Stéphane Jobic

The oA-La ₂ O ₂ S phase is a new metastable form of hP-La ₂ O ₂ S. As the latter acts as a host for various lanthanide-doped phosphors, this study explores how their structural differences affect their overall luminescence behavior.

The first use of the successive ionic layer adsorption and reaction (SILAR) method to grow CdS quantum dots on TiO₂ nanoparticles, achieving efficient photocatalytic alcohol oxidation and H₂ evolution without catalysts or sacrificial donors.

Using jointly experimental results and first-principles calculations, we unambiguously assign the underlying mechanisms behind two commonly observed luminescence bands for the Al₂O₃ material. Indeed, we show that the red band is associated with a Ti³⁺ d-d transition as expected, while the blue band is the combination of the Ti³⁺ + O^- → Ti⁴⁺ + O^2- and V_O^•+e^- → V_O^× de-excitation processes. Thanks to our recent developments, which take into account the vibrational contributions to the electronic transitions in solids, we were able to simulate the luminescence spectra for the different signatures. The excellent agreement with the experiment demonstrates that it should be possible to predict the color of the material with a CIE chromaticity diagram. We also anticipated the luminescence signature of Al₂O₃:Ti,Ca and Al₂O₃:Ti,Be that were confirmed by experiment.

Topochemistry refers to a generic category of solid-state reactions in which precursors and products display strong filiation in their crystal structures. Various low-dimensional materials are subject to this stepwise structure transformation by accommodating guest atoms or molecules in between their 2D slabs or 1D chains loosely bound by van der Waals (vdW) interactions. Those processes are driven by redox reactions between guests and the host framework, where transition metal cations have been widely exploited as the redox center. Topochemistry coupled with this cationic redox not only enables technological applications such as Li-ion secondary batteries but also serves as a powerful tool for structural or electronic fine-tuning of layered transition metal compounds. Over recent years, we have been pursuing materials design beyond this cationic redox topochemistry that was mostly limited to 2D or 1D vdW systems. For this, we proposed new topochemical reactions of non-vdW compounds built of 2D arrays of anionic chalcogen dimers alternating with redox-inert host cationic layers. These chalcogen dimers were found to undergo redox reaction with external metal elements, triggering either (1) insertion of these metals to construct 2D metal chalcogenides or (2) deintercalation of the constituent chalcogen anions. As a whole, this topochemistry works like a "zipper", where reductive cleavage of anionic chalcogen-chalcogen bonds opens up spaces in non-vdW materials, allowing the formation of novel layered structures. This Perspective briefly summarizes seminal examples of unique structure transformations achieved by anionic redox topochemistry as well as challenges on their syntheses and characterizations.

La2O2S2 was recently used as a precursor to prepare either a new metastable form of La2O2S by de-insertion of half of sulfur atoms of (S2) dimers or quaternary compounds by insertion of a coinage metal (e.g., La2O2Cu2S2). A strong structural relationship exists between the polysulfide precursor and the synthesized products, which highlights the topochemical nature of these reactions. Nevertheless, the crystal structure of the precursor material is still a matter of debate. Namely, several structural models were reported so far in the literature with different space groups and/or crystal systems. All these models were built upon infinite [Ln2O2] slabs separated from each other by a flat sul f u r layer of (S2) dumbbells. Nevertheless, all (S2) dimer s within a given sul f u r layer may rotate in phase by 90 degrees compared to the orientation along the stacking axis. This leads to some imbroglio and much confusion in the description of structural arrangement of Ln2O2S2 materials. Herein, the crystal structures of La2O2S2 and its Pr and Nd variants are revisited. We propose an alternative model that reconciles pre-existing structural descriptions of Ln2O2S2 (Ln = La, Pr, and Nd) materials and highlights the strong dependency of the degree of long-range ordering of the sulfur layers on the synthesis conditions.

This article reviews blue shortwave-excited luminescence (BSL) in natural minerals and synthetic materials. It also describes in detail the emission of seven minerals and gems displaying BSL, as well as three references in which BSL is caused by titanate groups (TiO₆): benitoite, Ti-doped synthetic sapphire and spinel. Emission (under 254 nm shortwave excitation) and excitation spectra are provided, and fluorescence decay times are measured. It is proposed that BSL in beryl (morganite), dumortierite, hydrozincite, pezzotaite, tourmaline (elbaite), some silicates glasses, and synthetic opals is due to titanate groups present at a concentration of 20 ppmw Ti or above. They all share a broad emission with a maximum between 420 and 480 nm (2.95 to 2.58 eV) (thus perceived as blue), and an excitation spectrum peaking in the short-wave range, between 230 and 290 nm (5.39 to 4.27 eV). Furthermore, their luminescence decay time is about 20 microseconds (from 2 to 40). These three parameters are consistent with a titanate emission, and to our knowledge, no other activator.

Herein, we present a methodology based on constrained density functional theory and vibrational mode computations to simulate and interpret the luminescence spectra of periodic solids. A multi dimension harmonic model is used to combine electronic and vibrational contributions into an overall vibrationally resolved emission spectrum. We applied it to Ti-doped BaZrO3 to accurately reproduce its blue luminescence and unambiguously assign the observed luminescence to a Ti3+ + O- -> Ti4+ + O2- charge transfer.