First-Principles Study of Antisite Defect Configurations in ZnGa₂O₄:Cr Persistent Phosphors

Zinc gallate doped with chromium is a recently developed near-infrared emitting persistent phosphor, which is now extensively studied for in vivo bioimaging and security applications. The precise mechanism of this persistent luminescence relies on defects, in particular, on antisite defects and antisite pairs. A theoretical model combining the solid host, the dopant, and/or antisite defects is constructed to elucidate the mutual interactions in these complex materials. Energies of formation as well as dopant, and defect energies are calculated through density-functional theory simulations of large periodic supercells. The calculations support the chromium substitution on the slightly distorted octahedrally coordinated gallium site, and additional energy levels are introduced in the band gap of the host. Antisite pairs are found to be energetically favored over isolated antisites due to significant charge compensation as shown by calculated Hirshfeld-I charges. Significant structural distortions are found around all antisite defects. The local Cr surrounding is mainly distorted due to a Zn_Ga antisite. The stability analysis reveals that the distance between both antisites dominates the overall stability picture of the material containing the Cr dopant and an antisite pair. The findings are further rationalized using calculated densities of states and Hirshfeld-I charges

Determination of the Nature of the Cu Coordination Complexes Formed in the Presence of NO and NH₃ within SSZ-13

Ammonia-selective catalytic reduction (NH₃-SCR) using Cu zeolites is a well-established strategy for the abatement of NO_<i>x</i> gases. Recent studies have demonstrated that Cu is particularly active when exchanged into the SSZ-13 zeolite, and its location in either the <i>6r</i> or <i>8r</i> renders it an excellent model system for fundamental studies. In this work, we examine the interaction of NH₃-SCR relevant gases (NO and NH₃) with the Cu²⁺ centers within the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption spectroscopy (XAFS), and density functional theory (DFT). This combined approach revealed that, upon calcination, cooling and gas exposure Cu ions tend to locate in the <i>8r</i> window. After NO introduction, Cu ions are seen to coordinate to two framework oxygens and one NO molecule, resulting in a bent Cu–nitrosyl complex with a Cu–N–O bond angle of ∼150°. Whilst Cu seems to be partially reduced/changed in coordination state, NO is partially oxidized. On exposure to NH₃ while the PD data suggest the Cu²⁺ ion occupies a similar position, simulation and XAFS pointed toward the formation of a Jahn–Teller distorted hexaamine complex [Cu­(NH₃)₆]²⁺ in the center of the <i>cha</i> cage. These results have important implications in terms of uptake and storage of these reactive gases and potentially for the mechanisms involved in the NH₃-SCR process

Bipyridine-Based Nanosized Metal–Organic Framework with Tunable Luminescence by a Postmodification with Eu(III): An Experimental and Theoretical Study

A gallium 2,2′-bipyridine-5,5′-dicarboxylate metal–organic framework, Ga­(OH)­(bpydc), denoted as COMOC-4 (COMOC = Center for Ordered Materials, Organometallics and Catalysis, Ghent University) has been synthesized via solvothermal synthesis procedure. The structure has the topology of an aluminum 2,2′-bipyridine-5,5′-dicarboxylate – the so-called MOF-253. TEM and SEM micrographs show the COMOC-4 crystals are formed in nanoplates with uniform size of 30–50 nm. The UV–vis spectra of COMOC-4 in methanol solution show maximal electronic absorption at 307 nm. This results from linker to linker transitions as elucidated by time-dependent density functional theory simulations on the linker and COMOC-4 cluster models. When excited at 400 nm, COMOC-4 displays an emission band centered at 542 nm. Upon immersion in different solvents, the emission band for the framework is shifted in the range of 525–548 nm depending on the solvent. After incorporating Eu³⁺ cations, the emission band of the framework is shifted to even shorter wavelengths (505 nm). By varying the excitation wavelengths from 250 to 400 nm, we can fine-tune the emission from red to yellowish green in the CIE diagram. The luminescence behavior of Eu³⁺ cations is well preserved and the solid-state luminescence lifetimes of τ₁ = 45 μs (35.4%) and τ₂ = 162 μs (64.6%) are observed