In Situ Neutron Diffraction Studies of the Metal Flux Growth of Ba/Yb/Mg/Si Intermetallics

Temperature-Dependent Products in Gallium Flux Reactions of Cerium and Transition Metals

Reactions of cerium and transition metals in excess molten gallium were carried out, exploring the formation of different cerium intermetallics as the flux reaction is cooled. Ce/T/Ga reactions with T = Ni, Cu, Pd, Ag, and Zn produce a high-temperature product, which converts into a low-temperature product in the flux. The phases present in the flux mixture were determined by quenching identical reactions at 750 and 300 °C and identifying the isolated products using elemental analysis and X-ray diffraction. The compounds CeGa2, CeCu0.37Ga3.63, CePd0.32Ga3.68, Ce5Ag1.76Ga17.29, and Ce5Zn1.37Ga17.73 were isolated by quenching at 750 °C. Upon cooling to 300 °C, the corresponding reactions instead yielded CeGa6, Ce2CuGa12, Ce2PdGa12, Ce2Ag0.7Ga9.1, and CeZnxGa7-x. All of these structures contain cerium in the ThCr2Si2-related layers. Large crystals of high-temperature products CeCu0.37Ga3.63, CePd0.32Ga3.68, Ce5Ag1.76Ga17.29, and Ce5Zn1.37Ga17.73 were used for magnetic susceptibility measurements. All of these materials show highly anisotropic ferromagnetic ordering of Ce3+ moments below 8 K, which is in contrast to the antiferromagnetism seen for the compounds that were isolated at 300 °C.

Flux Growth of Cerium Nickel Gallides Studied by In Situ Neutron Diffraction

Reactions of cerium and nickel in excess molten gallium were monitored by neutron diffraction during heating and cooling. The formation of binary intermediates CeGa₂ and Ni₂Ga₃ was observed during heating. During cooling of the molten mixture from 900 °C, precipitation of BaAl₄-type CeNi_0.74Ga_3.26 occurred at 850 °C. Upon cooling to 650 °C, this compound reacted in the flux to form Ce₂NiGa₁₀ and then Ce₂NiGa₁₂, the latter of which persisted to room temperature. Making use of this information, subsequent reactions were quenched at 750 °C to isolate crystals of CeNi_0.74Ga_3.26 for further study. Similar reactions replacing Ce with La and quenching above 750 °C yielded LaNi_0.35Ga_3.65 crystals. Magnetic susceptibility studies on CeNi_0.74Ga_3.26 indicate that the cerium is trivalent; the Ce³⁺ moments undergo a strongly anisotropic ferromagnetic ordering with moment perpendicular to the c axis below 7 K. Heat capacity data show little evidence of heavy fermion behavior. Resistivity measurements show that both LaNi_0.35Ga_3.65 and CeNi_0.74Ga_3.26 exhibit metallic behavior. Density of states calculations support this and indicate that Ni/Ga mixing in the compound stabilizes the structure.

How to Look for Compounds: Predictive Screening and in situ Studies in Na-Zn-Bi System

Here, the combination of theoretical computations followed by rapid experimental screening and in situ diffraction studies is demonstrated as a powerful strategy for novel compounds discovery. When applied for the previously “empty” Na−Zn−Bi system, such an approach led to four novel phases. The compositional space of this system was rapidly screened via the hydride route method and the theoretically predicted NaZnBi (PbClF type, P4/nmm) and Na₁₁Zn₂Bi₅ (Na₁₁Cd₂Sb₅ type, P1‾ ) phases were successfully synthesized, while other computationally generated compounds on the list were rejected. In addition, single crystal X‐ray diffraction studies of NaZnBi indicate minor deviations from the stoichiometric 1 : 1 : 1 molar ratio. As a result, two isostructural (PbClF type, P4/nmm) Zn‐deficient phases with similar compositions, but distinctly different unit cell parameters were discovered. The vacancies on Zn sites and unit cell expansion were rationalized from bonding analysis using electronic structure calculations on stoichiometric “NaZnBi”. In‐situ synchrotron powder X‐ray diffraction studies shed light on complex equilibria in the Na−Zn−Bi system at elevated temperatures. In particular, the high‐temperature polymorph HT‐Na₃Bi (BiF₃ type, Fm3‾m) was obtained as a product of Na₁₁Zn₂Bi₅ decomposition above 611 K. HT‐Na₃Bi cannot be stabilized at room temperature by quenching, and this type of structure was earlier observed in the high‐pressure polymorph HP‐Na₃Bi above 0.5 GPa. The aforementioned approach of predictive synthesis can be extended to other multinary systems.

New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides

Molten salts are promising reaction media candidates for the discovery of novel materials; however, they offer little control over oxidation state compared to aqueous solutions. Here, we demonstrated that when two hydroxides are mixed, their melts become fluxes with tunable solubility, which are surprisingly powerful solvents for ternary chalcogenides and offer effective paths for crystal growth to new compounds. We report that precise control of the oxidation state of Ni is achievable in mixed molten LiOH/KOH to grow single crystals of all known ternary K-Ni-S compounds. It is also possible to access several new phases, including a new polytope of β-K₂Ni₃S₄, as well as low-valence KNi₄S₂ and K₄Ni₉S₁₁. KNi₄S₂ is a two-dimensional low-valence nickel-rich sulfide, and β-K₂Ni₃S₄ has a hexagonal lattice. Moreover, using KNi₄S₂ as a template, we obtained a new layered binary Ni₂S by topotactic deintercalation of K. The new binary Ni₂S has a van der Waals gap and can function as a new host layer for intercalation chemistry, as demonstrated by the intercalation of LiOH between its layers. The oxidation states of low-valence KNi₄S₂ and Ni₂S were studied using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Density functional theory calculations showed large antibonding interactions at the Fermi level for both KNi₄S₂ and Ni₂S, corresponding to the flat-bands with large Ni-d_x²-y² character.

Ternary Zinc Antimonides Unlocked Using Hydride Synthesis

Three new sodium zinc antimonides Na₁₁Zn₂Sb₅, Na₄Zn₉Sb₉, and NaZn₃Sb₃ were synthesized utilizing sodium hydride NaH as a reactive sodium source. In comparison to the synthesis using sodium metal, salt-like NaH can be ball-milled, leading to the easy and uniform mixing of precursors in the desired stoichiometric ratios. Such comprehensive compositional control enables a fast screening of the Na-Zn-Sb system and identification of new compounds, followed by their preparation in bulk with high purity. Na₁₁Zn₂Sb₅ crystallizes in the triclinic P1 space group (No. 2, Z = 2, a = 8.8739(6) Å, b = 10.6407(7) Å, c = 11.4282(8) Å, α = 103.453(2)°, β = 96.997(2)°, γ = 107.517(2)°) and features polyanionic [Zn₂Sb₅]^11- clusters with unusual 3-coordinated Zn atoms. Both Na₄Zn₉Sb₉ (Z = 4, a = 28.4794(4) Å, b = 4.47189(5) Å, c = 17.2704(2) Å, β = 98.3363(6)°) and NaZn₃Sb₃ (Z = 8, a = 32.1790(1) Å, b = 4.51549(1) Å, c = 9.64569(2) Å, β = 98.4618(1)°) crystallize in the monoclinic C2/m space group (No. 12) and have complex new structure types. For both compounds, their frameworks are built from ZnSb₄ distorted tetrahedra, which are linked via edge-, vertex-sharing, or both, while Na cations fill in the framework channels. Due to the complex structures, Na₄Zn₉Sb₉ and NaZn₃Sb₃ compounds exhibit low thermal conductivities (0.97-1.26 W·m^-1 K^-1) at room temperature, positive Seebeck coefficients (19-32 μV/K) suggestive of holes as charge carriers, and semimetallic electrical resistivities (∼1.0-2.3 × 10^-4 Ω·m). Na₄Zn₉Sb₉ and NaZn₃Sb₃ decompose into the equiatomic NaZnSb above ∼800 K, as determined by in situ synchrotron powder X-ray diffraction. The discovery of multiple ternary compounds highlights the importance of judicious choice of the synthetic method.

Synthesis and Manipulation of Single-Crystalline Lithium Nickel Manganese Cobalt Oxide Cathodes: A Review of Growth Mechanism

Lithium nickel manganese cobalt oxide (NMC) cathodes are of great importance for the development of lithium ion batteries with high energy density. Currently, most commercially available NMC products are polycrystalline secondary particles, which are aggregated by anisotropic primary particles. Although the polycrystalline NMC particles have demonstrated large gravimetric capacity and good rate capabilities, the volumetric energy density, cycling stability as well as production adaptability are not satisfactory. Well-dispersed single-crystalline NMC is therefore proposed to be an alternative solution for further development of high-energy-density batteries. Various techniques have been explored to synthesize the single-crystalline NMC product, but the fundamental mechanisms behind these techniques are still fragmented and incoherent. In this manuscript, we start a journey from the fundamental crystal growth theory, compare the crystal growth of NMC among different techniques, and disclose the key factors governing the growth of single-crystalline NMC. We expect that the more generalized growth mechanism drawn from invaluable previous works could enhance the rational design and the synthesis of cathode materials with superior energy density.

Flux Growth of Phosphide and Arsenide Crystals

Flux crystal growth has been widely applied to explore new phases and grow crystals of emerging materials. To accommodate the needs of high-quality single crystals, the flux crystal growth should be reliable, controllable, and predictable. The selections of suitable flux and growth conditions remain empirical due to the lack of systematic investigation especially for reactions, which involve highly volatile components, such as P and As. Considering the flux elements, often the system in question is a quaternary or a higher multinary system, which drastically increases complexity. In this manuscript, on the examples of flux growth of phosphides and arsenides, guidelines of flux selections, existing challenges, and future directions are discussed. We expect that the field will be further developed by applying in situ techniques and computational modeling of the nucleation and growth kinetics. Additionally, leveraging variables other than temperature, such as applied pressure, will make flux growth a more powerful tool in the future.