In sharp contrast to molecular synthesis, materials synthesis is generally presumed to lack selectivity. The few known methods of designing selectivity in solid-state reactions have limited scope, such as topotactic reactions or strain stabilization. This contribution describes a general approach for searching large chemical spaces to identify selective reactions. This novel approach explains the ability of a nominally "innocent" Na 2 CO 3 precursor to enable the metathesis synthesis of single-phase Y 2 Mn 2 O 7 : an outcome that was previously only accomplished at extreme pressures and which cannot be achieved with closely related precursors of Li 2 CO 3 and K 2 CO 3 under identical conditions. By calculating the required change in chemical potential across all possible reactant-product interfaces in an expanded chemical space including Y, Mn, O, alkali metals, and halogens, using thermodynamic parameters obtained from density functional theory calculations, we identify reactions that minimize the thermodynamic competition from intermediates. In this manner, only the Na-based intermediates minimize the distance in the hyperdimensional chemical potential space to Y 2 Mn 2 O 7 , thus providing selective access to a phase which was previously thought to be metastable. Experimental evidence validating this mechanism for pathway-dependent selectivity is provided by intermediates identified from in situ synchrotron-based crystallographic analysis. This approach of calculating chemical potential distances in hyperdimensional compositional spaces provides a general method for designing selective solid-state syntheses that will be useful for gaining access to metastable phases and for identifying reaction pathways that can reduce the synthesis temperature, and cost, of technological materials.

Organic ligands with carboxylate functionalities have been shown to affect the solubility, speciation, and overall chemical behavior of tetravalent metal ions. While many reports have focused on actinide complexation by relatively simple monocarboxylates such as amino acids, in this work we examined Th(IV) and U(IV) complexation by 4-hydroxybenzoic acid in water with the aim of understanding the impact that the organic backbone has on the solution and solid state structural chemistry of thorium(IV) and uranium(IV) complexes. Two compounds of the general formula [AnO(OH)(HO)(4-HB)]· nHO [An = Th (Th-1) and U (U-1); 4-HB = 4-hydroxybenzoate] were synthesized via room-temperature reactions of AnCl and 4-hydroxybenzoic acid in water. Solid state structures were determined by single-crystal X-ray diffraction, and the compounds were further characterized by Raman, infrared, and optical spectroscopies and thermogravimetry. The magnetism of U-1 was also examined. The structures of the Th and U compounds are isomorphous and are built from ligand-decorated oxo/hydroxo-bridged hexanuclear units. The relationship between the building units observed in the solid state structure of U-1 and those that exist in solution prior to crystallization as well as upon dissolution of U-1 in nonaqueous solvents was investigated using small-angle X-ray scattering, ultraviolet-visible optical spectroscopy, and dynamic light scattering. The evolution of U solution speciation as a function of reaction time and temperature was examined. Such effects as well as the impact of the ligand on the formation and evolution of hexanuclear U(IV) clusters to UO nanoparticles compared to prior reported monocarboxylate ligand systems are discussed. Unlike prior reported syntheses of Th and U(IV) hexamers where the pH was adjusted to ∼2 and 3, respectively, to drive hydrolysis, hexamer formation with the HB ligand appears to be promoted only by the ligand.

Ternary nitride phase space holds great potential for new functional materials, as suggested by computational predictions of yet-to-be discovered stable phases. Here, we report a metathesis route to bulk powders of MgZrN2 and the solid solutions Mg x Zr2–x N2 (0 < x < 1). These ternary phases only result when lower temperature reactions are used, in contrast to previous work using the similar Mg-based metathesis reactions that resulted in the formation of exclusively ZrN. Thermochemical calculations illustrate why lower temperature metathesis reactions yield the incorporation of Mg, while higher temperature ceramic reactions yield exclusively ZrN. Experimental in situ X-ray diffraction of metathesis reactions during heating reveals two stages in the reaction pathway: initial consumption of the precursors to make an amorphous product (T rxn > 350 °C) followed by crystallization at higher temperatures (T rxn > 500 °C). Changing the ratio of the metathesis precursors (Mg2NCl and ZrCl4) controllably varies the composition of Mg x Zr2–x N2, which crystallizes as a cation-disordered rock salt, as evidenced by high-resolution synchrotron X-ray diffraction, electron microscopy, and bulk compositional analysis. Variation in composition leads to a gradual metal-to-insulator transition with increasing x, similar to other reports of analogous thin film specimens produced by combinatorial sputtering. Meanwhile, the optical behavior of these powders suggests nanoscale compositional inhomogeneity, as signatures of ZrN-like absorption are detectable even in Mg-rich samples. This metathesis approach appears to be generalizable to the synthesis of bulk ternary nitride materials.

The synthesis of inorganic metal nitrides poses a challenge due to the low reactivity of N2 gas at low temperatures, yet entropy driven formation of N2 gas at high temperatures. In contrast, synthetic approaches using more activated forms of nitrogen can be used to overcome the inertness of N2, but increased exothermicity can also result in diminished stoichiometric control and the activation of deleterious competing pathways. Here, kinetically controlled solid-state metathesis reactions are used to prepare Mn3N2 without the use of experimental conditions that increase the chemical potential of nitrogen and are known to produce phase impurity (e.g., NH3, N2-based plasma, azides, or high pressure). The solid-state metathesis reaction between MnCl2 and Mg2NCl or Mg3N2 is shown to generate Mn3N2, a phase on the border of stability. Highly exothermic control reactions performed with Li3N, Ca3N2, and Ca2NCl yield poorly crystalline, nitrogen-deficient Mn–N phases and N2 gas. The reactions with Mg2NCl and Mg3N2 do not self-propagate and have the lowest predicted free energies of reaction. A series of reactions performed at different times and temperatures, as well as in situ synchrotron X-ray diffraction, illustrate the importance of kinetic competence, and the results implicate the mechanism for this competence: the formation of a solid-solution, Mg x Mn1–x Cl2, between the halide precursor (MnCl2) and the halide product (MgCl2) coupled to a mildly exothermic reaction. Kinetically controlled solid-state metathesis continues to provide an avenue toward the synthesis of materials that cannot be prepared under traditional, high-temperature ceramic methods.

The adsorbed PVP enhances further the MOR activity on the O/T but suppresses it on the cubic Pt NPs.