Structure and luminescence behaviour of as-synthesized, calcined, and restored MgAlEu-LDH with high crystallinity

Abstract

Highly crystalline Eu³⁺-incorporated MgAl layered double hydroxides (LDHs) were synthesized by the homogeneous precipitation method. For the crystals as-prepared, after their calcination from 200–1000 °C, and, further, after restoration in a Na₂CO₃ solution, the structural and luminescent changes were investigated for the first time. Eu³⁺ ions with a coordination number of, probably, 8, were incorporated into the hydrotalcite layer, which led to a basal spacing (d_basal) increase, microstrain formation, and crystalline morphology imperfections, while retaining the original lattice symmetry, Rm. In the deconstruction process due to calcination, the Eu³⁺ ions restrained the formation of the spinel phase from the layered double oxide (LDO), but did not significantly change the memory effect, by which LDOs can convert to LDHs during the hydration process. For the reversible phase transformation between LDH and LDO, the morphology observation revealed that, in addition to the formation of pores on the surface, nano-slabs were formed, especially for the restored crystals. A layered phase with a d_basal of 5.8 Å, due to bridging bidentate carbonates with the hydrotalcite layer, was formed in the calcination process at low temperature (300 °C) before the formation of LDO, but could not be restored to a large spacing. Typical ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺ at 579, 593, 615, 653, and 698 nm were observed in the photoluminescence spectra and the intensity of the dominating 615 nm band decreased with the LDH deconstruction and the formation of free water, and then increased with the formation of LDOs in the calcination process, and vice versa in the reconstruction process. The Eu³⁺ ions had a probable 9- or 10-coordination mode in addition to the probable 8-coordination mode as the spinel phase appeared.