Two-Step Solid-State Synthesis of Ternary Nitride Materials.

Ternary nitride materials hold promise for many optical, electronic, and refractory applications; yet, their preparation via solid-state synthesis remains challenging. Often, high pressures or reactive gases are used to manipulate the effective chemical potential of nitrogen, yet these strategies require specialized equipment. Here, we report on a simple two-step synthesis using ion-exchange reactions that yield rocksalt-derived MgZrN2 and Mg2NbN3, as well as layered MgMoN2. All three compounds show almost temperature-independent and weak paramagnetic responses to an applied magnetic field at cryogenic temperatures, indicating phase-pure products. The key to synthesizing these ternary materials is an initial low-temperature step (300-450 °C) to promote Mg-M-N nucleation. The intermediates then are annealed (800-900 °C) to grow crystalline domains of the ternary product. Calorimetry experiments reveal that initial reaction temperatures are determined by phase transitions of reaction precursors, whereas heating directly to high temperatures results in decomposition. These two-step reactions provide a rational guide to material discovery of other bulk ternary nitrides.

Low-temperature synthesis of superconducting iron selenide using a triphenylphosphine flux.

Many functional materials have relatively low decomposition temperatures (T≤ 400 °C), which makes their synthesis challenging using conventional high-temperature solid-state chemistry. Therefore, non-conventional techniques such as metathesis, hydrothermal, and solution chemistry are often employed to access low-temperature phases; the discovery of new chemistries is needed to expand access to these phases. This contribution discusses the use of triphenylphosphine (PPh3) as a molten flux to synthesize superconducting iron selenide (Fe1+δSe) at low temperature (T = 325 °C). Powder X-ray diffraction and magnetism measurements confirm the successful formation of superconducting iron selenide while nuclear magnetic resonance spectroscopy and in situ X-ray diffraction show that the formation of superconducting FeSe at low temperatures is enabled by an adduct between the triphenylphosphine and selenium. Exploration of the Fe-Se-PPh3 phase space indicates that the PPh3-Se adduct effectively reduces the chemical potential of the selenium at high concentrations of triphenylphosphine. This contribution demonstrates that the use of a poorly-solvating yet reactive flux has the potential to enable the synthesis of new low-temperature phases of solid materials.