Rapid Microwave Synthesis, Characterization and Reactivity of Lithium Nitride Hydride, Li4NH

Lithium nitride hydride, Li4NH, was synthesised from lithium nitride and lithium hydride over minute timescales, using microwave synthesis methods in the solid state for the first time. The structure of the microwave-synthesised powders was confirmed by powder X-ray diffraction [tetragonal space group I41/a; a = 4.8864(1) Å, c = 9.9183(2) Å] and the nitride hydride reacts with moist air under ambient conditions to produce lithium hydroxide and subsequently lithium carbonate. Li4NH undergoes no dehydrogenation or decomposition [under Ar(g)] below 773 K. A tetragonal–cubic phase transition, however, occurs for the compound at ca. 770 K. The new high temperature (HT) phase adopts an anti-fluorite structure (space group Fm3¯m; a = 4.9462(3) Å) with N3− and H− ions disordered on the 4a sites. Thermal treatment of Li4NH under nitrogen yields a stoichiometric mixture of lithium nitride and lithium imide (Li3N and Li2NH respectively).


Introduction
The Li-N-H system is a promising hydrogen storage candidate, with the ability to store 11.5 wt % of H 2 reversibly [1]. This process occurs via two exothermic steps (Equations 1 and 2): However, it has been demonstrated that the reaction pathway may be more complex than originally indicated. In-situ powder neutron diffraction (PND) [2,3] revealed the possibility of a reaction pathway involving the formation of the lithium nitride hydride, Li 4 NH [4,5] from lithium nitride in addition to the hydrogenated phase, lithium imide, Li 2 NH. At a low partial pressure of hydrogen, the formation of LiH appears to be suppressed, leading to the overall reaction shown in Equation (3). The dehydrogenation behaviour of Li 4 NH itself, however, remains essentially unknown and Li 4 NH is the only nitride hydride currently known in the Li-N-H system.
Further, non-stoichiometric phases can be formed at 723 K from the reaction between the hydride and imide products in Equation (3). These complex non-stoichiometric phases thus contain N 3− , H − and (NH) 2− anions and form a solid solution [Equation (4)] [4]: A full understanding of the structure and reactivity of Li 4 NH is thus required in order to determine its role in the Li-N-H system and the process of hydrogen uptake and release. One of the problems in developing such an understanding centres on the reliable synthesis of single phase Li 4 NH. Preparation of the phase requires the solid state reaction of Li 3 N and LiH at high temperature under strictly anaerobic conditions while preventing side reactions with container materials.
In this work we demonstrate how microwave synthesis of Li 4 NH using both commercial multi-mode and single-mode microwave (MW) cavities can provide a solution to this problem. The result is a reproducible route for the synthesis of phase-pure Li 4 NH over timescales orders of magnitude shorter than those required for conventional heating methods, which are less energy-efficient and more difficult to control. This facile synthesis approach has allowed us to produce bulk powders of Li 4 NH for a subsequent comprehensive study of structure, stability and reactivity. This synthesis method may well be extrapolated successfully to other hydrogen storage materials.

Characterization
Powder X-ray diffraction (PXD) was conducted using a Bruker D8 diffractometer (Bruker Corporation, Billerica, MA, USA, Cu Kα source) or a PANalytical X'Pert Pro MPD powder diffractometer (Cu Kα 1 source) in capillary mode. The air-sensitive samples were ground into fine powders and placed in 0.5 mm diameter sealed glass capillaries for data collection. Data were collected in the range 5° ≤ 2θ ≤ 85° using a 0.0168° 2θ step size for 1 h for phase identification or 10° ≤ 2θ ≤ 110° for 12 h for structure refinement. PXD data were indexed and refined by least squares fitting using the CELREF software package [6]. Structural refinements were conducted via the Rietveld method using the GSAS and EXPGUI packages [7,8] The scale factor, zero point and background were refined in initial cycles, A shifted Chebyschev polynomial function (background function 1 in GSAS) was employed to model the background. The unit cell parameters, peak profile parameters and atomic parameters were refined subsequently. The peak shape was modelled using the pseudo-Voigt function (profile function 2 in GSAS). Constraints were applied to the thermal parameters of the N and H atoms within both the LT-and HT-Li 4 NH phases.
Simultaneous thermal analysis (thermogravimetric and differential thermal analysis; TG-DTA) was performed using a NETZSCH STA 409PC thermobalance coupled to a HIDEN HPR20 mass spectrometer (MS). Approximately 30 mg of Li 4 NH was placed in an alumina pan and heated from ambient temperature to either 773 K or 873 K at 5 K·min −1 under a flow of Ar or N 2 (60 mL·min −1 ), respectively. The maximum temperature was held for 1 h before cooling (5 K·min −1 ). Simultaneously, mass spectra for nitrogen, hydrogen, ammonia and water were recorded during heating.
IR spectra were collected at room temperature (20 scans/sample, 8 cm −1 resolution) using a Shimadzu FTIR 8400S instrument with a Pike MIRacle ATR sampling accessory. Raman spectra were collected at room temperature using a Horiba LabRAM HR confocal microscope system (Horiba Itd., Kyoto, Japan) with a 532 nm laser, 1200 gr·mm −1 grating and a Synapse CCD detector. A hole aperture of 50 μm and a 25 times reduced laser intensity were used in order to minimise sample decomposition.

Li 4 NH Synthesis Using a Multimode Microwave Reactor
MW synthesis in a commercial multimode cavity (MMC) reactor offers faster processing (over times of the order of minutes), increased energy efficiency and lower cost [9] than conventional high temperature approaches. To date, MW heating experiments with solid-state hydrogen storage materials have been limited to the study of the dehydrogenation properties of a small number of alkali, alkaline-earth and transition metal hydrides and of the alkali metal borohydrides, LiBH 4 , NaBH 4 and KBH 4 [10][11][12]. Nevertheless, given the difficulties in mapping the microwave field distributions in

Li 4 NH Synthesis Using a Single Mode Microwave Reactor
A summary of the reactions conducted in SMC system ( Figure 3) is shown in Table 2. As for the MMC syntheses described in section 3.1, cooling intervals were introduced between irradiation periods to avoid melting of the silica reaction ampoule (i.e., melting point 2073 K). Indeed, heating at 300 W for t > 240 s led to the destruction of the SiO 2 reaction vessel. It is evident from PXD data collected for sample 8 that single phase Li 4 NH could be successfully synthesized at 300 W in 180 s; no reflections from the starting materials α-Li 3 N and LiH were observed (Figure 4). The final product had the appearance of a yellow/beige pellet. Previously, Li 4 NH was synthesised from the reaction between Li 3 N and LiH at 763 K for 6 h under Ar [4] and thus with the synthetic approach described here, reaction times could be reduced by a factor of 100 and performed without the need for an inert cover gas.  The MW synthesis of lithium nitride hydride is possible due to the ability of the starting materials to absorb microwave energy and convert this into heat (as reflected in the loss tangent, tan δ). The ability of Li 3 N to produce heat in a microwave field may be attributed to its inherent fast ionic conductivity and semiconducting behavior [15,16]. In fact, it is well established that microwaves couple directly to charge carriers leading to extremely rapid reactions in many ionic conductors and semiconductors [17]. Conversely, LiH does not generate significant heat under a microwave field and, for example, no changes in temperature were observed when LiH was placed within SMC (400 W; 20 min) or MMC (500 W; 30 min) reactors [7,9]. In fact, in these previous studies among NaH, MgH 2 , CaH 2 , TiH 2 , VH 0.81 , ZrH 2 and LaH 2.48 only the transition metal and lanthanide hydrides showed a rapid increase in temperature, which even then only led to the desorption of a small percentage of hydrogen (< 0.5 wt %). During the reactions described here (samples 1-8), a purple plasma was observed along the length of the silica reaction tube. The purple plasma was followed on most occasions by yellow/orange flashes (Additional Supplementary Information). The observation of these plasmas/flashes provides evidence for the high local temperatures achieved in the reaction vessel (i.e., Li evaporation occurs at ca. 1573 K) [18].

Thermal Stability of Li 4 NH
The thermal stability of the nitride hydride was investigated by TG-DTA under flowing argon. TG-DTA of sample 8 showed no evidence of mass change and hence decomposition or dehydrogenation when the sample was heated to 773 K (Figure 5a). Moreover, it was also evident from mass spectra collected simultaneously while heating that no hydrogen or other gases were evolved over the entire m/z range (1 ≤ m/z ≤ 200) (Figure 5b). These results corroborate previous investigations conducted to 698 K under argon [4]. Indexing of the PXD pattern from sample 8 following the TG-DTA experiment (Figure 5c) yielded cell parameters for Li 4 NH of a = 4.891(2) Å and c = 9.9252(8) Å. These lattice parameters are within 2σ of those obtained for this sample prior to the TG-DTA and therefore no significant changes were noted. An Li 2 O impurity was noted in the post-TG-DTA diffractogram and was attributed to the presence of moisture in the Ar (g) and/or a reaction between a small amount of Li 4 NH and the alumina sample holder.
The DTA profile for sample 8 however reveals an interesting feature above 700 K with no corresponding simultaneous weight change. This endothermic peak at 770 K can thus be attributed to a structural phase transition in Li 4 NH. An equivalent exothermic peak in the DTA was observed at 755.6 K on cooling, demonstrating that the phase transition is reversible (and as corroborated by PXD where the tetragonal Li 4 NH is observed as discussed above).

Structur
Structure refinement d ncluded in t could be obt residuals wa see above). diffraction p Figure 7. The presence of the HT-phase in sample 8 can be rationalised by the relatively fast cooling rate from the SMC MW reaction (as compared to conventional heating), which allows some of the kinetically stable HT-Li 4 NH phase to remain in the sample at room temperature.  Table S1). Li atoms are tetrahedrally coordinated to N/H atoms in the HT-phase. There are also strong similarities between the anti-fluorite structures of HT-Li 4 NH (Li 2 N 0.5 H 0.5 ) and Li 2 NH (Li 2 (NH)). The Li-N 3− /H − bond lengths are shorter than the lithium-imido Li-N distances reported by Balogh et al. [19] in Li 2 NH. (2.205 Å).
Nitride hydrides are relatively rare but N 3− /H − anion ordering similar to that in the LT-Li 4 NH phase has also been observed in alkaline earth metal nitride hydrides such as Ca 2 NH(D) (cubic space group Fd 3 m) [20,21], Ba 2 NH(D) and Sr 2 NH (both hexagonal space group R 3 m) [22,23]. Although there are no previously reported examples of complete N 3− /H − disorder in the solid state, the anion disorder in HT-Li 4 NH is paralleled by the N 3− /F − distribution in nitride fluorides such as Ba 2 NF [24,25]. Further studies on deuterated LT-Li 4 NH and HT-Li 4 NH using powder neutron diffraction will be performed to elucidate the crystal structures more fully (i.e., determine accurate hydrogen (deuterium) occupancies and anisotropic thermal parameters).

Li 4 NH in Air
To determine the reactivity of Li 4 NH in air, a freshly made sample was exposed to the ambient atmosphere for different times and the as-formed products were analysed by PXD (Figure 9). After 4 h of air exposure, Li 4

Conclusions
In summary, Li 4 NH has been synthesized in multi-mode and single-mode cavity microwave reactors over unprecedented timescales. Single-mode microwave reactions demonstrate several advantages over multi-mode approaches, such as increased efficiency and higher reproducibility. This new synthetic approach can reduce reaction times by a factor of 100 compared to conventional synthesis methods. Diffraction data served to confirm the purity of the as-formed product and to provide a structural model for Li 4 NH by means of Rietveld refinement. Thermal treatment under argon showed that a phase transition to a high temperature cubic anti-fluorite phase occurs at ca. 770 K. HT-Li 4 NH contains disordered nitride and hydride anions. In addition, results on the reactivity of Li 4 NH under air and N 2 were also shown. In the former case, the nitride hydride reacts to form hydroxides (anhydrous and monohydrated) and subsequently lithium carbonate, under ambient conditions. In the latter case, Li 4 NH reacts to produce Li 3 N and Li 2 NH at high temperature.