As a vitally important thermodynamic parameter, high pressure has been considered to be a crucial tool to research the intrinsic properties of materials.^[1–9] New crystal structures and properties emerging through the application of high pressure also help interpret the elusive nature of novel phenomena or provide new routes for new materials. Recently, the great interest focusing on high pressure phase transitions of complex light-metal hydrides has been aroused based on a theoretical prediction of a new phase under high pressure by Vajeeston. The transition to a new high pressure phase was predicted with a 17% volume collapse at about 2.6 GPa in LiAlH₄.^[10] So they pointed out that it is worthwhile to investigate the reversibility and stabilization of high-pressure phases in those compounds, which have been predicted to exhibit superior hydrogen storage characteristics. If we can observe such a large volume collapse in experiment, and retain the high pressure phase to ambient pressure, then this would develop a new type of hydrogen storage material efficiently in aspect of gravity and volume. Interest in structural studies of these materials has therefore increased significantly during recent years.

There are many advantages of complex light-metal hydrides as the hydrogen storage material, such as high hydrogen content, relatively low decomposition temperature, environmental friendliness, and so on. Recently, there also has been significant interest in the properties of sodium amide (NaNH₂), although it has a relatively low theoretical hydrogen capacity (5.1 wt.%^[11]) compared with its counterpart LiNH₂.^[12] Nevertheless, it was proposed that NaNH₂ plays an important role in dehydrogenation with other light metal hydride systems when forming some composite hydrogen storage materials, for example, NaNH₂–LiAlH₄,^[12] NaNH₂–MgH₂,^[13] and NaNH₂–LiBH₄.^[14] Structural properties of the ambient-condition phase of NaNH₂ (hereafter, α-NaNH₂) have been widely studied by x-ray and neutron diffraction experiments.^[15,16] The α-NaNH₂ crystallizes into an orthorhombic phase belonging to space group Fddd (D224 h) with cell parameters a = 8.964 Å, b = 10.456 Å, c = 8.073 Å, and Z = 16.^[17] However, the high-pressure behavior of NaNH₂ has not been extensively studied, especially the x-ray diffraction method is limited due to the weak scattering from the low-Z atoms. Recently, Liu et al.^[18] observed two phase transitions at about 0.9 GPa and 2.0 GPa characterized by spectroscopical changes in both Raman and infrared (IR) experiments. Whereafter, the electronic structure and vibrational properties of NaNH₂ were calculated by Zhong^[19] and two high pressure phases (orthorhombic, and monoclinic, C2/c) were predicted by using the ab initio evolutionary structure prediction. However, no direct experimental evidence was reported, and detailed structural characterization of the high-pressure phases is extremely lacking.

The pressure-induced phase transitions in NaNH₂ have been observed through in situ high-pressure Raman spectra and synchrotron x-ray diffraction measurements up to 20 GPa at room temperature in this work. We observed interesting pressure-induced transformations and new phenomenon that have not been observed in the previous work. These high-pressure results may shed light on the lightweight hydrides.

The air and moisture sensitive NaNH₂ powder (Alfa Aesar, ) was loaded with ruby chips into a Mao-type symmetric diamond anvil cell (DAC). The diamond culets used in DACs were for both Raman and XRD experiments. A T301 stainless with a preindented 50- -thick was used as the gasket material, and a sample chamber was created by a laser drilling system. To prevent the sample reacting with water, oxygen, and common pressure-transmitting media, no pressure medium was used, and the sample loading was performed in a glove box with inert N₂ atmosphere, which kept the O₂ and H₂O residual contents both below 0.1 ppm. The pressure inside the sample chamber was gauged at room temperature using the standard ruby fluorescence method,^[20] the Raman ruby lines were sharp and well separated during the experimental pressure range. Thus, the quasi-hydrostatic conditions were confirmed to the highest pressure.

High-pressure angle-dispersive XRD measurements were carried out at 16-BM-D beamline of the High Pressure Collaborative Access Team (HPCAT) facility, at the Advanced Photon Source (APS), Argonne National Laboratory (ANL). The XRD experiments were performed at room temperature up to 20 GPa. The x-ray beam wavelength λ = 0.3979 Å. Diffraction images were obtained by a MAR3450 imaging plate detector, and then converted to XRD spectra (intensity versus 2θ patterns) through the FIT2D software.^[21] The indexing and refinement of the XRD patterns were carried out by Material Studio software packages.^[22] For the Raman experiment, we used a solid-state diode-pumped, frequency-doubled Nd: vanadate laser as the excitation laser, and the wavelength of the laser is 532 nm. In order to obtain high quality experimental data, we set the output power of the laser at 500 mW. The final Raman spectra were obtained through an ActonSpetraPro500i spectrograph with a liquid nitrogen cooled CCD detector (Princeton Instrument, 1340 × 100). All the Raman spectra were collected in the backscattering geometry.

As Raman spectroscopy is a powerful tool to study the structural transitions through the investigations on the changes of the lattice and internal vibrational modes, the Raman spectra of NaNH₂ have been measured up to 20 GPa. The spectrum in the bottom of Fig. 1 was recorded near ambient conditions inside the closed DAC, and it showed good agreement with a reference spectrum which was recorded directly from the NaNH₂ powder. Figure 1(a) shows that the lattice modes are located in the low frequency region with nine resolved Raman peaks from 100 cm⁻¹ to 600 cm⁻¹. Three expected peaks located at 1536 cm⁻¹, 3216 cm⁻¹, and 3267 cm⁻¹ can be seen in Figs. 1(b) and 1(c) in the internal region, which are assigned to the bending mode , the symmetric N–H stretching mode , and the asymmetric N–H stretching mode , respectively.^[18,23] Obviously, the Raman frequencies of all the internal modes of are lower than those in LiNH₂,^[24] indicating that the N–H distance in NaNH₂ is longer. Changes in the low-frequency region were observed by the abruptly weakened intensity for the librational modes during compression to 1.12 GPa. In addition, at 1.12 GPa, the mode rapidly developed into a triplet and the mode evolved into a tetrad as well. Such significant spectroscopic changes strongly suggest a phase transformation and a modification of the crystal structure from the initial phase at this pressure point, and the new phase is labeled as β-NaNH₂. Further compression to 1.93 GPa, the second phase transition from β- to γ-NaNH₂ was observed. During this structural transition, new lattice modes at lower frequencies were observed, and a marked decreased intensity for the H–N–H bending mode together with splitting into a doublet was observed at 1.93 GPa, and entirely vanished beyond 3.33 GPa. Up to the highest pressure, all the Raman peaks became rather weak and broad, only the peak mode was dominantly observable. The detailed pressure dependences of the measured Raman vibrational modes of NaNH₂ are shown up to 5.5 GPa in Fig. 2. In the region for the N–H symmetric and asymmetric stretching modes, all the modes show a strong upshift with pressure, and all the other modes exhibit slight pressure induced blue shifts. So there is no direct experimental evidence for the formation of hydrogen bonds either in the high pressure β- or γ-NaNH₂ phase from our Raman data.

Fig. 1.

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	Fig. 1. Representative Raman spectra of NaNH2 in the whole spectral regions upon compression at room temperature.

Fig. 2.

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	Fig. 2. The pressure dependence of vibration frequency for NaNH2 up to 5.5 GPa. The phase boundaries are indicated by vertical dotted lines.

To corroborate our observations in the Raman spectra experiments and further determine the symmetry information of the new structures, in situ high pressure synchrotron XRD measurements have been carried out. At ambient condition, NaNH₂ has an orthorhombic symmetry with space group Fddd. Figure 3(a) shows the Rietveld refinement of NaNH₂ carried out on an XRD pattern obtained at 0.05 GPa. The Rietveld refinement of NaNH₂ shows an excellent agreement with the ambient structure, which is shown in Fig. 3(b). The Na atoms are located at the 16f positions, N at the 16g positions, and H at the 32f sites, and the structure contains NaN₄ tetrahedra with Na–N distances of 2.4–2.5 Å. The lattice constants obtained from the refined results are a = 8.999 Å, b = 10.350 Å, c = 8.005 Å with unit cell volume V = 745.64 ų, and they are in good accordance with those previously reported in the literature.^[17]

Fig. 3.

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	Fig. 3. (a) Powder x-ray diffraction pattern and Rietveld full-profile refinement of α-NaNH2 structure at 0.05 GPa with λ = 0.3979 Å. The green vertical ticks indicate the expected Bragg positions and intensities of XRD peaks. (b) The crystal structures of α-NaNH2. The yellow (largest), grey (medium), and pink (smallest) spheres represent sodium, nitrogen, and hydrogen atoms, respectively.

The pressure dependent powder XRD patterns of NaNH₂ taken at room temperature are shown in Fig. 4(a). At 0.05 GPa, all the diffraction peaks can be resolved to the ambient phase α-NaNH₂ by the Rietveld refinement. With increasing pressure, all the obtained diffraction peaks shift to larger angles, indicating the shrinkage of the lattice. With the increase of the pressure to 1.07 GPa, several new peaks appear indicating the appearance of the first phase transition, which is coincident with the Raman results of structure phase transition at 1.12 GPa. The shape, intensity, and width of the peaks of the β-NaNH₂ phase are distinct from those of the α-NaNH₂ phase. There are new peaks emerged at low and high angles in the XRD patterns, indicating that a first order-like structural transformation has taken place at this pressure point. With continuous compression to 2.25 GPa, the XRD diffraction peaks again exhibit distinct changes both in the numbers, intensities, and sharpness, coinciding with the Raman data from the β-NaNH₂ phase transition into the γ-NaNH₂ phase. Up to 16.52 GPa, the XRD patterns of the γ-NaNH₂ phase remain stable with weak and broadened peaks. During the whole compression process, we can provide experimental evidences for two structure phase transformations from by both the XRD patterns and Raman spectroscopy. When we release the pressure from the highest pressure to ambient pressure, the high pressure γ-NaNH₂ transforms back to α-NaNH₂ as shown in the top of Fig. 4(a), reflecting the reversible of the phase transitions. The XRD rings of the sample at different pressure points in Fig. 4(b) show that NaNH₂ reveals a recrystallization phenomenon under high pressure, which has not been observed in other alkaline amide compounds. The sample at 1.84 GPa shows much smoother diffraction rings than that at 0.05 GPa. Intriguingly, during the compression to 2.25 GPa, the sample begins to melt, and upon compression to higher pressure, the diffraction rings imply more uniform crystalline grains of NaNH₂ after the subsequent recrystallization, the melting phase reverts back to the crystalline phase.

Fig. 4.

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	Fig. 4. (a) Representative angle dispersive x-ray diffraction patterns of NaNH2 during compression to 16.52 GPa at room temperature. The Miller indexes ( ) of the diffraction pattern peaks of ambient NaNH2 phase are shown by the numbers under the first pattern. (b) The XRD rings of the sample at 0.05 GPa, 2.25 GPa, and 16.52 GPa respectively.

The present XRD results provide strong evidence of the two pressure-induced phase transitions in NaNH₂. We try to obtain more information of the crystal structures of β-NaNH₂ and γ-NaNH₂. Zhong et al. have predicted that the structures of β-NaNH₂ and γ-NaNH₂ are with the space groups and C2/c, respectively.^[19] Therefore, we firstly make comparison between our obtained experimental high pressure XRD patterns and the predicted crystal structures from the theoretical calculation by Zhong et al. However, both of their two structures do not match with our high pressure XRD patterns. Besides this, the energetically competing structures proposed by Zhong et al., namely triclinic P-1, monoclinic C2, and orthorhombic Amm2 structures, are also considered for the high pressure structures of NaNH₂. Taken into the consideration of structural similarity with alkali metal amides, the tetragonal LiNH₂-type structure with I-4 space group and monoclinic KNH₂-type structure with space group are also explored. However, all of these structures fail to match with our high pressure XRD patterns. To index the peaks belonging to the unknown compound, the obtained high pressure XRD data of the unknown phases are analyzed by the program DICVOL06^[25] and PEAKFIT. For the β-NaNH₂ case, no indexing data can be resolved, probably because there are two phases mixed in β-NaNH₂. For phase γ, we select the data at 16.52 GPa to index, and the results show that it mainly belongs to an orthorhombic system. Several space groups are allowed for the orthorhombic system, for instance, PBAN, P2AN, PB2N, PBMN, and PBA2. Among them, PBAN is a strong candidate for phase , because it is most appropriate for comparison with the diffraction profile at 16.52 GPa. The obtained lattice parameters from the fitting results of γ-NaNH₂ with PBAN space group at 16.52 GPa are a = 8.4880 Å, b = 7.5655 Å, and c = 6.0678 Å, which is shown in Fig. 5. The reason for such poor agreement with those structures predicted by theoretical investigation may be the presence of recrystallization during compression.

Fig. 5.

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	Fig. 5. The indexing of the XRD patterns of γ-NaNH2 at 16.52 GPa.

A combination of synchrotron x-ray diffraction and Raman experiment has uncovered a complex sequence of phase transformation of NaNH₂ as a function of pressure. The ambient condition α-NaNH₂ first transforms into β-NaNH₂ at about 1 GPa and then into probably an orthorhombic γ-NaNH₂ with space group PBAN at about 2 GPa. Even though the intermolecular N–H distances become shortened during the compression process, all the vibrational modes exhibit blue shifts, so there is no hydrogen bonding effect within the pressure region. High pressure induced comminution and recrystallization may be the reason for the poor agreement with previous theoretical investigation.

The authors are grateful to Yue Meng and Jesse Smith for their technical support during the experiment. This work was performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. The Advanced Photon Source is a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.