Temperature Behavior of the AlH3 Polymorph by in Situ Investigation

ARTICLE pubs.acs.org/JPCA

Temperature Behavior of the AlH3 Polymorph by in Situ Investigation Using High Resolution Raman Scattering A. Giannasi,† D. Colognesi,† M. Fichtner,‡ E. R€ohm,‡ L. Ulivi,† C. Ziparo,† and M. Zoppi*,† † ‡

Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Italy Karlsruhe Institute of Technology, Institute of Nanotechnology, Hermann-von-Helmholtz-Platz 1, 76347 Eggenstein-Leopoldshafen, Germany

bS Supporting Information ABSTRACT: A Raman investigation of the AlH3 polymorph has been carried out at a low temperature (20 K) under helium atmosphere (2 bar). The pristine material was composed of three polymorphs, namely, the R, β, and γ phases. The β phase has been removed by warming the sample to 70 °C, while further heating at 100 °C was used to remove the γ phase. This allowed us to evidence, on a purely experimental basis, the characteristic Raman spectrum for each phase. Raman spectra, for the three phases, have been also calculated using density functional theory, and the results have been compared to the present experimental data, allowing for a univocal assignment, to each phase, of its characteristic spectral features.

’ INTRODUCTION Aluminum trihydride (alane) is a rather well-known material that was originally synthesized as a rocket propellant. More recently, a growing interest for this compound has been induced by its possible use as a material for hydrogen storage, thanks to its relatively large capacity, exceeding 10 wt % .1 Alane (AlH3) is known to be a metastable compound, in normal conditions of temperature and pressure and converts spontaneously (though with a slow kinetics) to Al and H2.2 As temperature increases, kinetics becomes faster, and samples of alane decompose in a short time. The thermodynamic behavior of this compound as a function of temperature, and its thermal decomposition path, have been thoroughly studied using in situ X-ray diffraction (XRD) and thermal desorption analysis.3 Aluminum hydride is among the candidate compounds for infiltrating nanoporous scaffolds to produce a material for reversible hydrogen storage. As such, it is important to carefully investigate its temperature behavior to check whether the thermal desorption (and reabsorption) of hydrogen follows a route similar to that observed in the bulk material. However, due to the nanostructuring, it would be difficult, if not impossible, to use the same experimental probe that was used for the bulk, namely, XRD. Thus, other diagnostic methods allowing us to investigate the sample in situ during the thermal treatment have to be devised. In this context, we are investigating the possibility of using Raman spectroscopy as an alternative tool for in situ diagnostics. r 2011 American Chemical Society

Raman spectroscopy studies of the AlH3 polymorphs (R and γ phases) have been carried out at room temperature4 and as a function of pressure from ambient to 32 GPa.5,6 However, no spectral information has been ever obtained at low temperatures, where a much higher peak discrimination is expected, that could be used to distinguish modes of different nature. Moreover, the low-frequency spectral region, where the lattice modes can be detected, has not been ever accurately measured to the authors' knowledge. Therefore, to acquire reliable information needed for the in situ diagnostics of the nanoscaffolded samples, we have performed a Raman spectroscopy experiment on the bulk sample, containing the R-, β-, and γ-phase of AlH3, at T = 20 K, so that the vibrational spectra pertaining to the different phases could be identified, taking advantage of the improved peak definition obtained at low temperatures. In addition, to identify the vibrational modes pertaining to the different phases of AlH3, we have recorded the Raman spectra of the same material, always at T = 20 K, after each of two successive heat treatments of the sample. The first (at 70 °C) was aimed to decrease, nearly to zero, the concentration of the β phase, while the second (at 100 °C) aimed to remove the γ phase, according to the results previously reported in the literature.2,3 The Received: September 21, 2010 Revised: December 22, 2010 Published: January 14, 2011 691

dx.doi.org/10.1021/jp109027h | J. Phys. Chem. A 2011, 115, 691–699

The Journal of Physical Chemistry A

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Figure 2. Raman spectrum of the pristine aluminum hydride measured at room temperature.

Figure 1. XRD pattern of the pristine material (top) is compared to the simulated ones of the pure R-, β-, and γ- phases (from top to bottom), calculated on the basis of the structural information given in refs 7, 8, and 9, respectively.

The spectroscopic Raman measurements were carried out using the green line (λ = 514.5 nm) of an Ar-Kr mixed gas ion laser. Spectra were acquired in the interval 150-2200 cm-1, using a Spex Triplemate spectrometer, equipped with high resolution holographic gratings, and a cooled charge coupled device (CCD) camera (Symphony Horiba Jobin Yvon). The overall spectral resolution was about 1.3 cm-1. The room temperature spectrum (spanning an energy range between 150 and 2200 cm-1) of the as-produced (pristine) material is reported in Figure 2. Here, 10 quite intense Raman peaks are clearly distinguishable, while the sharp line at 235 cm-1 is surely spurious, produced by a laser plasma emission. Among the most intense peaks, two can be easily attributed to the R phase (i.e., 506 and 715 cm-1), according to previous room temperature experiments4-6 and the available density functional theory (DFT) calculations .14,15 Group-theory analysis predicts four Raman active modes for the R phase. The remaining two Rphase peaks, which according to calculations should be placed around 1000 and 1500 cm-1, are more difficult to be identified in the measured spectrum and are likely to be masked by (or mixed with) the other evident lines observed at 1029, 1055, 1125, 1750, 1829, and 2030 cm-1, which should be related to the β and γ phase components. Other less intense peaks are also detected in the spectrum, and the results are summarized in Table 1, where they are compared to other experimental data, obtained at room temperature and ambient pressure.5,6 The Raman measurements described in the following were carried out on the sample, cooled to T = 20 K and under helium atmosphere. To test the homogeneity of the material, two different spectra were recorded from two distinct spots on the same sample. The two spectra show very similar features but reveal slightly quantitative differences, suggesting that the pristine material could be locally nonhomogeneous (see Supporting Information). Another feature, which is immediately appreciated by comparing the cold spectra to the room temperature ones, is that the observed peaks are much sharper in the former case. The greater spectral definition, available at low temperatures, is especially useful for weak peaks. Aiming to identify the Raman active modes related to the least stable β phase, the sample was heated at 70 °C for three hours, under helium atmosphere. According to the XRD results obtained by Graetz and Reilly,2 this thermal treatment transforms the β phase into the more stable R phase. It is well-known that, during this process, the R and γ phases slowly decompose into Al

comparison of the spectra with each other, with the results of a density functional theory (DFT) calculation, is described in detail in the following sections.

’ DESCRIPTION OF THE EXPERIMENTAL PROCEDURE The aluminum hydride sample was synthesized at the Karlsruhe Institute of Technology and was structurally characterized using X-ray powder diffraction in a Philips X'PERT diffractometer (Cu KR radiation). In Figure 1, we show the measured diffraction pattern (black line) together with the simulated ones pertaining to the three different pure phases whose structural parameters were obtained from the current literature, namely, Turley and Rinn for the R phase,7 Brinks et al. for the β phase,8 and Brinks et al. for the γ phase.9 As it has been already reported by Graetz and Reilly,2 the pristine sample appears to be composed of a mixture of the three phases, namely, R, β, and γ. A Rietveld refinement procedure, using the GSAS-EXPGUI software10,11 applied to the XRD pattern, allowed us to determine the relative phase concentration of the sample, which turned out to be composed of 29.4 wt % by the R phase, 20.4 wt % by the β phase, and 49.8 wt % by the γ phase. A slight amount of aluminum (0.4 wt %) was also detected in the pristine material. No presence of the R0 phase was evidenced by the XRD analysis carried out on the pristine material (to this aim we used the structural information given in ref 12). Taking into account that, to transform the R phase into the R0 , the sample should be heated at high pressure,8,12 we totally disregarded the R0 phase in our analysis. A small quantity of this material was carefully transferred into an optical scattering cell using a controlled atmosphere environment (glovebox filled with pure nitrogen) with measured contents of water and oxygen lower than 0.1 ppm. The sealed cell was then moved to the light scattering apparatus13 for further conditioning and in situ spectroscopic analysis. Here, the sample cell was evacuated, using a system composed of a turbo-molecular pump connected to a clean baking pump, and then filled with helium at 2 bar pressure. The presence of helium increases the thermal exchange between the sample and the environment thus 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. 692

dx.doi.org/10.1021/jp109027h |J. Phys. Chem. A 2011, 115, 691–699


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Table 1. Frequency of the AlH3 Modes Measured at Room Temperature and Compared to the Available Experimental Values, for the R Phase and γ Phase Polymorphs Wong et al. experiment5

present -1

experiment (cm )

R-phase

Tkacz et al. experiment6 R-phase

γ-phase

155 164 205 248

247

-

295

340

-

353

-

422

423

506

513

512

715 795

724

723

843

857

588

Figure 3. Raman spectra of aluminum hydride samples taken at T = 20 K. The bottom line (black) represents the spectrum of the pristine material and contains the three different phases of the polymorph. The intermediate spectrum (blue) pertains to the sample after the first thermal treatment (at 70 °C) and contains only the R and γ phases. The upper spectrum (red) is related to the sample after the second thermal treatment (at 100 °C) containing only the R phase.

545 807 885

-

912

-

952

temperatures (100 °C) to limit the loss of material. Thus, the sample was heated from room temperature, at a constant rate of 10 °C/min, up to T = 100 °C, and this temperature was maintained for about 30 min. According to Maehlen et al.,3 this treatment is sufficient to completely transform the γ-R polymorph into the most stable R phase. In this case too, the sample was monitored in situ taking a sequence of three Raman spectra, collected for 10 min each. Once again, we observed a situation very similar to that previously depicted, with an intensity decrease of the two principal bending modes and the emergence of a broad structure at 800 cm-1, which eventually disappeared when the sample was cooled back to room temperature. Following this stage, the sample was cooled again to 20 K to collect the high resolution Raman spectrum. In Figure 3 we report the spectra, taken at T = 20 K, of the three different samples. The first spectrum (black line, bottom) corresponds to the pristine material and contains the three different phases. In the second (blue line, middle), most of the β phase has been removed, and only the R and γ phases should contribute to the spectral features. Finally, the third spectrum (red line, top) should correspond to the R-phase alone. We observe that only two evident bending modes appear, though with different amplitudes, in all of the spectra and that moving from one spectrum to the next some features disappear.

1029 1055

1045

1043

1056

1501

1300 1478

1125 1483 1715 1756 1830

1821

2030

and H2. However, it is also known that, at this temperature, the decomposition rate is sensibly lower than that at 80 °C and much lower than that at 99 °C.2 Thus, considering the relatively short duration of the thermal treatment, we were probably able to get rid of the β phase, remaining with a substantial amount of R and γ phases. At any rate, to monitor the sample behavior, a series of in situ Raman spectra were recorded during this thermal treatment. The results of these measurements are detailed in the Supporting Information. Following this thermal treatment, the sample was cooled, once more, to T = 20 K, and the Raman spectrum was measured again. A more detailed analysis is presented later on, when all of the low-temperature Raman spectra are compared and discussed. A second thermal treatment was performed, to further increase the R phase concentration. According to the literature,16 this can be obtained rising temperature above 110 °C, where the γ phase converts spontaneously into the more stable R phase. In addition, we also take advantage from the fact that the decomposition rate of the γ phase is faster than that of the R phase.2 This implies a concentration increase in the R phase, although at the expenses of the loss of a certain amount of material. We point out that thermodynamic measurements using differential scanning calorimetry (DSC) proved that a fast γfR phase transition takes place at high temperature (i.e., 110 °C) .17 However, at this temperature, the decomposition rate of AlH3 into Al and H2 is also very high. Therefore, we decided to operate at lower

’ RAMAN SPECTRA ANALYSIS The low-temperature spectra, described in the previous section, have been carefully analyzed aiming to identify the vibrational peaks of the different samples and, eventually, to attribute each mode to the different phases. From an amplified view of the pristine material spectrum, shown in Figure 3 (cf. Supporting Information), it was possible to identify 42 distinct peaks in the spectral interval between 120 and 2200 cm-1. Four of these (in the low-frequency region) have been attributed to plasma lines of the Ar-Kr ion laser. These are located at 156, 170, 204, and 235 cm-1 and have been identified by analyzing the spectrum of the laser light scattered by a polished aluminum plate. Thus, we are left with 38 peaks, of different shapes and amplitudes, that are 693



Figure 4. Raman spectrum of the aluminum hydride R phase taken at T = 20 K. The frequency shift is reported on a logarithmic scale. The intensity scale is linear up to 0.20 and logarithmic beyond this value.

very likely ascribed to Raman lines and have been fitted with Lorentzian line shapes. We begin the analysis with the spectrum produced by the R phase component only (sample after the two thermal treatments), which is plotted again in Figure 4. Here, the Raman shift is reported on a logarithmic scale for a better visualization of the spectrum. In addition, the intensity scale is linear in the lowest part of the figure, to evidence the details of the weak Raman lines, and logarithmic in the upper part of the figure, so that the whole intensity range could be appreciated. We attribute the observed peaks at 512, 726, and 1060 cm-1 to the R phase of AlH3. The observed feature at 1837 cm-1 is located at a frequency too high for the R phase14,15 and is assigned to a small residual fraction of the γ phase in this sample. The broad structure observed at 1490 cm-1 can be assigned to the fourth active mode of the R phase or, possibly, to overtones and/or combination bands. Finally, considering that the Raman spectrum of the R phase does not possess low frequency active lattice modes, it is confirmed that the observed peaks at 156, 170, 204, and 235 cm-1 are due to laser plasma emission. According to the available theoretical calculations,14,15 only four Raman modes, one exhibiting an Ag symmetry and three an Eg symmetry, are active in the R phase (see Table 1). The calculated position of the former is 812 cm-114 or 738 cm-1 .15 The corresponding measured structure is at 726 cm-1 and appears in the form of an intense sharp peak, which is present in each measured spectrum. As for the other modes, the calculated positions are at 490, 993, and 1481 cm-1, according to Wolverton,14 while the simulation by Wang15 predicts slightly higher values at 497, 1034, and 1520 cm-1. Accordingly, we confirm the attribution of the first mode to the observed peak at 512 cm-1 and the other modes to the measured peak at 1060 and to the observed broad peak at 1486 cm-1 (cf. Figure 4). The calculation by Wang and co-workers15 gives also the symmetry and position of the Raman modes belonging to the γ phase. However, due to the not-perfect correspondence between the experimental findings and the theoretical results, a clear assignment of the β- and γ-phase observed modes is not an easy task, when based only on the available literature data.

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’ DFT CALCULATION AND COMPARISON TO THE EXPERIMENTAL RAMAN SPECTRA Given the difficulty experienced in the univocal assignment of the various Raman lines to the different phases, on the basis of the DFT published results (cf. Table 1), we performed original DFT calculations, taking advantage of the fact that some recent codes are also able to evaluate the Raman intensities, in addition to the corresponding frequency shifts. The structural information for the three alane phases is obtained from ref 7 (R-phase), from ref 8 (β-phase), and from ref 9 (γ-phase). In all of the three cases, we have used the CASTEP code, which employs a plane-wave basis set for the valence electrons, the atomic cores being incorporated through norm-conserving pseudopotentials. The code has been well-described elsewhere,20 and 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 keeping the cell size fixed (constant volume minimization) or imposing a selected value for the external pressure (constant pressure minimization). A gradient-corrected form of the exchange-correlation functional [i.e., the generalized gradient approximation of Perdew and Wang (GGA-PW91)21] was employed. The calculations were made using a plane-wave cutoff Ecut = 600.0 eV and a self-consistent field accuracy ε = 3.0 10-7 eV/atom. This cutoff yields well-converged properties of the fully relaxed structure with a Hellman-Feynman residual force lower than f = 0.007 eV/Å and an energy precision of e = 3.0 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 = 0.05 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 LO-TO splitting correction. After making sure that no imaginary frequencies were obtained (with the possible exception of the three immaterial acoustic modes), still making use of an internal CASTEP routine, the simulated infrared and Raman intensities were calculated for all the optically active modes. The results are summarized in Table 2, where the output of the DFT calculations is listed together with the experimental findings. Thanks to the calculated (DFT) band intensities, it is possible to draw a synthetic (theoretical) spectrum and compare it (visually) to the corresponding experimental one. To this aim, we have scaled the three experimental spectra, taken at 20 K, so that the intensity of some R-phase lines are almost the same in the three cases. By this procedure, the extra intensity in the pristine spectrum, with respect to the thermally treated ones, should correspond to all of the bands of the β-phase (that we assume to completely disappear after the first thermal treatment) and to the more intense bands of the γ-phase (that should decrease, even if not completely disappear, after the second thermal treatment). In the following, we will use this comparison to assign the observed modes to the different phases. Assignment of the β-Phase Bands. In Figure 5, we report the three experimental spectra in the region 1350-2200 cm-1 and compare these results with the synthetic DFT spectra. In particular, we concentrate on the green line which represents the DFT results for the β-phase. Looking at the stretching region (1600-2100 cm-1) we observe that the experimental spectra present a broad and structured band that is not much altered in shape by the thermal treatment. We are looking at a feature that disappears with the first thermal treatment to assign to the T2g 694



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Table 2. Frequencies of the AlH3 Pristine Material (Spectral Interval: 140-2100 cm-1) Measured at T = 20 K and Compared to the Present Theoretical DFT Calculations, for the R, β, and γ Phases R-phase (DFT calc.)

experiment

β-phase (DFT calc.)

γ-phase (DFT calc.)

freq. shift

A1g

Eg

A1g

Eg

T2g

Ag

B1g

B2g

B3g

148

-

-

-

-

-

-

-

-

-

193 223

-

-

-

-

-

-

-

-

235.0

253

-

-

-

-

-

251.6

-

-

-

269

-

-

-

-

-

-

-

258.1

-

296

-

-

-

-

-

-

288.8

-

-

313

-

-

-

-

-

-

-

-

301.6

334

-

-

-

-

-

-

-

-

-

354

-

-

-

-

-

427.8

-

-

-

429 445

-

-

-

-

-

-

450.3 -

487.2

-

476

-

-

-

-

-

-

510.8

-

-

512

-

472.1

-

-

-

530.9

-

-

-

544

-

-

-

-

-

-

-

-

-

597

-

-

-

-

-

-

-

689.3

-

679

-

-

-

-

-

-

-

-

730.2

726

721.6

-

-

-

721.2

-

770.5

-

-

781 806

-

-

-

842.0

-

787.1 -

-

851.3

-

853

-

-

-

-

-

-

-

-

875.2

882

-

-

-

-

-

-

-

-

-

913

--

-

-

-

-

-

-

-

957.4

963

-

-

-

-

-

-

-

958.7

-

980

-

-

-

-

-

-

-

-

-

1004

-

-

-

-

-

-

-

-

-

1030 1044

-

-

-

-

1019.4 -

1041.3

-

-

-

1060 1126

-

1023.2 -

1117.8

-

-

-

1064.8 -

-

-

1302

-

-

-

-

-

1308.3

-

-

-

1367

-

-

-

-

-

-

1326.1

-

-

1490

-

1473.4

-

-

-

-

1485.5

-

-

-

-

-

-

-

-

1487.0

-

-

-

1547 -

-

-

-

-

-

1544.7 -

-

1574.6

1581.9

-

-

-

-

-

-

-

-

-

1714

-

-

-

-

-

-

-

-

-

1766

-

-

-

-

-

-

1819.2

-

-

1837

-

-

-

-

-

-

1898.6

-

-

1920

-

-

-

-

1943.4

1951.0

-

-

-

2036

-

-

-

-

-

-

-

-

-

mode, calculated at 1943 cm-1 (green line in the figure). This may be tentatively identified with the shoulder detected at 1920 cm-1 (black dots). In Figure 6 the bending region between 750 and 1200 cm-1 is depicted. We notice that the three experimental bands at 805, 1030, and 1126 cm-1 (black dots), observed to disappear after the first thermal treatment, have an intensity ratio that mimics nicely the theoretical results for the β phase, so the assignment is not questionable. The calculated values for these modes are 842, 1019, and 1118 cm-1, respectively.

The spectral region between 600 and 900 cm-1 is reported in Figure 7. Here the most prominent feature is represented by the intense R-phase mode at 726 cm-1. At higher frequency, we recognize the β-phase line at 805 cm-1 and its theoretical counterpart calculation (green line) at 842 cm-1. In contrast, the T2g calculated mode at 721 cm-1 (cf. the weak green peak in the figure) is likely masked by the R-phase mode. However, looking again at all features that disappear with the thermal treatments, one can detect a shoulder in the interval 712-715 cm-1 that is present in the pristine spectrum 695



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Figure 7. Raman spectra of the aluminum hydride samples taken at T = 20 K (pristine material, black dots; after the 1st thermal treatment at 70 °C, blue dots; after the 2nd thermal treatment at 100 °C, magenta dots), in the frequency interval between 600 and 900 cm-1. The intensity scale is linear up to 2.0 and logarithmic beyond this value. The full lines represent the DFT calculation results (red: R phase; green: β phase; cyan: γ phase).

Figure 5. Raman spectra of the aluminum hydride samples taken at T = 20 K (pristine material, black dots; after the 1st thermal treatment at 70 °C, blue dots; after the 2nd thermal treatment at 100 °C, magenta dots), in the frequency interval between 1350 and 2200 cm-1. The full lines represent the DFT calculation results (red: R phase; green: β phase; cyan: γ phase).



(black dots) and already absent after the first thermal treatment (blue dots). Assignment of the γ-Phase Bands. With respect to the R and the β phases, the γ phase is characterized by a much larger number of modes. Of these, we were able to assign only the most intense ones and those pertaining to the lattice modes, at low frequency, where the other phases do not present Raman active excitations. According to the DFT simulation, the most intense mode of the γ phase should be located at 1041 cm-1. However, other weaker modes of the same phase, and one of the R phase, should also be present in this spectral region (cf. Figure 6). On the basis of the sample behavior following the thermal treatments, we are

led to think that some of the modes are superimposed. In essence, considering that the experimental peak at 1030 cm-1 was already unequivocally assigned to the β phase, we are led to conclude that the peak at 1060 cm-1 corresponds to the Ag mode of the γ phase (calculated at 1041 cm-1) and that the residual intensity, still visible after the second thermal treatment, should correspond to the weak mode at 1023 cm-1 of the R phase (red line). The other weak modes of the γ phase are probably undistinguishable in the experimental spectrum. The next frequency region where the evolution of the experimental spectra shows an evident band disappearance following the thermal treatments is in the interval between 400 and 550 cm-1. 696



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We now move to the analysis of the γ-phase spectral features in the region between 600 and 900 cm-1 (cf. Figure 7) which is dominated by the strong R-line at 726 cm-1. We are reminded that the peak at 806 cm-1 has been already assigned to the β phase. Therefore, the only remaining lines are those at 679 and 780 cm-1 (and possibly three even weaker ones placed toward the high frequency portion of the interval). These modes may be assigned to the theoretical lines of the γ phase (cyan line) that are visible in the figure. Concerning the spectral interval between 1400 and 1600 cm-1, we remind that the three low frequency modes of the R phase were already assigned in the previous sections, and we were left with the assignment of the fourth mode, which the present DFT calculation predicts to be located at a frequency shift of 1473 cm-1. In this spectral region, reported in Figure 5, we observe a broad band, which partially disappears with the thermal treatments. Accordingly, we assign this band to the presence of one broad R line and two γ modes at 1487 and 1545 cm-1. The interpretation of the high-frequency portion of the spectrum appears more involved. As we mentioned before, the experiments show a structured and relatively strong band around 1800 cm-1, which persists with the thermal treatments. The situation is depicted in the figure where, we are reminded, the intensity of the experimental spectra is normalized such that the intensity of the R bands are approximately the same in each case. We remember that the shoulder at 1930 cm-1 has been already assigned to the β phase (green line peak at 1943 cm-1), but nothing can be said for the rest, apart from the fact that the DFT calculation predicts two very weak B1g modes and one low-intensity Ag mode for the γ phase. We point out that the theory predicts some modes for the R phase which are not Raman active (IR active modes), and we can only guess that some optical activity for the Raman effect could be stimulated by crystal defects.

Figure 9. Raman spectra of the aluminum hydride samples taken at T = 20 K (pristine material, black dots; after the 1st thermal treatment at 70 °C, blue dots; after the 2nd thermal treatment at 100 °C, magenta dots), in the frequency interval between 190 and 450 cm-1. The orange full line represents the spectral response of a polished aluminum plate and evidence the presence of the laser plasma lines. The cyan line represents the DFT calculation results for the γ phase.

This region is seen in Figure 8. The intense line at 512 cm-1 is assigned to the R phase (calculated value at 472 cm-1, red line). The structured band, present in the pristine material spectrum (black dots), is apparently stronger than the calculated four bands of the γ phase at 428, 487, 511, and 531 cm-1 (cyan line in the figure), but its shape is suggestive of a composite band where the stronger line has the highest frequency. Therefore, we would be tempted to assign this band to these four modes, even though it is not clear why this band seems to disappear, after the first thermal cycle, as if it were belonging to the β phase. The γ phase is the only one that is characterized by lattice modes at a relatively low frequency. Some of these can be unambiguously assigned, on the basis of the calculated frequencies and intensities. In Figure 9 we compare the experimental spectra with the DFT calculation for the γ phase in the spectra interval between 190 and 450 cm-1. Here, the measured background, evidencing the presence of the plasma lines from the excitation laser, is also shown (orange line). This spectrum allows to attribute all of the peaks located at a frequency lower than 240 cm-1 to plasma lines. The observed line at 253 cm-1 is present in the experimental and the calculated spectra and is unequivocally assigned to the Ag mode of the γ phase. The line at 268 cm-1, given the behavior resembling that at 202 cm-1, can be ascribed to the plasma. The line at 313 cm-1 in the spectrum of the pristine material (black dots), is of more dubious origin. On one side, it appears rather separated from the plasma peak at 300 cm-1. In addition, the appearance in the spectrum of the thermally treated samples of the two weak structures at 333 and 353 cm-1 (with the former also present in the pristine material spectrum) is suggestive of a weak lattice mode that could be assigned to the two other modes calculated (at 288 and 302 cm-1) for the γ phase. However, we are not able to explain why, differently from the previous case, where theory and experiment did match rather well both for intensity and frequency, now the agreement appears much looser. Finally, the theoretical mode at 427 cm-1, which appears as a shoulder in the pristine material spectrum (black dots), becomes more visible in the experimental spectra of the thermally treated material (427 cm-1, red dots).

’ DISCUSSION Recent diffraction experiments18,19 have demonstrated that the structures of the R and γ phases can be described by different arrangements of the same building blocks. The building element is an AlH6 octahedron, where every Al atom is surrounded by six hydrogen atoms representing, each, the vertices of two adjacent octahedra, so as to respect the stoichiometry. In the structure of the R phase, the octahedra are connected simply by sharing the vertices, while for the γ phase some edges are also shared (cf. Figure 2 of ref 19). According to these experimental results, in the theoretical calculations for the γ phase, Wang and co-workers15 assume an orthorhombic unit cell with the space group Pnnm. This arrangement produces, as a consequence, the presence of two nonequivalent aluminum atoms in the network (Al1 and Al2) and four nonequivalent hydrogen atoms (H1-H4). Particularly the Al2 and H3 atoms involve a new type of double bridge bond (cf. Figure 1 of ref 15). The phonon density of states (PDOS), calculated by Wang and co-workers, evidence the contributions of different aluminum and hydrogen atoms to the spectrum. The reduced distance of the Al2 atom types (i.e., the increased interaction strength) introduces new features in the spectral region below 475 cm-1, which are absent in the R phase spectrum. In addition, the calculated peaks at 1043 and 1077 cm-1 are associated with the motions of the H4 type atoms, while the set of highest frequency peaks (i.e., around 1900 cm-1) are related to the vibrational modes of the H1 type atoms along the Al2-H1-Al2 direction. The resulting simulated Raman active 697


The Journal of Physical Chemistry A modes15 range from about 200 cm-1 (lattice modes) up to 1943 cm-1 (aluminum-hydrogen vibrational modes). The measured Raman spectra of the cold material qualitatively reproduce the features that were predicted by the DFT calculations. However, the experimental samples are not pure and contain the three different phases, namely, R, β, and γ, in different concentrations. Nonetheless, the spectral evolution of the material samples, from the pristine condition to the final composition obtained following two thermal treatments, confirms this picture. As a matter of fact, the measured composition of the as-produced sample was 49.8 wt % of γ, 29.4 wt % of R, and 20.4 wt % of β (see section on Experimental Procedure, above). In the second sample, obtained after a thermal treatment at 70 °C, the β phase is probably absent, and only the features characteristic of the R and γ phases are visible in the Raman spectrum. Finally, after the second thermal treatment (at 100 °C) only the R phase should be left. These considerations are qualitatively supported by the spectra shown in the Supporting Information. The experimental spectra for the three different samples, all taken at T = 20 K, have been quantitatively compared with the DFT results in Figures 5-9. The experimental intensities were normalized to each other using the intense Raman line of the R phase at 512 cm-1. The same line was used to normalize the DFT results of the R phase, and the same normalization constant (adjusted for the density change of the different phases) was applied to the theoretical results for the β and the γ phases. By this procedure we have been able to perform a quite significant comparison between theory and experiments, which was used to assign the various Raman lines to the different phases. This assignment turned out rather straightforward for the R and the β phases, taking into account the frequency and intensity distribution, as well as the observed differences between the pristine material spectrum and that obtained from the material after the first thermal treatment. The assignment of the experimental Raman lines to the γ phase turned out to be more involved. While the most intense lines could be assigned almost immediately, the task turned out more difficult for some weaker bands. Nevertheless, a quite satisfactory picture for the assignment of the Raman lines to the γ phase was obtained.

’ CONCLUSIONS An extensive spectroscopic investigation, using Raman scattering, has been carried out on polymorphic samples of alane (AlH3). Spectra have been collected at room temperature, at a low temperature (20 K), and during thermal treatments at 70 and 100 °C. The sample was always kept under helium atmosphere (2 bar) to avoid contamination. The pristine material, composed of the three polymorphs, namely, the R, β, and γ phases, has been analyzed by XRD, and its phase composition has been quantitatively determined by Rietveld analysis. The Raman spectra of this sample have been collected at room temperature and at T = 20 K. The sample was heated to 70 °C to remove the β phase. Raman spectra have been collected during the sample heating and, following the thermal treatment, at a low temperature (T = 20 K). A second thermal treatment at 100 °C was then applied to the sample to remove the γ phase. Again, Raman spectra have been collected during the sample heating and, after the thermal treatment, at a low temperature (T = 20 K).

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This procedure has allowed us to infer, from a purely experimental basis, the characteristic Raman spectrum for each phase. These features were compared to the available DFT calculations for the R and the γ phases.14,15 Because of some observed discrepancies between the experimental and calculated features, we decided to perform our own ab initio calculations using the CASTEP code, which simulates, besides the position of the Raman active modes, also their intensities. Even in this case, the emerging picture is not fully satisfactory, with the theoretical calculations showing some quantitative differences with respect to the corresponding measured properties. A possible explanation could be attributed to the less-thanoptimal performance of the code in the presence of many light nuclei. However, we should not refrain to mention that, also from the experimental point of view, some margin of uncertainty exists. On one hand, the sample homogeneity cannot be fully proven, and when experimentally tested, some slight differences could be measured by changing the measuring spot on the sample. On the other hand, one should keep in mind that, especially on optically dense materials, Raman spectroscopy is mostly sensitive to the sample surface. If we imagine that, for unknown reasons, the concentration of the R phase on the surface of the sample is different from that in the bulk, we are led to conclude that this effect could quantitatively affect the measured intensity distribution which, we are reminded, was normalized to the most intense line of the R phase. In conclusion, while a reasonable assignment of the various modes can be worked out from the experimental data, the remaining observed quantitative differences with respect to the DFT calculations call for a further analysis on more defined samples (e.g., single crystals). This task would represent an interesting research activity. However, it exceeds the scope of the present work.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details about the local homogeneity of the sample and its behavior during the thermal treatment. In addition, we also give an amplified view of the low temperature (T = 20 K) Raman spectrum, as well as more detailed concerning the DFT calculation and results. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been partially supported by the EU, seventh Framework Programme, under the project NANOHy (“Novel Nanocomposites for Hydrogen Storage Applications”, Contract No. 210092) and by Ente Cassa di Risparmio di Firenze under the project Firenze-Hydrolab. ’ REFERENCES (1) Sandrock, G.; Reilly, J. J.; Graetz, J.; Zhou, W. M.; Johnson, J.; Wegrzyn, J. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 687. (2) Graetz, J.; Reilly, J. J. J. Phys. Chem. B 2005, 109, 22181. 698



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(3) Maehlen, J. P.; Yartys, V. A.; Denys, R. V.; Fichtner, M.; Frommen, C.; Bulychev, B. M.; Pattison, P.; Emerich, H.; Filinchuk, Y. E.; Chernyshov, D. J. Alloys Compd. 2007, 446-447, 280. (4) Manciu, F. S.; Reza, L.; Durrer, W. G.; Bronson, A.; Lacina, D.; Graetz, J. J. Raman Spectrosc. 2010No. DOI: 10.1002/jrs.2722. (5) Wong, C. P.; Miller, P. J. Energy Mater. 2005, 23, 169. (6) Tkacz, M.; Palasyuk, T.; Graetz, J.; Saxena, S. J. Raman Spectrosc. 2008, 39, 922. (7) Turley, J. W.; Rinn, H. W. Inorg. Chem. 1969, 8, 18. (8) Brinks, H. W.; Langley, W.; Jensen, C. M.; Graetz, J.; Reilly, J. J.; Hauback, B. C. J. Alloys Compd. 2007, 433, 180. (9) Brinks, H. W.; Brown, C.; Jensen, C. M.; Graetz, J.; Reilly, J. J.; Hauback, B. C. J. Alloys Compd. 2007, 441, 364. (10) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), Report LAUR; Los Alamos National Laboratory: Los Alamos, NM, 1994; pp 86-748. (11) Toby, B. H. J. Appl. Crystallogr. 2001, 34, 210. (12) Sartori, S.; Opalka, S. M.; Løvvik, O. M.; Guzik, M. N.; Tang, X.; Hauback, B. C. J. Mater. Chem. 2008, 18, 2361. (13) Giannasi, A.; Celli, M.; Grazzi, F.; Ulivi, L.; Zoppi, M. Rev. Sci. Instrum. 2008, 79, 013105. (14) Wolverton, C.; Ozolins, V.; Asta, M. Phys. Rev. B 2004, 69, 144109. (15) Wang, Y.; Yan, J. A.; Chou, M. Y. Phys. Rev. B 2008, 77, 014101. (16) Graetz, J.; Reilly, J. J. J. Alloys Compd. 2006, 424, 262. (17) Graetz, J.; Reilly, J. J. J. Alloys Compd. 2007, 446-447, 271. (18) Yartys, V. A.; Denys, R. V.; Maehlen, J. P.; Frommen, C.; Fichtner, M.; Bulychev, B. M.; Emerich, H. Inorg. Chem. 2007, 46, 1051. (19) Brinks, H. W.; Browns, C.; Jensen, C. M.; Graetz, J.; Reilly, J. J.; Hauback, B. C. J. Alloys Compd. 2007, 441, 364. (20) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. Z. Kristallogr. 2005, 220, 567. (21) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671.

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Temperature Behavior of the AlH3 Polymorph by in Situ Investigation

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