Cathodo- and photoluminescence emission of a natural Mg-Cr carbonate layered double hydroxide

Spectral emission of a natural layered double hydroxide (LDH) studied by cathodo- and photoluminescence (PL), displays complex glow curves with wavebands that could be attributable to (i) the paramagnetic Non-bridging Oxygen Hole Center defects (peaked at 330 nm), (ii) Fo centers (oxygen vacancies trapping two electrons) associated with O vacancy-hole centers and Me-O bonding defects and dehydroxylation process (in the range of 360-500 nm), (iii) presence of Mn2+ (0.15%) substituting Mg2+ ions (530-640 nm) and (iv) a red-IR band mainly associated with Cr and Fe point defects. The presence of Cr3+ ion activators induces characteristic PL emission peaks in the red-infrared region at 681, 688 and 696 nm linked to 2E → 4A2 transitions. The sample (Mg6Cr2CO3(OH)16·4H2O) previously characterized by environmental scanning electron microscope, X-ray diffraction, and X-ray fluorescence indicates that this natural hydrotalcite-like compound is associated with hydroxyl-rich silicates (nimite and clinochlore) and Mg-Fe carbonate and Ni-Fe carbonate LDH (pyroaurite and reevesite).