On the viability of Mg extraction in MgMoN₂: a combined experimental and theoretical approach

Abstract

Layered MgMoN2 was prepared by solid state reaction at high temperature between Mo and Mg3N2 in N2 which represents a simple synthetic pathway compared to the previously reported method that used NaN3 as nitrogen source. The crystal structure of MgMoN2 was studied by synchrotron X-ray and neutron powder diffraction. The feasibility of oxidizing this compound and concomitantly extracting magnesium from the structure was assessed by both chemical and electrochemical approaches, using different protocols. The X-ray diffraction patterns of oxidized samples do not exhibit any relevant difference with respect to that of the as prepared MgMoN2 and no differences in the cell parameters are deduced from Rietveld refinements. No hints pointing at the presence of any amorphous phase are observed either. These results are rationalized through DFT calculated energy barriers for Mg 2+ ion migration in MgMoN2.

Two-step solid-state synthesis of ternary nitride materials

“…Figure 1a) depicts PXRD patterns of the reaction products, MgZrN 2 , Mg 2 NbN 3 , and MgMoN 2 , after washing with anhydrous methanol to remove MgCl 2 products. Quantitative crystallographic analysis using the Rietveld method reveals that the MgZrN 2 and Mg 2 NbN 3 crystallize in the rock-salt (Fm3m) structure as previously reported in thin film products, 9 whereas MgMoN 2 forms in the layered hexagonal crystal structure (P6 3 /mc), 17 as illustrated in Figure 1c). The simulated XRD patterns are shown in Figure 1b) for comparison, while structural parameters for each product are listed in Table 1 as compared to literature.…”

“…11,12 In bulk powder form, ternary nitrides have been synthesized by high pressure metathesis, 13,14 ammonolysis, 15 ammonothermally, 16 and from the elements under flowing N 2 . 17 For the ambient-pressure synthesis of magnesium metal nitrides, lower temperatures are required to avoid the loss of Mg from the ternary products. For example, in the synthesis of layered MgMoN 2 , high pressure autoclaves are used in conjunction with sodium azide at 700 °C to achieve crystalline products.…”

Ternary Nitride Materials: Fundamentals and Emerging Device Applications

“…Layered compounds are particularly of interest for ease of Li + (or other ion, see Ref. 56,57) intercalation, and layered LiMoN 2 and Li 7 MnN 4 (58) have been investigated as electrode materials, while anti-fluorite (59) and hexagonal lithium nitridosilicates (60) have been explored as promising solid state electrolytes with measured Li-ion conductivity as high as 5 × 10 −2 Ω −1 cm −1 at elevated temperatures. As seen in Figure 3 and discussed in Section 3.2, some ternary nitrides can be synthesized with disorder on the cation site, which may lead to additional benefits when used as electrode materials.…”

“…Closed systems are especially appropriate for reactions of binary nitrides, where all nitrogen needed for the final product is embedded in the precursor materials. These reactions are well-suited for ternaries where one or more metal is an alkali or alkaline earth metal due to the availability of N-containing precursors (e.g., Li 3 N, Mg 3 N 2 , Sr 2 N, and Ca 3 N 2 (56,59,108)), and can be extended to NaN 3 -based compounds with appropriate safety precautions for azide use (see sidebar). Reactions of binary nitrides are typically contained in welded, refractory metal tubes to avoid reaction of the metals with other containers (e.g., as occurs with SiO 2 ) and to retain a reasonable µ(N) for the endogenously-released N. However, for lower-temperature reactions (T < 1000 K), ion exchange or double exchange reactions can take place in an inert-atmosphere glovebox (119,120) Welded tubes are also used for reactions performed in molten, metallic fluxes, which have garnered success for crystal growth and materials discovery efforts.…”

On the Study of Ca and Mg Deintercalation from Ternary Tantalum Nitrides

“…Although few reports on the study of ternary alkali transition metal nitrides as Li-ion battery electrode materials exist, 12,15 such compounds have not been considered for multivalent technologies, the only exception being MgMoN 2 , for which Mg 2+ extraction was not possible. 16 Among the available alkaline earth metal transition metal nitrides, the ternary A–Ta–N (A: Ca, Mg) system represents an interesting playground for research, as the electronic configuration of tantalum allows in principle to explore a broad spectrum of phase compositions, with potentially high operating voltage and high specific capacities. Hexagonal Mg 1– x Ta 2+ x N 3 and cubic Mg 2.6– x Ta 2+ x N 3 phases were originally prepared by Brokamp and Jacobs by solid state reaction between Mg(NH 2 ) 2 or Mg 3 N 2 and Ta 3 N 5 .…”