Experimental and Theoretical Studies on a High Pressure

ARTICLE pubs.acs.org/JPCC

Experimental and Theoretical Studies on a High Pressure Monoclinic Phase of Ammonia Borane Yu Lin,*,† Hongwei Ma,†,^ Charles Wesley Matthews,‡ Brian Kolb,‡ Stanislav Sinogeikin,§ Timo Thonhauser,‡ and Wendy L. Mao†,|| †

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109, United States § High Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, United States Photon Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States

)

‡

bS Supporting Information ABSTRACT: The effect of pressure on the structure of ammonia borane NH3BH3 (AB) was investigated using a combination of high pressure X-ray diffraction (XRD) and density functional theory (DFT). In situ XRD was performed up to 15.0 GPa at room temperature in a diamond anvil cell, and two first-order phase transitions were observed at 1.6 and 12.9 GPa. The ambient pressure I4mm structure transformed into the high pressure Cmc21 phase at 1.6 GPa, and then experienced a second-order isostructural phase transition at 5 GPa, and further developed into a monoclinic P21 (Z = 4) phase at 12.9 GPa. The structure of the high pressure P21 phase was solved by powder diffraction data and further optimized using DFT calculations. The high pressure phase transitions were found to be reversible upon releasing pressure. The behavior of the N
H 3 3 3 H
B dihydrogen bonding framework, inter- and intramolecular interactions at high pressure was also investigated. The origin of the phase transition at 12.9 GPa is attributed to the reorganization of the dihydrogen bonding network and the change in the rotational dynamics of the NH3 and BH3 groups.

’ INTRODUCTION Over the past decade, AB has attracted considerable attention as a potential hydrogen-storage material, due to its high gravimetric (19.6 wt %) and volumetric H2 densities where H2 can be released by three single steps. A myriad of research has been conducted on how to effectively discharge H2 from AB, including lowering the decomposition temperatures and increasing the rate of H2 release through the use of acid1,2 or transition-metal-3,4 catalysts, ionic liquids,5 nanoscaffolds,6,7 etc. Recently, in the AB + H2 system, a novel high pressure compound was found which is able to store significant amounts of additional molecular H2 in AB, representing one of the most H-rich materials.8,9 AB is a solid at room temperature with a high melting point of +104 C when compared to isoelectronic compounds like ethane which melts at
181 C. This is primarily a result of the dipole
dipole interactions and a network of dihydrogen bonding. The short-range cooperative dipole
dipole interactions in the molecular AB crystal result in the length of B
N dative bond being significantly shorter in the solid state (1.58 Å) than in the gas phase (1.66 Å).10
12 The AB crystal also contains a unique class of unconventional H bonds which are considered dihydrogen bonds (DHBs) where both protonic H (Hδ+) and hydridic H (Hδ
) are present, which can be described as Nδ

Hδ+ 3 3 3 r 2012 American Chemical Society

Hδ

Bδ+. At ambient conditions, AB crystallizes into a tetragonal I4mm structure with two molecules in one unit cell. At temperatures below 225 K, it undergoes a first order phase transition to an orthorhombic space group Pmn21. A number of experimental and theoretical efforts have been devoted to study the nature of the structure changes as a function of temperature, as well as the evolution of the dihydrogen bonding network.13,14 The effect of pressure on the behavior of AB expands the fundamental understanding of this system and sheds further light onto its potential application as a hydrogen-storage material. In the past several years, extensive research has been carried out to elucidate its structural transformations upon compression by a suite of high pressure techniques, including Raman spectroscopy,15
18 synchrotron infrared measurements,18 powder X-ray diffraction (XRD),19
21 and neutron diffraction.20 In addition, theoretical calculations alone and in combination with experiments, Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: July 14, 2011 Revised: October 17, 2011 Published: January 03, 2012 2172

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The Journal of Physical Chemistry C such as group-theoretical analysis by Landau theory,19 density functional theory (DFT) calculations,19,22 and molecular dynamics (MD) simulations,23 were implemented for further understanding of this H-rich system. Pressure and temperature effects were also combined to investigate the stability of all the known AB phases.24,25 Although consensus has been reached on the transition from I4mm to Cmc21 at pressures below 2 GPa, how AB evolves with further compression still remains unresolved. Different transition pressures and higher pressure phases have been observed by experiments and predicted by calculations, including a P1 phase above 8 GPa by XRD and DFT,20 an unresolved phase at 5 GPa by XRD,21 and a P21 (Z = 2) phase above 12 GPa by MD simulation.23 In this study, we performed a systematic investigation of the structural evolution of AB as a function of pressure at room temperature up to 15.0 GPa by powder XRD and DFT calculations. Together with our earlier Raman spectroscopy results, we confirmed the I4mm to Cmc21 phase change around 1.6 GPa, suggested a second-order isostructural phase transition at 5 GPa, and found another structural transformation at approximately 12.9 GPa where Cmc21 changes into a high pressure monoclinic P21 phase.

’ EXPERIMENTAL AND THEORETICAL METHODS High-Pressure XRD Experiments. The AB starting material was synthesized at LANSCE, Los Alamos National Laboratory. Details of its synthesis can be found in previous work.17 A symmetric diamond anvil cell with 500 μm diamond culets was used for the high pressure experiment. A polycrystalline AB sample together with a ruby chip for pressure calibration26 were loaded into the sample chamber created by drilling a 150 μm hole in a preindented stainless steel gasket. No pressure transmitting medium was used because pure AB is quite soft and provides good quasihydrostatic conditions. In situ high pressure angle dispersive XRD experiments were performed at beamline 16-IDB of High Pressure Collaborative Access Team (HPCAT), Advanced Photon Source (APS), Argonne National Laboratory (ANL). Diffraction images were collected at a wavelength of λ = 0.3694 Å using a MAR345 image plate detector. The well collimated X-ray beam was approximately 5  7 μm2 centered within the gasket hole. All the measurements were carried out at room temperature. At each pressure point, diffraction images were recorded using multiimage scanning mode with an exposure time of 60 s per image. Pressure was measured before and after performing XRD experiments, and the average value was used throughout the data analysis. The two-dimensional Debye
Scherrer rings were integrated using the software package FIT2D.27 Jade 5 was further used to index the diffraction patterns and refine the lattice parameters as well as the unit cell volume.28 DFT Simulations. Using the experimentally determined structures as input, we have performed DFT29 calculations to further optimize all atomic coordinates while fixing the unit-cell lattice parameters. Since van der Waals forces may play an important role in the interaction between the NH3 and BH3 molecules in AB, we used the recently developed exchange-correlation functional vdW-DF,30,31 which includes van der Waals forces seamlessly and self-consistently. This functional has already been applied successfully to a wide variety of systems,32
38 including hydrogen-storage materials.39 Our calculations were performed with the PWscf code of the Quantum-Espresso package,40 using ultrasoft pseudopotentials with a wave function and charge-

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density cutoff of 35 and 210 Ry, respectively. During the optimization, all forces were converged to less than 2.5 meV/Å. We used a k-point sampling of the Brillouin zone according to Monkhorst
Pack41 utilizing a 6  6  4 mesh for the P21 phase and a 6  6  6 mesh for other phases. All the calculations correspond to T = 0 K.

’ RESULTS AND DISCUSSION 1. Structure Determination of the P21 Phase at High Pressure. In situ room temperature XRD patterns were collected

as a function of pressure up to 15.0 GPa, and two first-order phase transitions were identified upon compression (Figure 1). At 1.6 GPa, the parent tetragonal structure I4mm transformed to an orthorhombic Cmc21 phase. The pressure dependence of the unit cell dimensions and molecular volume are shown in Figure 2. As pressure was increased to 12.9 GPa, a new phase was observed which persisted to 15.0 GPa, the highest pressure reached in the experiment. The data set collected at 15.0 GPa was used for symmetry assignment and structure determination. The diffraction pattern was indexed with unit cell dimensions a = 7.713(6), b = 5.375(4), c = 3.898(4) Å, β = 97.22(8). Five possible monoclinic space groups are consistent with the diffraction systematic absences: P2, P21, Pm, P2/m, and P21/m. Generally speaking, a 21 screw axis and 2 rotation axis can be determined unambiguously according to the (0k0) reflections. Unfortunately, in this diffraction pattern, no observable (0k0) reflection was found. To determine the crystal structure, four NH3BH3 molecules were put into the unit cell with the same dimensions but different symmetries: P2, P21, Pm, P2/m, and P21/m, and then the parallel tempering technique implemented in the program FOX42 was used for global optimization of the structure against the observed diffraction pattern within the range of sin θ/λ < 0.21 Å
1. The geometrical configuration of the AB molecule was constrained to be identical with those at lower pressures, but relative rotation of the NH3 and BH3 groups was allowed. Final optimization results indicated that space group P21 gave the most plausible

Figure 1. Evolution of the XRD patterns of AB as a function of pressure up to 15.0 GPa. First-order phase transitions were observed at 1.6 and 12.9 GPa. The numbers on the right-hand side indicate pressure in GPa. The arrow shows the (101) reflection in the P21 phase. 2173

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√ Figure 2. (a) The pressure dependence of the unit cell parameters of the I4mm and Cmc21 phases. The a / 2 is used in the parent tetragonal phase for straightforward comparison to its high pressure orthorhombic phase. Unit cell parameters of the P21 phase are not shown here because the nonorthogonality of the a and c axes makes the direct comparison difficult. (b) Volume per molecule in the tetragonal I4mm, orthorhombic Cmc21, and monoclinic P21 phases as a function of pressure. The error bars associated with the lattice parameters and the molecular volume in the different phases are smaller than the symbol size.

arrangement of AB molecules in the unit cell. All the other four space groups resulted in significantly worse fits and unreasonable structure models. The P21 structure was further optimized with and without (201) preferred orientation (PO), denoted as models P21_PO and P21, respectively. The molecular volume of the P21 phase was obtained within the pressure range of 12.9
15.0 GPa and shown in Figure 2b. The two experimental structure models P21_PO and P21 were further optimized by DFT calculations, where we fixed the experimental cell parameters. The energy differences of the two structures (E(P21)
E(P21_PO)) with the exact experimental atomic positions and fully DFT-optimized coordinates are
1.1255 and
0.3096 eV/molecule, respectively. P21 is always lower in energy and represents the DFT ground state. The reason why the experimental P21_PO and P21 models have a somewhat large energy difference is mainly due to the unreliable H positions determined by the powder diffraction pattern. The DFT calculation where only H positions were optimized revealed that P21 was only 0.125 eV/molecule lower in energy than P21_PO. We also carried out further DFT calculations where the results confirmed that P21 is the ground state. After the P21 model was identified to be the structure with a lower total energy, all symmetry constraints were removed and the structure was lowered to be P1 symmetry. All the atoms were further fully relaxed. Interestingly, the resulting structure was identical to the one with enforced symmetry within numerical precision. This test confirms that P21 is indeed the symmetry this material prefers. The fully DFT-optimized P21 model with four molecules per unit cell is shown in Figure 3. The residual differences in intensity between the fully DFToptimized model and the experimental pattern are significant at 15 GPa, which can be due to a number of factors. First, the effect of nonhydrostaticity becomes increasingly pronounced at higher

Figure 3. Structure of the high pressure P21 phase based on the DFToptimized model at 15.0 GPa. N, B, and H atoms are denoted to be purple, green, and pink, respectively. The labels N1B1 and N2B2 represent the two types of AB molecules present in one unit cell.

pressures which are reflected in the broadening of the ruby R1 and R2 lines, the smooth but uneven intensity in the Debye rings of the two-dimensional diffraction patterns, and the intensity change of the (101) reflection in the pressure range 12.9
15.0 GPa. This reflection, indicated by the arrow in Figure 1, is lower in intensity than the stronger peak on its right-hand side in the DFT-optimized model at 15 GPa and introduces significant residual differences between the theory model and the experimental pattern. However, the residual differences can be greatly reduced by applying spherical harmonic preferential orientation in the Rietveld refinement using GSAS.43 Second, the 2174

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Table 1. The Structural Parameters of the Experimentally Suggested P21 Model, Fully DFT-Optimized [in Brackets], and Rietveld-Refined (in Parentheses) P21 Model in AB at 15.0 GPa and Room Temperature structure P21, Z = 4

N1

z

0

0

0

[
0.0460]

[0]

[0.0369]

b = 5.375(4)

(0.0188)

(0.0094)

(0.0603)

0.1429

0.1254

0.1178

[0.1236]

[0.0298]

[0.3060]

B1

β = 97.22(8)

stress-induced peak broadening in the experimental pattern especially in the higher 2θ region brings difficulty in precisely refining the intensity against the experimental pattern. The third possibility of causing the differences is the temperature effect involved in the experiments and calculations, which are 300 and 0 K, respectively. We can obtain the best Rietveld refinement with an R factor of 0.24 by including preferred orientation and modifying the peak profile (Figure 4). Figure 4 along with Figure 1 also suggest that a small fraction of the Cmc21 phase might coexist with the P21 phase at pressures above 12.9 GPa, which explains the residual peaks such as those at 2θ = 7.28 and 10.00. The experimental, fully DFT-optimized, and the Rietveld-refined structure information for the P21 model at 15.0 GPa are all listed in Table 1. 2. Pressure-Induced Structural Evolution and Development of Intermolecular Interactions. In the high pressure P21 phase, four AB molecules occupy one unit cell. The geometric characteristics of an individual AB molecule in P21 are comparable to those in I4mm and Cmc21. At 15.0 GPa, the N
H and B
H distances of AB molecules in the DFT-optimized structure are in the range 1.016
1.028 and 1.176
1.196 Å, respectively, which are slightly shorter than those in the lower pressure phases. Both the B
N
H and N
B
H angles are close to the tetrahedral coordination, in the range 109.5
114.9 and 106.5
109.1, respectively. The relatively robust geometry of the AB molecule indicates that the pressure-induced phase transition is attributed to the evolution of intermolecular interactions in the AB solid, which is represented by the unconventional dihydrogen bonding throughout the network. Constrained by the symmetry of space group P21, four AB molecules can be separated into two groups, which we denote as the N1B1 and N2B2 molecule. Due to the different environments associated with these two types of molecules, their dihydrogen bonding frameworks are quite different, and both are distinct from those in the low-temperature Pmn21 and highpressure Cmc21 phases. By setting the criterion for DHBs to be where the H 3 3 3 H distance is less than 2.2 Å, a previous theory study reported that the number of DHBs per molecule in the simulated Cmc21 phase increases from 12 to 14 at 6.4 GPa, and becomes 18 at 10.2 GPa.23 We found that in our DFT-optimized P21 phase the number of DHBs for both types of AB molecules is the same as the Cmc21 at 10.2 GPa (18 H 3 3 3 H contacts per molecule). Furthermore, we consider a DHB to be more rational only when the angles of N
H 3 3 3 H and H 3 3 3 H
B fall within

y

a = 7.713(6) c = 3.898(4) Å

Figure 4. Rietveld refinement profile for the P21 phase at 15.0 GPa compared to the experimental data (in pluses).

x

atom

(0.1638)

(0.1308)

(0.3348)

Hn11


0.0708 [
0.1403]

0.0549 [
0.1067]

0.1472 [0.1306]

(
0.0546)

(
0.1391)

(0.1294)

Hn12

0.0116


0.1874

0.0374

[
0.0255]

[
0.0753]

[
0.1957]

Hn13

Hb11

Hb12

Hb13

N2

B2

Hn21

Hn22

Hn23

Hb21

Hb22

Hb23

(0.0897)

(
0.0876)

(
0.0992)


0.0509

0.0334


0.2392

[
0.1044]

[0.1640]

[
0.0374]

(
0.0842) 0.0826

(0.1256) 0.2947

(0.0375) 0.2031

[0.0894]

[0.1764]

[0.5138]

(0.0960)

(0.2980)

(0.4650)

0.2273

0.1627


0.1102

[0.2411]

[0.0985]

[0.1582]

(0.2827)

(0.1790)

(0.1837)

0.2261

0.0218

0.3456

[0.1525] (0.1834)

[
0.1674] (0.0076)

[0.4388] (0.5934)

0.2724

0.6999

0.6509

[0.3914]

[0.7085]

[0.8800]

(0.3442)

(0.7087)

(0.8328)

0.3474

0.4594

0.6318

[0.3783]

[0.4804]

[0.6251]

(0.3727)

(0.4447)

(0.6685)

0.2660 [0.3647]

0.7068 [0.6665]

0.8816 [1.1249]

(0.2147)

(0.7670)

(0.8216)

0.3539

0.8397

0.5971

[0.5167]

[0.7723]

[0.9158]

(0.3468)

(0.7231)

(1.0945)

0.1498

0.7247

0.5228

[0.3147]

[0.8551]

[0.7891]

(0.4325) 0.4177

(0.8277) 0.4449

(0.7418) 0.8892

[0.5183]

[0.4542]

[0.5384]

(0.4981)

(0.4623)

(0.5215)

0.4446

0.4536

0.4185

[0.2686]

[0.5216]

[0.3942]

(0.2364)

(0.4209)

(0.4928)

0.2378

0.3028

0.5827

[0.3320] (0.3763)

[0.2990] (0.2593)

[0.7665] (0.8480)

the ranges 117
171 and 90
171, respectively.23 The number of DHBs was then reduced to 14 for the N1B1 molecule and 10 for the N2B2 molecule, with every H atom involved in 2175

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Table 2. The Distances of the H 3 3 3 H Contacts and the Angles of N
H 3 3 3 H and H 3 3 3 H
B in Each DHB Associated with the N1B1 Molecule in the DFT-Optimized P21 Model at 15.0 GPa DHB

distance (Å)

Hn11 3 3 3 Hb23

1.66

Hn11 3 3 3 Hb12

2.04

Hn11 3 3 3 Hb11

1.82

Hn12 3 3 3 Hb11

1.85

Hn12 3 3 3 Hb11

2.04

Hn13 3 3 3 Hb13

1.81

Hb12 3 3 3 Hn22

2.14

Hb12 3 3 3 Hn23

2.08

Hb13 3 3 3 Hn23

1.74

angle (deg) N1
Hn11
Hb23

161.3

Hn11
Hb23
B2

100.9

N1
Hn11
Hb12

118.3

Hn11
Hb12
B1

104.3

N1
Hn11
Hb11

122.3

Hn11
Hb11
B1

154.8

N1
Hn12
Hb11 Hn12
Hb11
B1

145.4 94.9

N1
Hn12
Hb11

110.9

Hn12
Hb11
B1

95.1

N1
Hn13
Hb13

135.9

Hn13
Hb13
B1

93.6

N2
Hn22
Hb12

131.6

Hn22
Hb12
B1

158.4

N2
Hn23
Hb12 Hn23
Hb12
B1

116.3 115.5

N2
Hn23
Hb13

123.2

Hn23
Hb13
B1

110.9

Table 3. The Distances of the H 3 3 3 H Contacts and the Angles of N
H 3 3 3 H and H 3 3 3 H
B in Each DHB Associated with the N2B2 Molecule in the DFT-Optimized P21 Model at 15.0 GPa DHB

distance (Å)

Hn21 3 3 3 Hb22

1.57

N2
Hn21
Hb22

154.1

Hn21
Hb22
B2

104.6

2.15

N2
Hn21
Hb21

106.9

Hn22 3 3 3 Hb23

1.60

Hn21
Hb21
B2 N2
Hn22
Hb23

103.9 136.4

Hn22
Hb23
B2

101.5

Hn22 3 3 3 Hb12

2.14

N2
Hn22
Hb12

131.6

Hn22
Hb12
B1

158.4

N2
Hn23
Hb13

123.2

Hn23
Hb13
B1

110.9

2.08

N2
Hn23
Hb12

116.3

1.66

Hn23
Hb12
B1 N1
Hn11
Hb23

115.5 161.3

Hn11
Hb23
B2

100.9

Hn21 3 3 3 Hb21

Figure 5. The dihydrogen bonding frameworks (represented by dashed lines) associated with the N1B1 (a) and the N2B2 (b) molecule in the DFT-optimized model at 15.0 GPa. The molecules marked with N1B1 and N2B2 are the ones of interest. The orientation of all the molecules is the same as that in Figure 3 which illustrates the unit cell information.

Hn23 3 3 3 Hb13 Hn23 3 3 3 Hb12

dihydrogen bonding networks. Figure 5 shows the dihydrogen bonding contacts for the two types of molecules while considering both the distance and angle. The geometric characteristics of each DHB associated with the N1B1 molecule and the N2B2 molecule are listed in Tables 2 and 3, respectively. The N
H 3 3 3 H groups tend to be more linear, while the H 3 3 3 H
B groups tend to more bent, consistent with previous studies for N
H 3 3 3 H
B containing structures and suggesting that the NH proton not only interacts with H in the B
H bond but the B
H bond itself.44
46 The wide spread in length and directionality associated with dihydrogen bonding frameworks may explain the increased spectral complexity with pressure in our earlier Raman study.17 Meanwhile, the observed splitting of both the B
N stretching mode and an overtone of the low frequency NH3 rocking mode at 12 GPa could be understood as being due to the two distinct AB molecules in the P21 phase.

angle (deg)

Hb23 3 3 3 Hn11

1.74

In addition, we found that the N1B1 molecule has an almost staggered conformation, while the N2B2 molecule has an eclipsed conformation in the DFT-optimized P21 phase. In contrast, AB molecules in the Pmn21 and Cmc21 phases have a perfect staggered conformation. The relative rotation between the NH3 group and BH3 group could be partially responsible for the phase transition which we previously suggested.17 The apparent spectral changes in the N
H and B
H bending modes at 12 GPa imply the complexity of the rotational dynamics in this molecular solid at higher pressure. However, whether the rotational energy barrier is lower in the NH3 group or the BH3 group 2176

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than our calculated value and consistent with the reported result of 10.3 GPa.19 V0 was found to reduce to 65.93(34) Å3 from 69.32(3) Å3. The fitting curve is shown as the black solid line in Figure 6a. However, the residual differences between the experimental molecular volume and the calculated molecular volume (Vexp
Vcal) show the most negative residual at 5 GPa with the two ends continuously rising (black squares and the black solid line in Figure 6b). On the other hand, when fitting the data below and above 5 GPa separately, the residual differences have a random distribution with good fitting statistics (red and green symbols in Figure 6b). With K0 being 6.4, the obtained bulk moduli for the phases below and above 5 GPa are 8.0 ( 0.3 and 12.8 ( 1.1 GPa, respectively, and V0 changes to 67.45(33) Å3. Although the diffraction patterns in this pressure range have no characteristic change, the different compressibility data associated with the phases below and above 5 GPa, much improved fitting statistics by separately considering the two regions, and apparent changes in our previous Raman study all suggest that a second-order isostructural phase transition occurs at 5 GPa.21

Figure 6. (a) The 3OBM EOS fitting results for the indicated pressure range of the Cmc21 phase (open circles). The black curve represents the result by using all the data points in the pressure range of 1.6
11.8 GPa. The obtained K0 is 9.7 ( 0.4 GPa with K0 fixed at 6.4. The red and green curves represent the results by separately fitting the data points below and above 5 GPa. The obtained K0 are 8.0 ( 0.3 and 12.8 ( 1.1 GPa, respectively, with K0 fixed at 6.4. (b) The residual differences of the experimental molecular volume and the calculated molecular volume derived from the fitting results (Vexp
Vcal). The black square, red star, or green star symbols correspond to the residual differences between the experimental values and the calculated values from the black, red, or green curve in part a, respectively. The trend revealed by the solid black line indicates a systematic mismatch which is asymmetrically centered at 5 GPa.

requires further investigation, with a few recent studies suggesting very different rotation energies for these two groups.19,23 3. A Second-Order Isostructural Phase Transition at 5 GPa. In the pressure range 1.6
11.8 GPa, all 11 diffraction patterns can be positively indexed to the orthorhombic Cmc21 phase. Fitting to a third-order Birth
Murnaghan (3OBM) equation of state (EOS)47 was attempted to obtain the bulk modulus K0 and its pressure derivative K0 of this phase. However, the fitting results did not converge. We found that the fitting statistics kept improving by progressively increasing K0 until the K0 and as-fitted V0 and K0 became physically unreasonable; for example, with K0 to be 10, the obtained V0 was found to be the same as the V0 at ambient pressure with a very small K0 of 4.8 GPa. Since our DFT calculations predicted that the Cmc21 phase has a bulk modulus of 8.1 GPa and its pressure derivative of 6.4, we fixed K0 to be 6.4 to obtain the experimental K0 using the 3OBM EOS, yielding the value of K0 to be 9.7(4) GPa which is slightly larger

’ CONCLUSIONS The effect of pressure on the structural evolution of pure AB was investigated up to 15 GPa at room temperature. We confirmed the phase transformation from the parent I4mm to the Cmc21 structure below 2 GPa, and observed another high pressure phase above 12 GPa. By combining high pressure powder XRD experiments and DFT calculations, we solved the high pressure structure above 12 GPa and found it to be a monoclinic P21 phase. Together with our previous Raman studies, we clarified the origin of the phase transition at 12 GPa which is triggered by the reorganization of the dihydrogen bonding frameworks and the change in the rotational order in the NH3 and BH3 groups. This study demonstrates the power of coupling experiments with theoretical calculations in understanding the structure of H-rich materials. ’ ASSOCIATED CONTENT

bS

Supporting Information. Four figures, showing a raw 2D diffraction pattern, and simulated patterns associated with P21, P21_PO, and DFT-optimized models at 15.0 GPa as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses ^

Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA.

’ ACKNOWLEDGMENT This work was supported by the Department of Energy (DOE) through the Stanford Institute for Materials & Energy Science DEAC02-76SF00515. The experiments were performed at HPCAT (Sector 16), APS, ANL. HPCAT is supported by CIW, CDAC, UNLV, and LLNL through funding from DOE-NNSA, DOE-BES, and NSF. APS is supported by DOE-BES, under Contract No. DEAC02-06CH11357. All DFT simulations were performed on the 2177

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The Journal of Physical Chemistry C WFU DEAC cluster. We thank L. L. Daemen for synthesizing the AB sample and S. Hirai for assistance with GSAS refinement. Y.L. acknowledges support from the Stanford Graduate Fellowship and Florida International University for travel under Award Number DE-FG02-07ER46461.

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