Ce³⁺-Activated γ‑Ca₂SiO₄ and Other Olivine-Type ABXO₄ Phosphors for Solid-State Lighting

Yellow-emitting phosphors activated by Ce³⁺ are key components of white light-emitting diodes (LEDs) based on the blue (Ga,In)N LED. In these phosphors, the electronic environment around Ce³⁺ determines its photoluminescence wavelengths. Placing Ce³⁺ in octahedral sites in oxides can potentially lead to yellow or orange phosphors that can compete with the important commercial phosphor, Y₃Al₅O₁₂:Ce. However, there are very few examples of such materials. In this article, we explore the photoluminescent behavior of Ce³⁺ in oxides with the olivine structure, whereby two distorted octahedral doping sites exist. We demonstrate that the promising new yellow phosphor γ-Ca₂SiO₄:Ce requires a second dopant, Al³⁺, to avoid the formation of the undesired β polymorph. Our results indicate that the yellow emission is indirectly caused by the complex polymorphism of Ca₂SiO₄, particularly the low temperature formation of the γ-phase. The dramatic shift in emission from yellow to light-blue when this phosphor is heated to 800 <b>°</b>C is explained in terms of the redistribution of Ce³⁺ in the lattice. A similar effect is observed when Si⁴⁺ is substituted by Ge⁴⁺. The light-blue emission of Ca₂GeO₄:Ce,Al is reported as well as the photoluminescent properties of the Ca₂(Ge,Si)­O₄:Ce,Al solid solution

Thermochemistry of Zeolitic Imidazolate Frameworks of Varying Porosity

The first thermochemical analysis by room-temperature aqueous solution calorimetry of a series of zeolite imidazolate frameworks (ZIFs) has been completed. The enthalpies of formation of the evacuated ZIFsZIF-zni, ZIF-1, ZIF-4, CoZIF-4, ZIF-7, and ZIF-8along with as-synthesized ZIF-4 (ZIF-4·DMF) and ball-milling amorphized ZIF-4 (<i>a</i>_mZIF-4) were measured with respect to dense components: metal oxide (ZnO or CoO), the corresponding imidazole linker, and <i>N</i>,<i>N</i> dimethylformamide (DMF) in the case of ZIF-4·DMF. Enthalpies of formation of ZIFs from these components at 298 K are exothermic, but the ZIFs are metastable energetically with respect to hypothetical dense components in which zinc is bonded to nitrogen rather than oxygen. These enthalpic destabilizations increase with increasing porosity and span a narrow range from 13.0 to 27.1 kJ/mol, while the molar volumes extend from 135.9 to 248.8 cm³/mol; thus, almost doubling the molar volume results in only a modest energetic destabilization. The experimental results are supported by DFT calculations. The series of ZIFs studied tie in with previously studied MOF-5, creating a broader trend that mirrors a similar pattern by porous inorganic oxides, zeolites, zeotypes, and mesoporous silicas. These findings suggest that no immediate thermodynamic barrier precludes the further development of highly porous materials

High-Throughput Computational Screening of Metal–Organic Frameworks for Thiol Capture

We report high-throughput computational screening of 137 953 hypothetical metal–organic frameworks (hMOFs) and 4764 computation-ready experimental MOFs (CoRE-MOFs) for the capture of thiols (methanethiol and ethanethiol) from air. To minimize the competitive adsorption of moisture, 31 816 hydrophobic MOFs are first identified on the basis of a threshold criterion in the Henry constant of water, and then used to assess the adsorption capacity of thiol (<i>N</i>_SH) and the selectivity of thiol over air (<i>S</i>_SH/Air). The highest <i>N</i>_SH and <i>S</i>_SH/Air are predicted to be 70.86 mg/g and 2.6 × 10⁷, respectively. Most of the high-performance MOFs are found to be hMOFs. The structure–property relationships are derived for <i>N</i>_SH and <i>S</i>_SH/Air with MOF descriptors (including the isosteric heat, the largest cavity diameter, surface area, and void fraction). While the relationship with isosteric heat tends to be monotonic, there exist optimal ranges in the other relationships. Principal component analysis is applied to assess the interrelationships among the four descriptors; then multiple linear regression is used to quantitatively determine the respective effects of descriptors on <i>N</i>_SH and <i>S</i>_SH/Air. It is revealed that the isosteric heat is a key descriptor governing thiol capture. Moreover, decision tree modeling is employed to define a clear effective path to screen high-performance MOFs. Finally, the best MOFs are identified. The microscopic insights obtained from our bottom-up approach are useful toward the development of MOFs and other nanoporous materials for thiol capture from air or in a variety of environmental and industrial situations

Chiral, Racemic, and <i>Meso</i>-Lithium Tartrate Framework Polymorphs: A Detailed Structural Analysis

Following our previous report of five anhydrous lithium tartrates <b>1</b>–<b>5</b> (tart = C₄H₄O₆ ^2–), we have synthesized and solved the single crystal structures of four new I¹O² inorganic–organic frameworks, all with the same chemical formula, Li₂(tart). Reactions between lithium acetate and the meso, chiral, and racemic isomers of tartaric acid in water:ethanol mixtures have yielded two new polymorphs of Li₂(<i>meso</i>-tart) in space groups <i>P</i>2₁/<i>c</i> <b>6</b> and <i>Cc</i> <b>7</b>, racemic Li₂(d,l-tart) in <i>P</i>2₁/<i>c</i> <b>8</b>, and chiral Li₂(l-tart) in <i>C</i>2 <b>9</b>. Hydrogen bond graph set analysis was adapted for use with framework materials and employed here to examine the motifs displayed by the eight anhydrous dilithium tartrates <b>2</b>–<b>9</b>. A variety of hydrogen-bonding patterns and dimensionalities are observed in this system, and the relative hydrogen bond strengths are found to correlate well with O–H stretching frequency shifts in the FTIR spectra. The relative formation energies of the framework isomers have been calculated by DFT methods, using schemes that include dispersion correction, zero-point vibrational energy, and thermal vibrations at room temperature. Although the energy ordering depends slightly on the scheme used, it is generally found to relate to the differences in crystallographic density and hydrogen bond strength rather than other structural features

Stacking Faults and Mechanical Behavior beyond the Elastic Limit of an Imidazole-Based Metal Organic Framework: ZIF‑8

We determine the nonlinear mechanical behavior of a prototypical zeolitic imidazolate framework (ZIF-8) along two modes of mechanical failure in response to tensile and shear forces using first-principles simulations. Our generalized stacking fault energy surface reveals an intrinsic stacking fault of surprisingly low energy comparable to that in copper, though the energy barrier associated with its formation is much higher. The lack of vibrational spectroscopic evidence for such faults in experiments can be explained with the structural instability of the barrier state to form a denser and disordered state of ZIF-8 seen in our analysis, that is, large shear leads to its amorphization rather than formation of faults

Hybrid Nanosheets of an Inorganic–Organic Framework Material: Facile Synthesis, Structure, and Elastic Properties

We report a new 2-D inorganic–organic framework material, MnDMS [Mn 2,2-dimethylsuccinate], featuring weakly bound hybrid layers in its bulk crystals that can be readily exfoliated into nanosheets <i>via</i> ultrasonication. The fully exfoliated hybrid nanosheets correspond to a unilamellar thickness of about 1 nm, while the partially exfoliated nanosheets (multilayer films) exhibit a typical thickness on the order of 10 nm. We used atomic force microscopy to characterize their surface topography and to map the variation of nanomechanical properties across the surface of the delaminated nanosheets. The morphology and crystallographic orientation of the exfoliated layers were further studied by transmission electron microscopy. Additionally, we investigated the elastic anisotropy underlying the bulk host material by means of single-crystal nanoindentation, from which the critical resolved shear stress (τ_crit) needed for the micromechanical delamination of individual layers was determined to be relatively small (≲0.4 GPa)

Insulator-to-Proton-Conductor Transition in a Dense Metal–Organic Framework

Metal–organic frameworks (MOFs) are prone to exhibit phase transitions under stimuli such as changes in pressure, temperature, or gas sorption because of their flexible and responsive structures. Here we report that a dense MOF, ((CH₃)₂NH₂)₂­[Li₂Zr­(C₂O₄)₄], exhibits an abrupt increase in proton conductivity from <10^–9 to 3.9 × 10^–5 S/cm at 17 °C (activation energy, 0.64 eV) upon exposure to humidity. The conductivities were determined using single crystals, and the structures were analyzed by X-ray diffraction and X-ray pair distribution function analysis. The initial anhydrous structure transforms to another dense structure via topotactic hydration (H₂O/Zr = 0.5), wherein one-fourth of the Li ions are irreversibly rearranged and coordinated by water molecules. This structure further transforms into a third crystalline structure by water uptake (H₂O/Zr = 4.0). The abrupt increase in conductivity is reversible and is associated with the latter reversible structure transformation. The H₂O molecules coordinated to Li ions, which are formed in the first step of the transformation, are considered to be the proton source, and the absorbed water molecules, which are formed in the second step, are considered to be proton carriers

Mechanical Properties of a Calcium Dietary Supplement, Calcium Fumarate Trihydrate

The mechanical properties of calcium fumarate trihydrate, a 1D coordination polymer considered for use as a calcium source for food and beverage enrichment, have been determined via nanoindentation and high-pressure X-ray diffraction with single crystals. The nanoindentation studies reveal that the elastic modulus (16.7–33.4 GPa, depending on crystallographic orientation), hardness (1.05–1.36 GPa), yield stress (0.70–0.90 GPa), and creep behavior (0.8–5.8 nm/s) can be rationalized in view of the anisotropic crystal structure; factors include the directionality of the inorganic Ca–O–Ca chain and hydrogen bonding, as well as the orientation of the fumarate ligands. High-pressure single-crystal X-ray diffraction studies show a bulk modulus of ∼20 GPa, which is indicative of elastic recovery intermediate between small molecule drug crystals and inorganic pharmaceutical ingredients. The combined use of nanoindentation and high-pressure X-ray diffraction techniques provides a complementary experimental approach for probing the critical mechanical properties related to tableting of these dietary supplements

[Am]Mn(H₂POO)₃: A New Family of Hybrid Perovskites Based on the Hypophosphite Ligand

A family of five hybrid ABX₃ perovskites has been synthesized using hypophosphite (H₂POO)⁻ as the X-site ion. These compounds adopt the general formula [Am]­Mn­(H₂POO)₃, where Am = guanidinium (GUA), formamidinium (FA), imidazolium, triazolium, and dabconium. We explore the diverse structural and phase transition behavior of these materials through single-crystal diffraction measurements and demonstrate contrasting magnetism in two of the phases, Am = GUA and FA, that arises from structural distortions. The results show that hypophosphite perovskites offer a promising platform for generating new functional materials

Understanding of Electrochemical Mechanisms for CO₂ Capture and Conversion into Hydrocarbon Fuels in Transition-Metal Carbides (MXenes)

Two-dimensional (2D) transition-metal (groups IV, V, VI) carbides (MXenes) with formulas M₃C₂ have been investigated as CO₂ conversion catalysts with well-resolved density functional theory calculations. While MXenes from the group IV to VI series have demonstrated an active behavior for the capture of CO₂, the Cr₃C₂ and Mo₃C₂ MXenes exhibit the most promising CO₂ to CH₄ selective conversion capabilities. Our results predicted the formation of OCHO^• and HOCO^• radical species in the early hydrogenation steps through spontaneous reactions. This provides atomic level insights into the computer-aided screening for high-performance catalysts and the understanding of electrochemical mechanisms for CO₂ reduction to energy-rich hydrocarbon fuels, which is of fundamental significance to elucidate the elementary steps for CO₂ fixation