Large Volume Collapse during Pressure-Induced Phase Transition in

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Large Volume Collapse during Pressure-Induced Phase Transition in Lithium Amide Xiaoli Huang, Da Li, Fangfei Li, Xilian Jin, Shuqing Jiang, Wenbo Li, Xinyi Yang, Qiang Zhou, Bo Zou, Qiliang Cui, Bingbing Liu, and Tian Cui* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People’s Republic of China ABSTRACT: The structural studies of lithium amide (LiNH2) have been performed by synchrotron X-ray diffraction measurements and ab initio density functional theoretical calculations up to 28.0 GPa. It is revealed that LiNH2 undergoes a reversible pressure-induced phase transitions from tetragonal phase (I-4) into the monoclinic phase (P21), which starts from about 10.3 GPa and completes at about 15.0 GPa. This transition is accompanied by about 11% large volume collapse, and this volume collapse is much larger than other complex ternary hydrides. The experimental pressure−volume data for the two phases of LiNH2 are fitted by third-order Birch−Murnaghan equation of state, yielding B0 of 37.2 (1.7) GPa for the tetragonal phase and 7.6 (4.9) GPa for the monoclinic phase with the pressure derivatives at 3.5. We also have calculated the total and partial density of states of the two phases in order to explore the mechanism of the volume reduction.

I. INTRODUCTION In order to achieve the targets for mobile hydrogen fuel cell applications,1,2 it is important to develop hydrogen carrying systems with a high weight percentage and high capacity of hydrogen that can be readily released and recharged. The complex hydrides have recently attracted growing attention due to their high gravimetric densities of hydrogen providing a possible solution to the storage problem.3−12 One potential new type of complex hydrides has been recently proposed by Chen et al.,13 and it is demonstrated that lithium-based amides and imides can reversibly store hydrogen with a theoretical capacity of 6.5 wt % H2: LiNH2 + LiH ↔ Li2NH + H2. For this reaction, experimental investigations give the standard enthalpy of 65.6 kJ/mol H2 yielding an equilibrium temperature of 230 °C at normal pressure. LiNH2 is crucial in this hydrogen desorption/absorption process, so it would be of great importance to understand the structural stability of LiNH2 under high pressure, as a primary step toward their experimental characterization, and also a prerequisite for finding or designing material with specific storage thermodynamics. According to previous literature, structural phase transitions under high pressure are very common in alkali and alkalineearth complex hydrides. The ambient structure of LiBH4 with Pnma symmetry initially transforms at 1.2 GPa into a tetragonal phase with Ama2 symmetry and then to a cubic (Fm-3m) phase at 10.0 GPa.14 Furthermore, the tetragonal phase (Ama2) was suggested as a promising candidate of hydrogen storage materials. Recent theoretical calculations have predicted that the α-LiAlH4 (P21/c) phase transforms into a new phase with α-NaAlH4 structure (I41/a) at 2.6 GPa with a large volume collapse of 17% and then changes into a KGaH4 type structure (Pnma) at 33.8 GPa.15 Lately, the structural stability of © 2012 American Chemical Society

Mg(BH4)2 under high pressure has been investigated by George et al., and they found a structural phase transition around 2.5 GPa and again around 14.4 GPa by synchrotron Xray diffraction (XRD) and Raman spectroscopy.16 As for LiNH2, there is little investigation on the structural stability of LiNH2 under high pressure, in comparing with much literature focusing on the synthesis and the reaction mechanism.17−21 So far, there is only one high pressure research performed by Chellappa et al.22 They found that a phase transition occurred at about 12 GPa by means of in situ high-pressure Raman spectroscopy studies. However, the crystal structure of the high pressure phase has not been obtained. Besides, it is still an unknown question that whether the high pressure phase of LiNH2 has more favorable properties than ambient structure. In the present work, we report a combined experimental and theoretical study of LiNH2 as a function of pressure. To elucidate the influence of pressure on the structural stability of LiNH2 and find more favorable hydrogen storage materials, we have performed in situ high-pressure angle-dispersive XRD measurements up to 28.0 GPa. Further, ab initio calculations are also employed to determine the exact atomic postions and gain insight into the new structure at high pressure, providing a better understanding of the structural stability of the LiNH2 under high pressure. Both experimental and theoretical results discoved a phase transition around 10.0 GPa and confirmed the crystal structure with space group P21 of the new high pressure phase (β-LiNH2). Received: December 29, 2011 Revised: April 12, 2012 Published: April 16, 2012 9744

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Figure 1. (a) Selected angle dispersive X-ray diffraction patterns of LiNH2 with increasing pressure up to 28.0 GPa at room temperature (incident wavelength λ = 0.6199 Ǻ ). The numbers under the first pattern are Miller indexes (h k l) of the diffraction peaks of α-LiNH2. The asterisk marked in the profiles at 10.3 GPa represents the peak from the new phase β-LiNH2. (b) Variation of the d-spacing of main peaks under high pressure, and the two dashed lines indicate the phase transition pressure range.

Perdew−Zunger28,29 (CA-PZ) to describe the exchange and correlation potential. Convergence tests give a kinetic energy cutoff Ecutoff as 550 eV and Brillouin zone sampling with the Monkhorst−Pack grid30 of spacing 2π × 0.03 Å−1 for all phases.

II. EXPERIMENTAL AND THEORETICAL METHODS Commercially available LiNH2 powder (Alfa Products, purity >95%) without further purification was loaded into a symmetric diamond anvil cells with 500 μm diamond anvils. A 140-μmdiameter hole was drilled through the center of a preindented 80-μm-thick T-301 stainless steel gasket to form a sample chamber. Considering the sample is air and water sensitive and readily reacts with most common pressure-transmitting media, so no pressure medium was used in the experiment, and the sample loadings were performed in a glovebox (nitrogen atmosphere). Pressure in the diamond anvil cell was determined by the standard ruby fluorescence method,23 and the ruby fluorescence peaks were found to be sharp and well separated up to the highest pressure of 28.0 GPa, which confirmed a quasi-hydrostatic condition in sample chamber over the whole pressure range. Angle-dispersive XRD at room temperature were performed at the 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF). An image plate detector (MAR-3450) was used to collect diffraction patterns, and the two-dimensional XRD images were analyzed using the FIT2D software, yielding onedimensional intensity versus diffraction angle 2θ patterns.24 The average acquisition time was 600 s. The X-ray beam was focused by a pair of Kirkpatrick−Baez mirrors. The sample to detector distance and geometric parameters were calibrated using a CeO2 standard. The XRD patterns were fitted by Rietveld profile matching using the Material Studio program. During each refinement cycle, the fractional coordinates, scale factor, background parameter, isotropic thermal parameters, profile function, and cell parameters were optimized. The first-principles calculations described here were performed with the CASTEP code,25 based on density functional theory (DFT) using Norm-conserving pseudopotentials.26 All the structural optimizations under high pressure, including the atomic positions and the lattice constants, were performed by the Broyden−Fletcher−Goldfarb−Shanno algorithm.27 Convergence was achieved when the forces acting on the atoms are less than 0.01 eV/Å and all the stress components are less than 0.02 GPa. We use the local density approximation (LDA) in the scheme of Ceperley−Alder−

III. RESULTS AND DISCUSSION Representative XRD patterns of LiNH2 with increasing pressure are shown in Figure 1a. Under compression, all diffraction peaks shift toward higher 2θ angles, indicating a decrease of interplanar distance of crystal planes. Up to 10.0 GPa, the intensity of the peaks decreases with pressure increase. At about 1.0 GPa, the diffraction pattern is most easily indexed by using tetragonal symmetry with the largest figure of merit (FOM) (about 41.0). Below 10.3 GPa, although some weak peaks become very weak and are nearly invisible, the crystal structure does not change until the appearance of new peaks. At about 9.5 GPa, the indexing of the XRD pattern is performed. And the main indexing results show that the FOM of tetragonal system is the largest one with 61.1. So we confirm that the XRD pattern at about 9.5 GPa is still tetragonal crystal, meaning that the crystal structure does not change. Above 10.3 GPa, it is clearly observed that the relative intensity of the main peaks such as (101), (112), and (200) of the tetragonal type LiNH2 decreases with increasing pressure. Simultaneously, a new diffraction peak centered at 2θ = 18.6° emerges, which is marked with asterisk in Figure 1a. The variation of the dspacing of main peaks under high pressure can be seen much more clearly in Figure 1b. The original peaks of the α-LiNH2 linearly deceased with increasing pressure until the new peak appeared at about 10.3 GPa. These results indicate that a pressure-induced phase transformation occurs at 10.3 GPa. However, the peaks belonging to α-LiNH2 persist until 15.0 GPa, and then the sample is fully converted to β-LiNH2. The large pressure coexistence region (10.3−15.0 GPa) of the two phases is also commonly found in other complex hydrides. In addition to nonhydrostatic stresses, the large coexistence region may be largely due to slow transition kinetics.31−33 With further pressure increasing, no structural changes are observed in the XRD profiles up to 28.0 GPa. Besides, during decompression to zero pressure, the high pressure phase β-LiNH2 transforms 9745

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back to the α phase indicating the phase transition is fully reversible. The schematic ambient crystal structure of the tetragonal LiNH2 (α-LiNH2) with space group I-4 is shown in Figure 3a. There are eight formula units in the unit cell. In this structure, three inequivalent Li atoms occupy Wyckoff 2a, 2c, and 4f positions, the N atom is located at the 8g position, and two nonequivalent H atoms are at the 8g1 and 8g2 positions, respectively. Figure 2a shows the Rietveld refinement of LiNH2

Figure 3. Unit cell crystal structures of (a) α-LiNH2 and (b) β-LiNH2 phases. The purple (largest), blue (medium), and white (smallest) spheres represent lithium, nitrogen, and hydrogen atoms, respectively. (c and d) Views of the α and β phases along the a−c plane direction.

= 4.844 (1) Å, b = 3.703 (1) Å, c = 4.794 (1) Å, and β = 87.28° (1). The corresponding atomic positions at 16.2 GPa are listed in Table 1. As is illustrated in Figure 4, the calculated enthalpy Figure 2. Rietveld full-profile refinement of the XRD patterns of (a) αLiNH2 at 1.0 GPa and (b) β-LiNH2 at 16.2 GPa at room temperature. The difference between the observed and fitted patterns is also shown on the same scale. Vertical bars under the patterns represent the calculated positions of reflections from the α- and β-LiNH2 phases. The two fit are good for the diffraction pattern shown with Rwp = 5.73% and 6.80%, respectively.

Table 1. Atomic Fractinal Coordinates, and Isotropic Displacement Factors Uiso (Å2) of the β phase for LiNH2 at 16.2 GPa atom Li1 Li2 N1 N2 H1 H2 H3 H4

carried out on an XRD pattern obtained at 1.0 GPa. Rietveld refinement of LiNH2 shows a good agreement with the tetragonal structure. The refined lattice constants are a = b = 4.978 Å and c = 10.135 Å with unit cell volume V = 251.109 Å3, which are in good agreement with those previously reported in the literature.17,19 The present X-ray results provide strong evidence of a pressure-induced phase transition in LiNH2. However, in case of β-LiNH2, none of the known structure types coincides with the experimental XRD results. By means of the crystal structure prediction method-USPEX (Universal Structure Predictor: Evolutionary Xtallography),34−36 we obtained a new structure of β-LiNH2, which is monoclinic with space group P21, as shown in Figure 3b. The β-LiNH2 structure has four molecules with two inequivalent Li and two different N atoms in the unit cell. The mechanical and dynamical stabilities of the monoclinic structure of β-LiNH2 have been confirmed by the calculations of elastic constants and phonon structure. As is illustrated in Figure 3, panels c and d, the arrangements order of [NH2]−1 ions in the lattice changes from the α-LiNH2 phase transforming into β-LiNH2 phase, which is related to the multiple mode splitting in Raman spectra of β-LiNH2.22 Moreover, we have performed the Rietveld refinement of the XRD pattern at 16.2 GPa directly based on the predicted structure by ab initio calculations. Figure 2b presents the Rietveld refinement of LiNH2 performed at 16.2 GPa, showing good agreement with a monoclinic cell with space group P21 with lattice parameters a

x 0.217 0.953 0.748 0.322 0.199 0.661 0.630 0.207

y (2) (3)

(3) (4)

0.674 0.503 0.346 0.253 0.228 0.200 0.577 0.427

(2) (5) (7) (1) (7)

(2)

z

Uiso

0.939 (4) 1.182 (3) 0.529 (2) 1.008 1.191 (4) 0.53948 0.528 (3) 0.894

0.0397 0.1367 0.8604 0.3913 0 0 0 0

Figure 4. Enthalpy difference per fomular unit as a function of pressure for LiNH2 in the α-LiNH2 (full line) and β-LiNH2 (dashed line) crystal structures. The enthalpy of the α-LiNH2 phase is taken as the reference energy. 9746

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Figure 5. (a) Lattice parameters of LiNH2 at different pressure. (b) Pressure-volume data of α and β phases at 300 K. Solid black and red symbols represent experiment data, and solid black lines are third-order Birch−Murnaghan equation of state fitting data.

showed that the α phase transformed into β phase at about 10.0 GPa, which is consistent with our experimental phase transition pressure (10.3−15.0 GPa). The lattice parameters of the α- and β-LiNH2 phases as a function of pressure are shown in Figure 5a. It is observed that all lattice parameters in two phases decrease monotonically with increasing pressure except angle β in the β-LiNH2 phase. Figure 5b shows the pressure dependence of the volume changes obtained from experimental data for the two phases. There is about 11% volume shrinkage during the phase transition at about 12.0 GPa, indicating the first-order nature of the phase transition. More importantly, it is likely that hydrogen can be stored more efficiently in the β-LiNH2 phase owing to its large volume shrinkage. Although other hydrogen storage materials was also observed volume collapse under high pressure12,14,15 our obtained β-LiNH2 phase is with much larger volume collapse than other complex hydrogen storage material such as LiBH414 and NaAlH4.12 The ambient structure of LiBH4 with Pnma symmetry transforms into a tetragonal phase with Ama2 symmetry at 1.2 GPa, which shows a remarkable volume collapse by 6.6%. So the tetragonal phase (Ama2) was suggested as a promising candidate for stabilization at ambient conditions. Vajeeston et al. has predicted that there is a huge volume collapse during the phase transition by theoretical calculations, so the β-LiAlH4 phase was considered as a potential hydrogen storage material. Similarly, we suggest that the dense structure β-LiNH2 (P21) found in our experiments is more favorable for obtaining improved hydrogen-storage materials by means of various chemical substitutions. Perhaps it is also a way to improve the kinetics of reversible hydrogen absorption/desorption because the structural behavior of the β phase is drastically different from the α phase. The experimental pressure-volume data were fitted by thirdorder Birch−Murnaghan (BM) equation of state (EOS)37

P=

−7/3 ⎡ ⎛ V ⎞−5/3⎤ 3B0 ⎢⎛ V ⎞ ⎥ −⎜ ⎟ ⎜ ⎟ ⎥⎦ 2 ⎢⎣⎝ V0 ⎠ ⎝ V0 ⎠

⎧ ⎫ ⎡⎛ ⎞−2/3 ⎤⎪ ⎪ V 3 ⎨1 + (B0 ′ − 4)⎢⎜ ⎟ − 1⎥⎬ ⎢⎣⎝ V0 ⎠ ⎥⎦⎪ ⎪ 4 ⎩ ⎭

where V0 is the volume per formula unit at ambient pressure, V is the volume per formula unit at pressure P given in GPa, B0 is the isothermal bulkmodulus, and B0′ is the first pressure derivative of the bulk modulus. We obtain B0 of 37.2 (1.7) GPa for the α phase and 7.6 (4.9) GPa for the β phase with B0′ fixed at 3.5, as is listed in Table 2. The calculated bulk modulus of Table 2. Unit Cell Volumes Per Formula Unit (V0), Unit Cell Volumes (Vu), and Equation of State Parameters (B0 and B0’) of the α-LiNH2 and β-LiNH2 Phases V0

Vu

B0 (GPa)

B0′

37.2 (1.7) 32.7 (1) 33.0

3.5 3.5

7.6 (5) 11.0 (1)

3.5 3.5

α-LiNH2 present exp. data present calc. data calc. data24 present exp. data present calc. data

32.7 (1) 261.6 30.1 (1) 240.8 32.4 β-LiNH2 55.0 (10) 220.0 42.9 (1) 171.6

the two phases is in agreement with the experimental ones. Comparing with the bulk modulus of high pressure phases in LiBH4,14 the β-LiNH2 phase is much lower, which means with a higher compressiblility. It is interesting to note that the bulk modulus of the high-pressure phase decreases about 70% compared to that of the ambient-pressure phase, indicating an increasing compressibility of LiNH2 at high pressure. For providing some insight on the bulk modulus of both α- and βLiNH2, we have added theoretical calculations of their elastic constants. Since the fitted Birch−Murnaghan EOS gives the bulk modulus of ambient pressure, so we calculated their elastic 9747

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hybridization between the N 2p and H 1s states by the covalent bond between N and H in LiNH2; the highest valence band is mostly arising from N 2p electrons. The conduction band is mainly attributed to H 1s and Li 2s electrons. The main difference of DOS between the two phases is that the β phase has a broader band gap compared with that of the α phase. Moreover, the gap in the highest valence band of N 2p electrons disappears from the α phase to the β phase. With regard to the mechanism of volume collapse in LiAlH4 induced by phase transition, Vajeeston et al. has considered that the sto-p electronic transition within the Al atom in the β-LiAlH4 causes the huge volume collapse.15 However, the DOS of LiNH2 is apparently distinct from LiAlH4, so the mechanism for large volume collapse of LiNH2 is different from lithium alanate and perhaps relates with the disappearance of gap in the highest valence band of N 2p electrons. More experimental and theoretical studies are required to explore the mechanism, which is of great importance to understanding more hydrogen storage materials under high pressure. Although the β-LiNH2 phase cannot be preserved to ambient pressure by decompression pressure, it is also of great help for finding a better hydrogen storage material with new structure and provide insights into effects of pressure on the N−H bonding and its implications for hydrogen desorption/absorption. Since the crystal structure of β-LiNH2 phase is known by our work, there are many ways to achieve this phase including chemical substitutions or low temperature cooling, which is our next work under way.

constants at ambient pressure shown in Table 3. From this table, it is seen that the four elastic constants (C11, C12, C33, and Table 3. Calculated Elastic Constants of the α-LiNH2 and βLiNH2 Phases at Ambient Pressure phase

elastic constants

C11 = 54.12 GPa, C33 = 62.57 GPa, C44 = 20.96 GPa, C66 = 28.74 αGPa, C12 = 11.57 GPa, C13 = 16.36 GPa, C16 = −0.78 GPa LiNH2 βC11 = 41.34 GPa, C22 = 99.51 GPa, C33 = 52.78 GPa, C44 = 39.89 LiNH2 GPa, C55 = 28.74 GPa, C66 = 22.45 GPa, C12 = 9.60 GPa, C13 = 3.33 GPa, C15 = −0.05 GPa, C23 = 19.72 GPa, C25 = −0.34 GPa, C35 = 0.14 GPa, C46 = 0.04 GPa

C13) of α phase, which determined the bulk modulus of αLiNH2, are larger than that of β-LiNH2. Moreover, the C22 of βLiNH2 is much larger than C11 and C33 in β-LiNH2, which reflects the larger anisotropic comparing with the case of αLiNH2. So the larger anisotropic in elastic constants is the main reason for β-LiNH2 phase with lower bulk modulus. Deep reasons can be found by Mulliken population analysis, the N− H covalent bond of α-LiNH2 is with larger population, so it is a little stronger than that of β-LiNH2. So it is proposed the stronger N−H bond contributes to the larger bulk modulus of α-LiNH2. Figure 6 shows the calculated density of states (DOS) of αand β-LiNH2 phases at 1.0 and 16.2 GPa, respectively. From this figure we can see that both α- and β-LiNH2 are insulators with a band gap of about 3.7 and 5.2 eV, respectively. There are three main valence bands and one conduction band in the DOS of α- and β-LiNH2 structures. The lowest valence band is contributed by the hybridization between the N 2s and H 1s states; the higher energy valence bands are mainly of the strong

IV. CONCLUSIONS In conclusion, the experimental synchrotron XRD measurements have identified a reversible phase transition in LiNH2.

Figure 6. Calculated total and partial density of states (DOS) for (a) α-LiNH2 at 1.0 GPa and (b) β-LiNH2 at 16.0 GPa. Fermi levels are set at zero energy and marked by dotted vertical lines. 9748

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The α-LiNH2 (I-4) transforms into the β phase (P21) at about 12.0 GPa with a huge volume reduction of 11%. The observed phase stability and transition pressure are in agreement with our first-principles calculations. The experimental pressure− volume data were fitted to a third-order Birch−Murnaghan equation of state. With B0′ fixed at 3.5, we obtain B0 of 37.2 (1.7) GPa for the α phase and 7.6 (4.9) GPa for the β phase, respectively. From our calculated total DOS of the α- and βLiNH2 phases, the disappearance of gap in the highest valence band of N 2p electrons in the β phase is associated with the volume collapse. The β-LiNH2 structure (P21) with huge volume shrinkage comparing with α phase is more favorable for obtaining improved hydrogen-storage materials by means of various chemical substitutions or low temperature cooling. We suggest that the dense structure found in our high-pressure experiments to be targeted for obtaining improved hydrogenstorage materials. The experiments to stabilize it under ambient conditions are needed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-431-85168825. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Xiaodong Li and Lingyun Tang for their help during the experimental research. ADXRD experiments of this work were performed at 4W2 HP-Station, BSRF assistance in the synchrotron measurement. This work was supported by the National Basic Research Program of China (Nos. 2011CB808200), the National Natural Science Foundation of China (Nos. 51032001, 11074090, 10979001, 51025206, and 11004074), and Changjiang Scholar and Innovative Research Team in University (No. IRT1132). Thanks for the High Performance Computing Center (HPCC) of Jilin University.

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