Article pubs.acs.org/JPCA
Raman Spectra of Ammonia Borane: Low Frequency Lattice Modes C. Ziparo, D. Colognesi, A. Giannasi, and M. Zoppi* Consiglio Nazionale delle Ricerche - Istituto dei Sistemi Complessi, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy S Supporting Information *
ABSTRACT: We have measured the Raman spectrum of ammonia borane at low temperature (T = 15 K) and across the orthorhombic-to-tetragonal phase transition at T = 225 K. A comprehensive study of the low frequency lattice modes using Raman spectroscopy has been carried out. Data analysis has been complemented by a density functional theory calculation of which the results have been used for a detailed assignment of the Raman active modes. The analysis of the spectroscopic measurements taken across the phase transition seems to be consistent with the increasing orientational disorder of the molecular components and seems to be compatible with the equalization of the a and b lattice constants characteristic of the tetragonal phase.
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INTRODUCTION Ammonia borane (H3N−BH3) is a molecular solid appearing as a white powder stable at room temperature and ambient pressure. This material, which was intended to be used as a solid fuel for jet propellers, was first synthesized in 1955 at the University of Michigan.1−3 More recently, ammonia borane has gained renewed interest as a convenient solid-state material for hydrogen storage.4 As a matter of fact, as shown by its chemical formula, each molecule contains six protons, and the two coordination atoms, namely, nitrogen and boron, are characterized by a low atomic weight resulting in a material which can theoretically store up to 19.6 wt % of hydrogen.5 In recent years, ammonia borane has been the object of wide research activity aiming to gain all the possible insights into its thermal behavior upon heating4,6−11 as well as into its microscopic structural2,3,12−17 and dynamical18−27 properties. The molecular structure of H3N−BH3 is isoelectronic to that of ethane. However, as this substance decomposes upon heating, its single-molecule features were practically unknown until recently when an infrared experimental investigation was carried out in the vapor phase.28 At ambient conditions, the material is solid and relatively stable (vapor pressure lower than 1 μm Hg at room temperature).29 The melting temperature of ammonia borane has been determined to be T = 107 °C.30 However, the decomposition, and the consequent release of hydrogen, starts rather earlier (T ≃ 70 °C),4 and becomes faster at higher temperature (T ≃ 120 °C) when the polymerization of (−NH2−BH2)n begins to be effective.6 The high-temperature thermal behavior of ammonia borane and its decomposition processes have been experimentally studied using thermo-analytical methods.9 The lattice group of H3N−BH3 is tetragonal at room temperature with the N−B bond oriented parallel to the crystallographic c-axis,12 and the compound undergoes a © 2012 American Chemical Society
structural transition to an orthorhombic phase at about 225 K12,31 with the N−B bonds oppositely tilted with respect to the c-axis.13,14 A rotationally order−disorder character has been hypothesized for this transition12,18 with a possible displacive component because of a distortion in the NH3 unit.31 It has been suggested that this transition is triggered by the slowing down of the NH3 motion,31 but, at present, the driving mechanism of the tetragonal-to-orthorhombic transformation is still unknown, and little is also known on its kinetics as well as on the observed hysteretic features. A careful analysis of the crystal modes connected to the microscopic molecular dynamics can surely shed some light on (and suggest the driving mechanism of) this phase transition. In particular, we expect that the low frequency lattice modes could be effective for this investigation in case of the presence of the so-called soft modes. To our knowledge, not too much has been published on this topic apart from some Raman and infrared (IR) spectra taken at room temperature as a function of pressure.19,23,27 More recently, a high-pressure study of this material was carried out in the pressure range 0−15 GPa and in the temperature interval 80−350 K.32 In this paper, we report on a Raman spectroscopy experiment aiming at a careful investigation of the vibrational features of ammonia borane as a function of temperature, across the structural phase transition, focusing on the low frequency lattice modes. For a better interpretation of the results, the experiment has been complemented (when appropriate) with density functional theory (DFT) calculations, which are known to give, in general, correct results at low temperature. Received: April 24, 2012 Revised: July 9, 2012 Published: July 13, 2012 8827
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(2) a medium-low spectral region (500−900 cm−1) containing the B−N stretching modes; (3) an intermediate region (900−1300 cm−1) which contains the BH3 deformations; (4) a medium-high spectral region (1300−1700 cm−1) where the NH3 deformations are observed; (5) a high-frequency region (2200−2500 cm−1) containing the B−H stretching modes; (6) a very high frequency region (3100−3400 cm−1) containing the N−H stretching modes. The presently measured Raman spectrum agrees with the previous experimental determinations, which are summarized in the following: (a) Room-temperature spectra, extending from about 100 to 4000 cm−1, were measured at ambient pressure and high pressure (p = 35.9 Kbar) with a resolving power of 2.6 cm−1.19 (b) Other room-temperature spectra were measured (estimated resolving power ≃1 cm−1) as a function of pressure, still at room temperature, from p = 1 bar to 40 Kbar and covering regions 1, 2, 5, and 6.20 (c) The temperature dependence of the Raman spectra in the interval 600−3700 cm−1 (spectral regions 2−6) was measured between T = 88 K and T = 330 K aiming to evidence the phase transition at 225 K between the orthorhombic and the tetragonal phase of ammonia borane.22 (d) More room-temperature spectra (spectral regions 1−6) were measured as a function of pressure between 0.7 and 22.3 GPa23 and between ambient pressure and 13.5 GPa.27 (e) Pressure-dependent spectra, covering the whole spectral region (1−6), were measured as a function of pressure (0−15 GPa) in the temperature range between 80 and 350 K with the aim of investigating the structural evolution of the material.32 In the following, we will concentrate our interest in the low frequency spectral region (i.e., the interval 100−450 cm−1), which contains the lattice modes only. For a detailed discussion of the higher frequency vibrational modes, we address the reader to ref 22. With reference to Figure 2, we observe a decreasing feature at the beginning of the measured frequency interval, which is likely attributable to a weak low frequency
LOW-TEMPERATURE MEASUREMENTS Ammonia borane powder (99% assay) obtained from Aviabor was purified by vacuum sublimation. A small quantity of this compound was carefully transferred into an optical scattering cell using a controlled atmosphere environment (i.e., a glovebox filled with pure nitrogen) with measured contents of water and oxygen both lower than 0.1 ppm. The sealed cell was then moved to the light-scattering apparatus33 for further conditioning and for in situ spectroscopic analysis. Here, the sample cell was evacuated, using a system composed of a turbomolecular pump connected to a clean baking pump, and then was filled with helium at 2 bar pressure. The presence of helium increased the thermal exchange between the sample and the environment and so avoiding, or at least greatly reducing, the local heating possibly induced by the laser beam. At any rate, the laser power was kept rather low, that is, between 3 and 5 mW, on the sample. Spectroscopic Raman measurements were carried out using the green line (λ = 514.5 nm) of an Ar ion laser. Spectra were collected in several frequency intervals starting from 100 cm−1, to avoid interference with the strong stray light component, up to about 4000 cm−1 using a Spex Triplemate spectrometer equipped with high resolution holographic gratings and a cooled charge coupled device (CCD) camera (namely, a Symphony Horiba Jobin Yvon). The overall spectral resolution was about 1.3 cm−1. Initially, the sample was cooled to T = 15 K under helium atmosphere, and a full set of spectral data was collected. At this low temperature, Raman peaks are much sharper than at higher temperature and so a better peak identification can be obtained. The greater spectral definition available at this low temperature is especially useful for the weak peaks. After this measurement, the temperature was raised to 200 K, and then several spectra were collected at different temperatures, namely, at T = 210 and T = 220 K, and across the orthorhombic-to-tetragonal structural phase transition at intervals of 2.5 K between 220 and 230 K. The complete low temperature (i.e., orthorhombic phase) Raman spectrum is shown in Figure 1. Here, six different spectral regions are identified (we have added a low frequency lattice mode region to the five already defined in ref 22): (1) a low-frequency region (100−500 cm−1) which contains the lattice modes;
Figure 2. Low-frequency Raman spectrum (i.e., lattice modes) at T = 15 K.
Figure 1. Measured Raman spectrum of H3N−BH3 at T = 15 K. 8828
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mode that, masked by the spectrometer filter, cannot be fully accounted for. A second broad feature is visible, peaked at 116.5 cm−1, which however seems too broad to represent a single Raman mode. The next sharp peak appears at 149.8 cm−1 followed by a second sharp feature at 159.4 cm−1. Then, a broad asymmetric structure, peaked at 185 cm−1, is followed by the highest intensity sharp peak, placed at 211.8 cm−1, and by a weak satellite line at 221 cm−1. From here, the spectrum shows a slow decrease, which is very likely a signature of a broad structure underneath and, finally, an extremely broad band, centered around 337 cm−1, which slowly decreases up to 400− 450 cm−1, where the spectral interval attributable to the lattice modes ends. Concerning the frequency interval considered here, the presently measured lattice-mode spectrum appears quite richer than the previous measurements. Actually, with respect to the available literature results, at ambient pressure (and room temperature), we observe that only one composite peak was measured at around 100 cm−1 in ref 19. Similarly, a broad band extending from 160 to 240 cm−1 was measured in ref 20, while a weak peak around 170 cm−1 was measured in ref 23, and a lattice mode at 141 cm−1 was measured in ref 27. In ref 32, instead, a low-temperature measurement (at T = 180 K) evidenced two lattice modes at 94 and 212 cm−1.
report the most likely assignment of the observed Raman peaks on the basis of the frequency shift and observed intensities (cf. Figure 2). Table 1. DFT Results for Ammonia Borane Lattice Modesa mode N.
char.
frequency (cm−1)
intensity (I /Å3/ cm−1)
L-1 L-2 L-3 L-4 L-5 L-6 L-7 L-8 L-9 L-10 L-11
A2 A2 A1 B1 B2 A1 B1 A2 B2 B2 A2
77.2 92.9 103.4 143.8 154.2 167.0 173.5 216.4 237.8 320.9 343.3
0.2312 0.1550 0.0312 0.0004 0.1877 0.0849 0.0118 0.2705 0.0572 0.0577 0.0798
obsd frequency (cm−1) ≃100 116 150 159 185 212 221 ≃337 ≃337
Shifts are expressed in cm−1, while intensities are in arbitrary units per cubic Å per cm−1. In the last column, we report the most probable assignment of the observed peaks.
a
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DENSITY FUNCTIONAL THEORY CALCULATIONS To help with the assignment of the observed spectral features to their respective lattice modes, we have carried out a DFT calculation. To this aim, we have used the CASTEP code, which employs a plane-wave basis set for the valence electrons, with the atomic cores being incorporated through normconserving pseudopotentials. One of the advantages of this code resides in the ability of calculating an estimate of the Raman transitionsʼ intensities. The structural information on the low temperature orthorhombic phase of ammonia borane was obtained from ref 13. The code has been well described elsewhere,34 and so this part is not repeated here. One of the main features of the CASTEP code is that the internal coordinates can be automatically relaxed so that the structure with the minimum total energy is obtained either by keeping the cell size fixed (constant volume minimization) or by imposing a selected value for the external pressure (constant pressure minimization). A gradient-corrected form of the exchange-correlation functional [i.e., the approximate functional by Perdew, Burke, and Ernzerhof (GGA - PBE)35] was employed. Calculations were made using a plane-wave cutoff Ecut = 770.0 eV and a selfconsistent field accuracy ε = 5.0 × 10−7 eV/atom. This cutoff yields well-converged properties of the fully relaxed structure with a Hellman-Feynman residual force lower than f = 0.01 eV/ Å and an energy precision of e = 5.0 × 10−6 eV/atom. The Brillouin zone sampling was performed using special k-points automatically generated by the code with a grid separation of Δk = 0.07 Å−1. Once the final structures (still complying with the respective symmetry constraints) were obtained, the phonons at the Γ point were evaluated including the important Longitudinal Optical−Tranverse Optical (LO−TO) splitting correction. After making sure that no imaginary frequencies were obtained (with the possible exception of the three immaterial acoustic modes) and still making use of an internal CASTEP routine, the simulated infrared and Raman intensities were calculated for all the optically active modes. The results, limited to the lattice modes only, are summarized in Table 1, where we also
Unfortunately, the calculation gives no information about the various line widths, and therefore, the large differences observed in the experimental spectrum cannot be easily explained. Nonetheless, the observed broad band at around 337 cm−1, which we attribute to the combined action of the L10 and L-11 modes, shows an experimental width that is not much dissimilar from that measured by inelastic neutron scattering on the same substance in similar thermodynamic conditions.36 A detailed description of the lattice modes, complemented by some clarifying pictures, is reported in the Supporting Information.37
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RAMAN MEASUREMENT ACROSS THE PHASE TRANSITION In the second part of the experiment, the sample was warmed up to a temperature of T = 200 K, and new Raman spectra were collected. The temperature evolution of the spectral shape in the region of interest for the present work is reported in Figure 3. Because of the higher temperature range, the lines appear much broader and many details are lost with respect to Figure 2. Nonetheless, two clear broad lines centered around 133 and 200 cm−1 are still visible at the lowest temperature (T = 200 K). Looking at the intensities and at the observed down-shift as temperature increases, we assign the less intense peak to the L8 mode, while the other band is assigned to the merging of L-5 and L-6 modes (cf. Table 1). In correspondence with the structural phase transition (at T = 225 K), the still observable lattice peaks (characteristic of the orthorhombic phase) tend to merge into a broader band centered at lower frequency (i.e., mode softening). This is even more evident in Figure 4, where we have concentrated our attention on the low-frequency features related to the behavior of the lattice modes. Here, the three vertical dotted lines, which simply represent a guide for the eyes, show how the lowest frequency modes (around 133 cm−1) undergo a softening process larger than the intermediate peak (at around 200 cm−1), while the broad band centered at around 343 cm−1 remains almost fixed as the temperature increases from 200 to 222.5 K. 8829
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K), these are assigned, in agreement with ref 22, to the symmetric stretching C2v A1 symmetry mode for the two boron isotopes for which the DFT calculation predicts an isotopic shift of 17.4 cm−1. Incidentally, the same calculation predicts a much less intense (1 order of magnitude) B1 asymmetric stretching mode characterized by an isotopic separation of ≃2 cm−1 only. The temperature evolution of these peaks is nevertheless interesting as shown in Figure 5. Here, vertical lines represent
Figure 3. Low-frequency spectrum (lattice modes) measured as a function of temperature across the structural phase transition occurring at T = 225 K. Higher temperature spectra (not shown) do not substantially differ from that measured at T = 227.5 K.
Figure 5. Enlarged view of the high-frequency range of spectra shown in Figure 3. The vertical lines are guides for the eyes drawn through the peaks and suggest a different behavior for the two groups of modes.
guides for the eyes drawn through the principal peaks. As far as the pair of peaks at higher frequency is concerned, the two lines run more or less parallel confirming that the distance between the peaks is simply due to the isotopic shift and has nothing to do with the phase transition. Conversely, there is a clear convergence between the pair of lines on the left, with an even clearer merging of all peaks in a single broad one, when temperature grows beyond the phase-transition value at T = 225 K. Because these lines are representative of the rocking modes, they are clearly affected by the change of the lattice structure. Incidentally, according to ref 31, the orthorhombic structure is stabilized by the presence of H···H bonds.
Figure 4. Expanded view of the lowest frequency spectrum as a function of temperature approaching the structural phase transition occurring at T = 225 K. Dotted lines are guides for the eyes and suggest that the lower frequency Raman bands undergo a more consistent down-shift.
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As it is evident from Figure 3, the two experimental spectra collected at 225 and 227.5 K (i.e., in correspondence and immediately above the structural phase transition) show a broad band, the center of which is placed around 174 cm−1 and still decreases with increasing temperature. Another interesting feature emerging from the spectra reported in Figure 3 concerns the peak evolution in the spectral region 700−850 cm−1, which is the region of the B−N symmetric stretching and asymmetric rocking modes. 22 According to ref 22, the lower frequency peaks are originating from the B−N rocking modes, and three of them are clearly visible in the low-temperature spectra. Incidentally, the DFT calculation predicts (assuming for boron an average atomic mass determined by the natural isotopic composition) four peaks in this region, namely, at 712.6, 724.1, 727.2, and 739.3 cm−1. Here, the calculated isotopic shift is about 2 cm−1, which is comparable to the instrument resolving power (1.3 cm−1), so that further attempts to better identify these modes appear to be impossible. As far as the two other peaks observed in this region are concerned, namely, at 790 and 806 cm−1 (at T = 200
DISCUSSION The two optical phonon modes at 320.9 and 343.3 cm−1 describe, according to the DFT results, molecular torsional modes where the three nitrogens’ and the three borons’ hydrogen atoms oscillate in counterphase around the B−N bond which, in turn, remains almost fixed in the space. Thus, an increase in temperature and the consequent increase in the amplitude of the atomic motions are not expected to excessively affect the structure of the lattice system. A similar argument applies to the L-9 mode, where the molecules undergo a similar, even though rigid, torsional motion The L-8 mode, instead, seems to be different. Here, the torsional motion of the NH3 group is only partially balanced by the corresponding motion of the BH3 group with a consequent oscillation of the whole molecule in a direction almost perpendicular to the B−N bond (and to the c-axis). A similar situation, though for different reasons, applies to the L-7 and L6 modes where the most relevant oscillation turns out to be 8830
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placed along a direction perpendicular to both the c-axis and the previous oscillation (L-8) direction. As a matter of fact, in the first case, the molecule oscillates almost rigidly around the center-of-mass position resulting in a rigid tilting of the whole molecule perpendicular to the c-axis. In the second case, the whole molecule undergoes a rigid center-of-mass oscillation in the same direction. Similarly, the L-5 mode produces a resulting oscillating motion of the rigid molecule in a plane perpendicular to the c-axis with an effect similar to that of L-8. The L-4 mode substantially produces a molecular oscillation along the c-axis . Therefore, it should be less sensitive to the structural phase change and to the temperature. Unfortunately, this mode is extremely weak and almost invisible even at very low temperature. The lowest frequency modesL-3, L-2, and L-1turn out to give rise to an effective molecular motion in a plane perpendicular to the c-axis with the prevalent direction of mode L-3 perpendicular to that of the other two. Thus, all these three modes are expected to be favored by the transition to the tetragonal configuration, where the difference between the two crystal axes a and b vanishes. Concerning the high-temperature spectra, that is, above T = 225 K, the spectral features appear much less detailed. On the one side, the high temperature (and the consequent disorder due to the expected librational or hindered rotation of the hydrogen groups) makes it impossible to distinguish between the various lattice modes which appear as a unique broad band placed around 170 cm−1. On the other side, also the higher frequency peaks (i.e., the B−N rocking modes) appear much less structured as it is evident from Figure 5 where the triplet around 720−750 cm−1 evolves into a single broader structure peaked at 726 cm−1, describing the complex motion of the hydrogens along the direction of the B−N bond, under the influence of a probable orientational disorder of the −NH3 and −BH3 groups. Here, it is interesting to note the different temperature behavior of the leftmost mode, which appears more stable with respect to the other two rocking modes. Because of the expected orientational disorder in the tetragonal phase,31 our attempts to simulate the hightemperature spectra using the DFT lattice dynamics calculations turned out to be unsuccessful. This is not surprising at all as it is well-known that standard lattice dynamics is not applicable to highly anharmonic solids, like the so-called plastic crystals,38 which high-temperature ammonia borane surely belongs to. Nevertheless, the aforementioned qualitative considerations about the increasing orientational disorder of the two −NH3 and −BH3 groups do agree with the results discussed by Parvanov et al.15 on the basis of neutron diffraction experiment and DFT calculations.
crossing the structural phase transition temperature at 225 K. At these high temperatures, spectral features appear much broader. However, some interesting considerations are still possible. In particular, by increasing the temperature and crossing the phase transition, we have observed a clear downshifting of certain lattice modes, which testifies to the softening of the corresponding intermolecular bonds. Similar effects are also observed among the lower frequency intramolecular modes, for example, those involving the rocking motion of the −NH3 and −BH3 groups, while the highest frequency B−N stretching modes, which involve the strongest bonds, appear spectrally more stable (cf. Raman Measurement section). Thus, the present Raman experimental data seem to confirm the previous conclusions that the transition to the tetragonal phase is induced by the orientational disorder of the molecular groups H3N−BH3 which, in turn, favors the alignment of the intramolecular B−N symmetry axis with the lattice c-axis. Unfortunately, Raman data do not allow to determine whether the librations of the −NH3 and −BH3 groups are actually correlated.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed description of the low frequency Lattice Modes. Isotopic Effects. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge fruitful discussions with Thomas Autrey and Mark Bowden (Pacific Northwest National Laboratory), Rosario Cantelli (University of Roma, La Sapienza), and Annalisa Paolone and Oriele Palumbo (CNRISC, Roma). This work has been partially supported by Ente Cassa di Risparmio di Firenze under the Firenze-Hydrolab project.
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REFERENCES
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CONCLUSIONS In this work, we have measured the Raman spectral features of ammonia borane in a wide range of temperatures. The high frequency results, which are related to the intramolecular modes of H3N−BH3, turned out to be very similar to the previous experimental determinations and have not been discussed here. Instead, we have concentrated our interest on the low frequency lattice modes that are located in the spectral region between 100 and 450 cm−1. We have used the lowtemperature determination (T = 15 K) as well as the results of a DFT simulation to assign the observed modes in the orthorhombic phase. Subsequently, we have measured several spectra in the temperature range between 200 and 230 K 8831
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