Synthesis of Layered Group IV Nitride Materials by Soft Chemical Anion Metathesis

Abstract

Traditionally, solid state chemistry has been dominated by high temperature methods that only lead to thermodynamically stable products. More recently, soft chemical methods have emerged as a means to new kinetically stable materials that cannot be attained by traditional methods. Ion exchange, or metathesis, has become a powerful soft chemical synthetic tool, especially in oxide chemistry. Although cation exchange has become extremely common in many solid state materials, anion exchange is much more uncommon due to effects such as size and polarizability. This work developes rare anion metathesis as a synthetic tool in the MNX (M = Zr, Hf; X = Cl, Br, I) system, making new materials that cannot be prepared by traditional methods. Topochemical reactions between beta-ZrNCl and A2S ( A = Na, K, Rb) in the solid state provide alpha-Zr2N2S (P-3m1, 800oC), which neatly transforms to beta-Zr2N2S (P63/mmc) at elevated (> 850oC) temperatures. At low temperatures (300 - 400oC), this reaction yields ACl intercalated phases of the formula AxZr2N2SClx (0 < x < ~ 0.15) that are reversibly hygroscopic. Reactions of ZrNCl with excess A2S at 400 - 1000oC gives A2S intercalated phases of the formula A2xZr2N2S1+x (R-3m) where 0 < x < 0.5. Recrystallized β-ZrNCl reacts with excess NaF in the solid state via anion exchange to give AxZrNF1+x (A = Na, K; x ~ 0.3), the first fluoride structural analogue (P63/mmc crystal symmetry) of the MNX system (M = Zr, Hf; X = Cl, Br, I). Organic/inorganic hybrid materials have also been obtained by chloride for alkoxide exchange. Excess AOR (A = Na, K; OR = OMe, OEt, On-Bu) reacts with ZrNCl solvothermally in THF (200-250oC) yielding a series of ZrN(OR) compounds, which retain the trigonal, lamellar nature of the ZrNCl parent. Full structural and analytical characterization is described for all phases as well as implications of the results.