoped BaAl 2 O 4 (BA1) that was annealed in air.…”

“…As mentioned in Ref. [6] it is impossible to identify one factor that is directly responsible for the different values of r in Table 1; however, it seems that the combination of adding a co-dopant and annealing at low temperature is maximizing r. Brito et al [22] suggested that the origin of the band at 430 nm is due to the effect of water adsorption to the surface of the BaAl 2 O 4 crystals. This hypothesis could not be verified by our analysis of the CL-spectrum of BaAl 2 O 4 :Eu that had a shelf life of about 3 years in air at room temperature [3], because that sample did not show a luminescence band at 430 nm.…”

“…The situation regarding BaAl 2 O 4 :Eu 2+ is summarised in Table 1, which is an extended version of the Table shown in Ref. [6]. The differences between the spectra in the literature are represented in Table 1 as the ratio r, which is the spectral radiance at about 430 nm divided by the spectral radiance at 500 nm.…”