186 Search Results

Crystalline materials, characterized by their well-defined lattices, typically exhibit a unique global thermodynamic minimum for a specific composition. However, in this study, we discover a quaternary chalcogenide family, A₂M₂M’Q₄ (A: alkali metals; M: coinage metal; M’: transition or group-IVA metals; Q: chalcogens), that exhibits pervasive energy (near-)degeneracy. For a given composition, multiple structurally distinct polymorphs exist within a formation enthalpy window of only a few milli-electron volts per atom. We quantify this inherent structural flexibility using a dedicated descriptor, σ_f: the standard deviation of formation enthalpies among degenerate (meta)stable polymorphs. The consistently low σf observed across the A₂M₂M’Q₄ family signifiesmore » a characteristically shallow and frustrated potential energy landscape, which drives pronounced lattice anharmonicity, marking these materials as prime candidates for ultralow lattice thermal conductivity (κ_L). Employing an advanced high-throughput computational framework that integrates thermodynamics, lattice dynamics, and thermal conductivity calculations, we screen 1215 A₂M₂M’Q₄ compounds, identifying 30 stable candidates with κ_L < 0.5 W m^–1 K^–1 at 300 K. Among them, Rb₂Ag₂SnTe₄ and Rb₂Au₂HfTe₄, two representatives from the IVA and TM subgroups, are predicted to show ultralow room-temperature κ_L of 0.174 W m^–1 K^–1 and 0.295 W m^–1 K^–1, respectively. A systematic analysis suggests that the nonbonding and antibonding states induced by “dual rattlers” are the origin of low thermal conductivity in these compounds. Our results position the A₂M₂M’Q₄ family as a rich source of intrinsic thermal insulators and suggest that polymorphic energy degeneracy may serve as a valuable signpost for identifying crystalline families with potential anharmonicity.« less

Formaldehyde (HCHO) is a fundamental chemical feedstock with widespread industrial applications. The direct oxidation of methane by oxygen to formaldehyde (CH₄ + 1/2O₂ → H₂ + HCHO) under mild conditions represents an attractive but challenging transformation, as it requires both activation of the inert C–H bonds of CH₄ and suppression of overoxidation to products such as carbon dioxide. In this work, mass spectrometry experiments combined with theoretical calculations reveal that CoMoO⁺ cations can efficiently mediate this transformation at room temperature. The unique electronic structure of CoMoO⁺ facilitates the formation of a crucial CoMoOCH₂⁺ intermediate during the reaction with CH₄ andmore » prevents methanol formation. In the subsequent oxidation reaction, the Mo atom in CoMoO⁺ serves as the active site for O₂ adsorption, and both Mo and Co atoms act as electron donors to activate O₂, leading to the formation of the C–O bond in formaldehyde. This work reports the first gas-phase example of achieving conversion of CH₄ to HCHO and its radical derivatives by O₂ at room temperature using heteronuclear non-noble metal cations. Remarkably, the CoMoOCH₂⁺ cation maintains high reactivity after adsorbing one or two CH₄ molecules. Finally, these findings provide new mechanistic insights into selective methane activation and conversion.« less

The incorporation of multiple cations on a single lattice site in the high-entropy oxides is considered the key driving factor for modifying the known atomic-level response to energetic ion irradiation due to the presence of structural disorder; however, these effects are not well-understood yet. In this work, we present atomic-level insight into irradiation-induced nanoscale phase transformations in a perovskite-structured high-entropy oxide, Sr(Zr_0.2Sn_0.2Ti_0.2Hf_0.2Nb_0.2)O₃ (Sr(HE)O₃), subjected to 774 MeV swift Xe heavy ions, where damage is dominated by inelastic ion−lattice interactions. While these ions generally are known to create nanoscale disordered channels, “ion tracks”, along the penetration direction in the material, thismore » study shows the formation of discontinuous and partially recrystallized ion tracks in Sr(HE)O₃. Compared to SrTiO₃ irradiated under identical energy loss conditions, the ion tracks in Sr(HE)O₃ exhibit significantly reduced diameters and a markedly different interfacial structure. Notably, the crystalline−amorphous interface in Sr(HE)O₃ shows minimal lattice distortion, confined to approximately 2−3 monolayers, in contrast to the extended disordered shell commonly observed in SrTiO₃. Using in situ atomic-resolution electron microscopy, we further demonstrate that the amorphous/disordered regions within Sr(HE)O₃ ion tracks remain highly stable under electron irradiation, whereas tracks in SrTiO₃ readily recrystallize. This enhanced stability is attributed to the dominance of structural and chemical complexity arising from multiple B-site cations, which suppress defect migration and templated recrystallization driven by electronic excitations and local heating. Overall, this study highlights how high-entropy oxide chemistry fundamentally reshapes irradiation damage evolution, offering insights into defect formation and phase stability under extreme conditions.« less

Alkali metal (AM) cations are often taken for granted as counterions in coordination chemistry and organometallic reactions. However, the AM cation can be more than a bystander in inorganic transformations. This Account focuses on research that has elucidated several types of AM cation effects and how these can be exploited to achieve novel structures and reactivity pathways. Here, a particular focus is on AM cation effects in low-coordinate iron β-diketiminate complexes, though we address general trends and potential applications in systems with other supporting ligands.

Ionic liquids with special functionalities are synthesized with the specific purpose of creating new patterns of interaction in the condensed phase. This Letter discusses the case of alcohol-functionalized ILs, the so-called HFILs, which are part of the larger cohort of task-specific ionic liquids. We find that this small chemical modification can cause massive changes in the liquid landscape when the cationic tails are longer. For prototypical ionic liquids, larger alkyl tails act as separators of charge networks, but in the case of HFILs these become physical charge network linkers. The OH functionality adds a large repertoire of interactions and correlationsmore » that were mostly unavailable to traditional ILs.« less

The oxygen evolution reaction often limits the efficiency of renewable fuel syntheses due to its sluggish reaction kinetics. Of the factors that have been studied to improve this important reaction is how the choice of the electrolyte may alter the reaction kinetics. Despite its importance, sys-tematic studies of this effect have been relatively rare. Herein, we report an effort toward correcting this deficiency by investigating the effect of nitrate on water oxidation catalyzed by IrO_x. The results show that nitrate can suppress the reaction, resulting in a decrease of the rate and an increase in the Tafel slope. The effectmore » was found to be consistent with a microkinetic model incorporating competitive adsorption between reac-tion intermediates and nitrate, suggesting that the reaction mechanism was unaffected by anion identity. Moreover, this blocking effect exhibited dependence on the cations, following a trend Li⁺ ≈ Na⁺ ≈ K⁺ > Cs⁺ > TEA⁺. The results are expected to find broad applications in electrocatalysis.« less

For this work, a series of six quinary rare-earth sulfides Ce⁴⁺_1.85Eu²⁺_1.15Na_0.30SiS₇, Ce⁴⁺_1.91Eu²⁺_1.09K_0.18SiS₇, Ce⁴⁺_1.96Eu²⁺_1.04Rb_0.08SiS₇, Ce⁴⁺_1.98Eu²⁺_1.02Cs_0.05SiS₇, Ce⁴⁺_1.97Eu²⁺_1.03Ag_0.06SiS₇, and Ce⁴⁺_1.50Eu²⁺_1.50CuSiS₇ were obtained in an alkali iodide flux using the boron-chalcogen mixture (BCM) method. Single crystal X-ray diffraction was used to determine the structures of the high quality single crystals that were grown; their elemental compositions were confirmed by energy-dispersive spectroscopy (EDS). The compounds crystallize in the hexagonal crystal system in the noncentrosymmetric space group P63. The crystal structure consists of a three-dimensional network composed of mixed cerium and europium bicapped trigonal prisms, isolated SiS4 tetrahedra, and monovalent metals (Na, K, Rb, Cs, Ag,more » and Cu) located in cavities created by linked Ce/EuS₈ polyhedra. The structures are charge-balanced when Ce and Eu are in their +4 and +2 oxidation states, respectively. The effective magnetic moment of Ce_1.50⁴⁺Eu_1.50²⁺CuSiS₇ determined from the temperature dependence of the magnetic susceptibility data is consistent with the presence of Ce⁴⁺ and Eu²⁺. Clear correlations between the alkali ion site occupancy, the ionic radius of the alkali cations, and the average bond length of Ce⁴⁺/Eu²⁺–S, were established. UV–vis diffuse reflectance data were collected for Ce_1.50⁴⁺Eu_1.50²⁺CuSiS₇ and a band gap of 1.9(1) eV was established.« less

Direct coupling of N₂ with abundant feedstocks like benzene to form N-containing organic compounds is a promising strategy for N₂ fixation pathways. The challenge of coupling N₂ activation and C–H bond oxidative addition was recently solved by introducing a reversible benzene C–H bond activation process mediated by a low-valent Fe(0) complex, which gave an organometallic product that could couple with partially reduced N₂. Interestingly, the energetics of the C–H oxidative addition/reductive elimination step depends on the choice of alkali metal. However, the reason why the alkali metal influences the C–H bond activation remained elusive. In this work, we present amore » comprehensive study on this Fe(0)-mediated reversible C–H activation. Through density functional theory combined with high-level coupled cluster calculations, we discovered that the intermediate-spin triplet (S = 1) controls the energy of the transition state for C–H cleavage, while the high-spin quintet (S = 2) controls the position of the equilibrium. Na⁺ drives the equilibrium toward oxidative addition due to an electrostatic effect, while K⁺ and Rb⁺ are dominated by a steric effect that favors the iron(0) species. The key role played by nonbonding interactions in the Fe-mediated C–H activation provides a conceptual model for alkali control over organometallic transformations.« less

Ni-rich single-crystalline cathodes are pivotal for advancing lithium-ion battery technology due to their high energy density and mechanical stability. However, Ni-rich single-crystalline particles face intrinsic structural heterogeneity due to excessively high sintering temperature required to shape micron-sized morphologies─typically over 150 °C above the polycrystalline optimum, leading to rapid electrochemical decay and unsatisfied rate performance that hinder their practical application. Here, in this work, we propose a lithium-deficient presintering strategy to synthesize cation-ordered single-crystalline LiNi_0.83Co_0.12Mn_0.05O₂ (S-NCM83), effectively minimizing lattice chemical heterogeneity and defect formation. The resulting cation-ordered percolation network enhances the structural stability of the bulk, reduces the energy barrier formore » Li⁺ migration, and stabilizes Li⁺ diffusion pathways. Consequently, S-NCM83 demonstrates significantly improved cycling stability across various operating temperatures and achieves exceptional rate performance, delivering 206 mAh g^–1 at 0.1 C and 170 mAh g^–1 at 5 C, without requiring surface coatings or doping. This work introduces a universal strategy to address the long-standing structural instability issues in single-crystalline cathodes, paving the way for simplified and scalable approaches to long-life and high-energy lithium-ion batteries.« less

Facilitating different chemistries between the rare earth (RE = La–Lu, Sc, Y) ions is of significant interest for their separations. While the bulk of attention has been on maximizing the small differences in their ground state chemistry, interest is beginning to shift toward the differences in their electronic excited states. In this work, we demonstrate modulation of the photostationary state of an azobenzene derivative, Na1, via chelation to a series of RE^IIIDO3A (DO3A = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) complexes. The extent of photoisomerization of 1^– follows the trend in REIII Lewis acidity with two exceptions: Sm^III and Er^III. UV–vis spectroscopy, titration experiments,more » and computational analysis show that these exceptions are a result of energy transfer rather than differences in ground state chemistry. Finally, these results open a pathway to differentiate REs by new photochemical means.« less