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Until about two hundred years ago, no gemological distinction was made between ruby and spinel. Red spinel and red ruby are not infrequently found together and though gem cutters and engravers noticed and commented on the difference in hardness, the assumption was that spinel was simply an \"unripe\" version of ruby. Additionally, ruby and sapphire are both versions of the mineral corundum, distinguished only by color and minute traces of the metal oxides that caused these different colors. Sapphires may be pink, yellow, and green as well as blue, while rubies come in many shades of red which, inevitably causes confusion as one person's pale red ruby is another's pink sapphire--there are no absolutes. Consequently, the nomenclature is confused, both within early texts, and also in later translations of those texts. The ancient authors could only report on the basis of the information available to them at the time, while those writing the later translations were fine textual scholars or epigraphers, but not infrequently poor gemologists, not familiar with the mineralogical distinctions between the gems. It has often been difficult to get an overarching view of the many different factors that all played a part in the spread of precious gems and of the dissemination of knowledge about them. Given the paucity of available information concentrating exclusively on the use of ancient precious gemstones, the author combed the literature for relevant references. A surprising amount of descriptive and factual information was found, mostly scattered throughout early texts. The most interesting passages were selected and wherever possible the original authors' words were quoted rather than paraphrased. The early translations in the languages used by 17th-19th century scholars are given, names of people, places or objects that otherwise might have remained obscure are explained. Gems travel. They follow wealth and because of their natural immutability, the only way they can be identified by culture is by the way man has affected their appearance, deliberately or accidentally. The dating of gems that are still in original period settings is easier because the dated typology of rings and jewelry settings generally, is more secure than the study of gem shapes, while the study and dating of specific faceting styles of unmounted stones is still in its infancy.

The paper describes a heterobimetallic mixed-ligand hexanuclear precursor [NaMn[sub.2](thd)[sub.4](OAc)][sub.2] (1) (thd = 2,2,6,6-tetramethyl-3,5-heptadionate; OAc = acetate) that was designed based on its lithium homoleptic analogue, [LiMn[sub.2](thd)[sub.5]], by replacing one of the thd ligands with an acetate group in order to accommodate 5-coordinated sodium instead of tetrahedral lithium ion. The complex, which is highly volatile and soluble in a variety of common solvents, has been synthesized by both the solid-state and solution methods. The unique “dimer-of-trimers” heterometallic structure consists of two trinuclear [NaMn[sup.II] [sub.2](thd)[sub.4]][sup.+] units firmly bridged by two acetate ligands. X-ray diffraction techniques, DART mass spectrometry, ICP-OES analysis, and IR spectroscopy have been employed to confirm the structure and composition of the hexanuclear complex. Similar to the Li counterpart forming LiMn[sub.2]O[sub.4] spinel material upon thermal decomposition, the title Na:Mn = 1:2 compound was utilized as the first single-source precursor for the low-temperature preparation of Na[sub.4]Mn[sub.9]O[sub.18] tunnel oxide. Importantly, four Mn sites in the hexanuclear molecule can be potentially partially substituted by other transition metals, leading to heterotri- and tetrametallic precursors for the advanced quaternary and quinary Na-ion oxide cathode materials.

The development of efficient and durable oxygen evolution reaction (OER) catalysts from earth-abundant materials is essential for advancing alkaline water electrolysis. Herein, nanograss-like CoMn[sub.2]O[sub.4] electrode films are directly grown on stainless-steel substrates via a temperature-controlled hydrothermal approach, and their OER performance is systematically investigated. The CoMn[sub.2]O[sub.4] obtained at 120 °C (CMO-120) delivers the best catalytic activity in 1.0 M KOH, requiring an overpotential of 292 mV at 10 mA cm[sup.−2], which is lower than those synthesized at 150 (CMO-150) and 90 °C (CMO-90). Notably, activity of CMO-120 becomes even more pronounced at elevated current densities, achieving the low overpotential of 434 mV even at 300 mA cm[sup.−2], substantially outperforming both CMO-90 and CMO-150 electrodes. The enhanced activity is attributed to an interconnected nanograss architecture with mixed Co[sup.2+]/Co[sup.3+] and Mn[sup.2+]/Mn[sup.3+] redox couples and abundant defect-related oxygen species, which result in increased electrochemically active surface area and improved charge transportation throughout the nanograss architecture that facilitate OH[sup.−] adsorption and OER intermediate transformation. Furthermore, CMO-120 demonstrates excellent durability (100 h) after electro-oxidation-induced surface activation. These findings highlight precise temperature regulation as an effective strategy for optimizing Mn-Co spinel for efficient alkaline OER applications.

Spinels are important materials for an application in bioimaging. The key advantage with spinel-type hosts is the presence of antisite defects, which act as charge reservoirs for trapping electrons and holes at complementary defect sites. This makes them a host system similar to a molecular system. Herein, we present a systematic approach to modulating the band gap of an inverse Zn[sub.2]TiO[sub.4] spinel. With a change in ZnO concentration, the absorption band at 375 nm diminishes and disappears at a ZnO:TiO[sub.2] concentration of 1.40:1.00. The band gap of the material is modified from 3.30 to 4.40 eV. The crystal structure of the sample does not change drastically as determined using X-ray diffraction and Rietveld refinement. The Zn[sub.2]TiO[sub.4] emits in the UV-A region with a lifetime in the time domain of ‘ns’. The sample also shows persistent luminescence of at least 15 min upon excitation with 254 nm with prominent emission in the UV-A region (300–390 nm). The present results open a new avenue for the synthesis of spinel hosts where the band gap can be modified with ease. The UV emission thus observed is expected to find usage in interesting applications like photocatalysis, anti-counterfeiting, water disinfecting, etc.

Cuprate delafossite phases such as CuMnO[sub.2] (crednerite) and CuFeO[sub.2], as well as iron- and manganese-doped mcconnellite composites, were investigated as candidates for producing intense black ceramic pigments via conventional solid-state synthesis. Both electric kiln and fast dielectric (microwave) firing methods were employed, with mcconnellite (CuCrO[sub.2]) used as a reference pigment. Microwave firing led to a marked improvement in sample blackness compared to conventional electric firing. Among the delafossite phases, only mcconnellite subjected to microwave-assisted firing (R[sub.Vis] = 1.40%, corresponding to 98.60% visible light absorption) emerges, pending further optimization, as a promising candidate for an ultra-black ceramic pigment (R[sub.Vis] < 1%) under optimized glaze conditions (ZnO-free) and a firing temperature of 1000 °C. Considering the pigments in powder form, microwave-fired crednerite (R[sub.Vis] = 4.85%, 95.15% absorption) and iron- and iron–manganese-doped mcconnellite composites (R[sub.Vis] = 3.27% and 3.23%, respectively) appear as potential candidates for deep-black pigments (R[sub.Vis] < 3%), benefiting from the composite effect between the delafossite phase and the associated chromium spinel. Moreover, microwave-fired crednerite also demonstrates noteworthy potential for deep-black coloration in glazed samples (R[sub.Vis] = 4.27%, 95.73% absorption).

The catalytic performance of spinel oxide NiCo.sub.2O.sub.4 for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was investigated. It shows that NiCo.sub.2O.sub.4 synthesized via hydrothermal treatment at 90 â followed by calcination at 350 â exhibits excellent catalytic activity. Furthermore, Fe-doped MnO.sub.2 (marked as Fe-M) was introduced as a modifier to further improve the performance of NiCo.sub.2O.sub.4. As expected, the 20% Fe-M hybrid catalyst (NiCo.sub.2O.sub.4 modified with 20 wt% Fe-M) displays the best ORR and OER catalytic activity. This is evidenced by its high OER current density of 265 mA cm.sup.-2, high ORR current density of 220 mA cm.sup.-2, low OER Tafel slope of 119 mV dec.sup.-1, low ORR Tafel slope of 167 mV dec.sup.-1. These exceptional results can be attributed to the synergistic effect between NiCo.sub.2O.sub.4 and Fe-M.

This research deals with the synthesis of undoped and Sm.sup.3+-doped ZnAl.sub.2O.sub.4 using the citrate sol-gel method. Various complementary techniques have extensively studied the consequences of samarium ion (Sm.sup.3+) doping on the crystallinity and emission mechanism. XRD analysis endorses the formation of Sm.sup.3+-doped ZnAl.sub.2O.sub.4 spinel powder within the size range of 14-26 nm. The granular surface morphology of the prepared samples and their particle size distribution were examined by SEM and TEM, respectively. The direct bandgap of the doped samples was calculated (2.77-3.65 eV) using the Kubelka-Munk function on the diffuse reflectance data. At high concentrations of Sm.sup.3+, photoluminescence (PL) results demonstrate orange-red emission at 597 nm after providing energy corresponding to direct excitation (396 nm) of Sm.sup.3+ ion. The blue-green emission is obtained first time in Sm.sup.3+-doped ZnAl.sub.2O.sub.4 at [lambda].sub.ex = 320 nm excitation wavelength. The CIE chromaticity coordinates indicated that the prepared samples could be applied as an efficient phosphor in the visible range.

Nonradical Fenton-like catalysis offers opportunities to overcome the low efficiency and secondary pollution limitations of existing advanced oxidation decontamination technologies, but realizing this on transition metal spinel oxide catalysts remains challenging due to insufficient understanding of their catalytic mechanisms. Here, we explore the origins of catalytic selectivity of Fe–Mn spinel oxide and identify electron delocalization of the surface metal active site as the key driver of its nonradical catalysis. Through fine-tuning the crystal geometry to trigger Fe–Mn superexchange interaction at the spinel octahedra, ZnFeMnO₄ with high-degree electron delocalization of the Mn–O unit was created to enable near 100% nonradical activation of peroxymonosulfate (PMS) at unprecedented utilization efficiency. The resulting surface-bound PMS* complex can efficiently oxidize electron-rich pollutants with extraordinary degradation activity, selectivity, and good environmental robustness to favor water decontamination applications. Our work provides a molecule-level understanding of the catalytic selectivity and bimetallic interactions of Fe–Mn spinel oxides, which may guide the design of low-cost spinel oxides for more selective and efficient decontamination applications.

The influence of Mg doping in α-Al[sub.2]O[sub.3] crystals is investigated in this article by first-principles calculations and formation energies, density of states, and computed absorption spectra. Three models related to Mg[sup.2+] substituting for Al[sup.3+] doping structures were constructed, as well as spinel structure models with varying aluminum-magnesium ratios. The formation energy calculations confirmed the rationality of the Mg[sub.Al]V[sub.O] model, which means that Mg substitutional doping incorporating oxygen vacancies is most likely to form in crystals. The combined action of magnesium and oxygen vacancies introduced new defect energy levels in the bandgap. The calculated absorption spectra of the Mg[sub.Al]V[sub.O] and Mg-rich spinel structures exhibited various color centers. The experimental absorption spectra and thermoluminescence characteristics of α-Al[sub.2]O[sub.3]:Mg and alumina-magnesium (Al-Mg) spinel crystal samples were tested. The thermoluminescence peak of the Al-Mg spinel was significantly stronger than that of the α-Al[sub.2]O[sub.3]:Mg crystal. The consistency between the model-calculated absorption spectra and the experimental results confirmed the theoretical predictions. Based on the experimental and computational results, the influence of Mg[sup.2+] substitutional doping in α-Al[sub.2]O[sub.3] and the impact of the locally Mg-rich spinel on the optical and radiation performance of α-Al[sub.2]O[sub.3]:Mg crystals are elucidated.

This study developed a highly efficient one-step catalytic reductive amination method, achieving the highly selective conversion of 5-hydroxymethylfurfural (HMF) to 2,5-bis(aminomethyl)furan (BAMF). A series of NiZnAl catalysts were prepared via the coprecipitation method and demonstrated excellent catalytic performance in HMF conversion. The Ni[sub.4]Zn[sub.4]Al[sub.8]O[sub.x] catalyst achieved up to 100% substrate conversion with 83.71% BAMF selectivity. Their structure–activity relationship was elucidated through a comprehensive characterization using XRD, H[sub.2]-TPR, NH[sub.3]-TPD, XPS, and TEM techniques. The study reveals that the unique synergistic interactions between the metal and acidic sites on the ZnAl[sub.2]O[sub.4] spinel structure is crucial for catalytic performance: on the one hand, Zn introduction forms the spinel structure and promotes electron enrichment at Ni active sites, significantly enhancing the activation capability of HMF hydroxyl groups; on the other hand, the moderately acidic sites in the catalyst form “metal-acid” dual-functional synergistic centers with the metal sites, simultaneously promoting substrate activation and effectively regulating the transformation pathways of reaction intermediates. This precise matching between the metal active sites and acidic sites enables the efficient sequential progression of all steps in the reaction, offering a novel and more selective solution for the efficient reductive amination of HMF.