Vibrational Properties of MBH4 and MBF4 Crystals (M= Li, Na, K): A

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Vibrational Properties of MBH4 and MBF4 Crystals (M = Li, Na, K): A Combined DFT, Infrared, and Raman Study Olena Zavorotynska,* Marta Corno,* Alessandro Damin, Giuseppe Spoto, Piero Ugliengo, and Marcello Baricco Dipartimento di Chimica IFM and NIS, Universita degli Studi di Torino, via P. Giuria 7/9, 10125 Torino, Italy

bS Supporting Information ABSTRACT: In this work vibrational properties of alkaline-metal borohydrides and of the corresponding tetrafluorborates are studied by comparing DFT harmonic vibrational IR and Raman spectra of the crystals with the experimental ones, obtained by infrared attenuated total reflection and Raman techniques. The computed internal bending frequencies of the [BX4] anions are found to be in good agreement with the experiment, and the computed stretching frequencies of tetrafluorborates are slightly underestimated. As expected, due to the neglecting of anharmonicity in the DFT spectra, the computed stretching frequencies of borohydrides are overestimated. The peak assignment of the experimental spectra is carried out in terms of factor group theory. For borohydrides, it is mostly in agreement with previously published data but for a peak observed at ca. 1400 cm1. The peak assignment for KBF4 and LiBF4 was carried out for the first time in terms of factor group theory. This work is the first step on the way to determining the vibrational properties of the MBH4 + MBF4 solid solutions for hydrogen storage materials with enhanced H2-release/uptake properties and solid-state electrolytes.

’ INTRODUCTION Light metal borohydrides are attractive materials for solid state hydrogen storage due to their high gravimetric (up to 18.5 wt %) and volumetric (up to 122 kg H2 m3) hydrogen content.1 Nevertheless, high decomposition temperature, complex decomposition pathways, and the questioned reversibility hamper their practical application at present. Therefore, the vast majority of current scientific research on borohydrides is focused on finding ways to extract hydrogen conveniently as well as ensuring safety and the repeatable de/rehydrogenation cycling. Several theoretical2,3 and experimental49 studies evidence the positive effect of fluorine-containing additives on the thermodynamic, de/rehydrogenation properties, and/or decomposition pathways of complex hydrides or of reactive hydride composites (RHCs). Although negative effects, such as, for example, loss in hydrogen absorption/desorption reversibility, were also reported.10 Following these results one can conclude that the hydrogen absorption/release properties of the modified complex hydrides depend strongly on many factors, e.g. on the additive source, sample preparation procedure, and the structure of the new mixture. Whether the additive can dissolve or react with the borohydride, occupy defect sites in the crystal lattices, or only exists as a separate phase are the questions to answer in order to explain the differences in hydrogenation/dehydrogenation paths and thermodynamics for different fluorine-containing hydrogen storage systems. Raman and infrared spectroscopies are often used to characterize the structure of complex hydrides and their mixtures. Added value of these techniques in the studies of borohydrides is r 2011 American Chemical Society

related to the sensitivity of the [BH4] vibrational frequencies to the local environment in the solid. Free (gaseous) [BX4] type molecules have Td symmetry with four active vibrational modes in Raman and two in infrared. It is however a matter of fact that in a solid the vibrational features of the [BX4] units can be altered due to the surrounding crystal field. For example, the vibrational spectra of [BH4] were found substantially modified depending on the borohydride chemical composition,11,12 lattice symmetry,13,14 and the nature of bonds15 in the solid they were imbedded in. In this way, the vibrational spectrum of a metal borohydride is indeed an informative source to determine the changes in the structure and composition of the material. This explains why Raman and IR techniques have been extensively employed for characterization of pure metal borohydrides,11,13,14,1621 their mixtures, and mixtures with other compounds;2225 the decomposition pathways; and the reaction products.6,26,27 Due to the complexity of the spectra, quantum-mechanics simulations are necessary for the detailed assignment of the experimental frequencies. In this work the vibrational properties of alkali metal borohydrides (MBH4, M = Li, Na, K) are studied via DFT calculations (CRYSTAL code) and by means of infrared and Raman spectroscopies. The vibrational properties of the corresponding metal tetrafluorborates (MBF4, M = Li, Na, K) are studied as these materials can be the source of fluorine for the fluorine-substituted Received: June 21, 2011 Revised: August 17, 2011 Published: August 17, 2011 18890

dx.doi.org/10.1021/jp2058244 | J. Phys. Chem. C 2011, 115, 18890–18900

The Journal of Physical Chemistry C

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Table 1. Lattice Parameters and Most Relevant Bond Lengths for Optimized DFT Models of MBX4 Crystals (M = Li, Na, K; X = H, F), Compared to the Corresponding Available Experimental Valuesa LiBH4Pnma b

a

b

c

volume

BH1

BH2

BH3

exp.

7.121

4.406

6.674

209.4

1.213

1.224

1.208

1.215

109.3

DFT NaBH4cubic

7.328

4.379

6.494

208.4

1.230

1.233

1.229

1.231

109.3

exp. Fm3mc

6.131

230.4

1.094

109.5

DFT F43m

6.116

228.8

1.230

109.5

KBH4cubic exp. Fm3mc

6.690

299.4

1.220

109.5

DFT F43m

6.693

299.8

1.237

109.5

LiBF4P3121

a

b

c

4.892 4.960

4.892 4.960

11.002 11.173

exp.e

6.837

6.262

DFT

6.941

6.357

exp.f

8.659

DFT

8.848

d

exp. DFT P3121

BF1

BF2

228.0 238.0

1.387 1.421

1.391 1.422

6.792

298.8

1.386

1.386

6.956

306.9

1.420

1.420

5.480

7.030

333.6

1.377

5.554

7.170

352.3

1.410

BF3

1.389 1.421

109.5 109.2

1.392

1.389

109.1

1.424

1.422

109.4

1.382

1.391

1.385

109.2

1.418

1.432

1.423

109.3

NaBF4Cmcm

KBF4Pnma

a

All data expressed in Å, except volume in Å3. H and F labels refer to symmetry different atoms in the tetrahedron. Structures are displayed in Figure 1af. b Reference 51. c Reference 52. d Reference 53. e Reference 54. f Reference 55.

borohydrides. In recent years several studies have appeared on a similar topic, using the spectroscopic techniques and the DFT calculations to investigate different features of the Li, Na, and K borohydrides (see, for example, refs 20, 28, and 29). The majority of these computational studies have been conducted within the plane waves scheme, whereas in the present research paper, a local Gaussian basis set was adopted, using the CRYSTAL code, an established powerful tool in simulating vibrational features of crystalline materials.30 Whereas alkali metal borohydrides were extensively explored in the past via spectroscopic and theoretical methods, less so is for the tetrafluorborates. Available results are almost all focused on NaBF4, with significantly less information on Li and K salts, even though LiBF4 was recognized as one of the “electrolyte salts of the 21st Century.”31 The early studies on ammonia, sodium, and potassium borofluorides did not consider the site symmetry effects when interpreting the vibrational spectra.3234 The spectra were only discussed in terms of the Td symmetry of the free [BF4] ion with general considerations on the degeneracy removal of the vibrational modes due to the crystal field effect. In a more recent Raman study of NaBF435 the site symmetry effects were taken into account so that the splitting was adequately explained, but to our knowledge, similar studies on KBF4 and LiBF4 are still missing, though IR spectroscopy is employed sometimes to characterize LiBF4-containing electrolytes.3639 The results obtained in this paper are discussed with respect to the dependence of the [BX4] (X = H, F) internal vibrations on the crystal symmetries of the studied solids. One added value of the present work is the adoption of a single computational code (CRYSTAL09) to treat in a numerically coherent way (same basis set quality, same DFT functional, same numerical procedure for computing the frequencies, etc.) six different systems which were then subjected to IR and Raman measurements ensuring a robust and consistent comparison. This is particularly important, as the present results are the foundation

for the interpretation of the vibrational features of the more complicated fluorine-substituted borohydrides, which are of relevant interest in hydrogen storage field.

’ EXPERIMENT AND METHODS Materials and Experimental Techniques. Alkali-metal borohydrides and tetrafluorborates were obtained from SigmaAldrich: LiBH4 > 95.0%, KBH4 99.9%, LiBF4 99.998%, NaBF4 g 98.0%, and KBF4 99.999%. NaBH4 (98% purity) was obtained from Alfa Aesar. Received samples were transferred directly inside the glovebox and further handled in nitrogen atmosphere with minimal traces of water and oxygen (H2O < 0.1 ppm and O2 < 0.14 ppm). Infrared spectra were recorded on the single-reflection ALPHA-Platinum ATR (attenuated total reflection) instrument (BRUKER) with diamond crystal accessory. All infrared measurements were held in the glovebox. The spectra were obtained in the 4000400 cm1 range at 2 cm1 resolution, and 64 scans were averaged for each spectrum and the background. Raman spectra were obtained with an inVia Raman microscope, with a 514 nm laser excitation line and a 20 UltraLongWorking Distance MSPlan Olympus objective. The spectra were recorded in the 3000150 cm1 range, with 10 acquisitions of 40 s per each spectrum. Samples for Raman measurements were closed in quartz cells in nitrogen atmosphere. Powder X-ray diffraction patterns were obtained for all of the samples to ensure their phase purity. The patterns were collected with a PW3050/60 X’Pert PRO MPD diffractometer (PANalytical) in DebyeScherrer geometry. The powdered samples were placed inside the 0.8 or 0.5 mm boron-silicate capillaries and sealed in inert atmosphere. All patterns revealed the samples’ phase purity. In the pattern of LiBH4 two extra peaks at 30 and 27.9° (2θ) of a very small relative intensity were found. The patterns are available in the Supporting Information. 18891

dx.doi.org/10.1021/jp2058244 |J. Phys. Chem. C 2011, 115, 18890–18900


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Figure 1. Optimized crystal structures of MBX4 (M = Li, Na, K; X = H, F): for each case the best view has been chosen. BX4 units are displayed as solid tetrahedra, and cell borders are reported in black.

All of the experimental measurements were carried out at room temperature. Calculations. DFT calculations based on the GGA PBE functional were carried out with the CRYSTAL09 periodic code40,41 adopting localized basis set functions of polarized double-ζ quality. For all borohydrides and borofluorides, boron

was described by a 6-21G(d) basis set42 (αsp = 0.124 bohr2 for the most diffuse shell exponent and αpol = 0.800 bohr2 for polarization), whereas for hydrogen and fluorine a 31G(p)43 (αsp = 0.1613 bohr2 for the most diffuse shell exponent and αpol = 1.1 bohr2 for polarization) and a 7-311G(d)44 were considered, respectively. As for the different cations, Li was described 18892


The Journal of Physical Chemistry C with a 5-11G(d) basis set45 (αsp = 0.479 bohr2 for the most diffuse shell exponent and αpol = 0.600 bohr2 for polarization), Na with 8-511G (αsp = 0.323 bohr2 for the most diffuse shell exponent), and K with 86-511G (αsp = 0.389 and αd = 0.394 bohr2 for the most diffuse shell exponent of sp and d functions).46 For both NaBH4 and KBH4, the crystal simulation has required a lowering of the symmetry, as the hydrogen atom occupancy was imposed to 0.5 in the experimental crystal structure, due to static disorder. For the calculations, in order to avoid spurious replica by the action of symmetry operators, a supercell was adopted by doubling the primitive cell, removing the nonphysical hydrogen atoms, and reducing the symmetry space group to F43m. Within this space group the number of normal modes increases compared to the original set due to symmetry lowering. As a guide to facilitate the comparison with the experimental IR spectra, the modes with the highest intensity were considered. For the Raman-active and the IR, Raman-active modes, the closest to the experimental values were chosen. Phonon frequencies calculations of all of the reported models for borohydrides and borofluorides were computed at the Γ point in the harmonic approximation. Indeed, experimental infrared and Raman spectroscopy refers to the Γ point. Frequencies of the whole system were calculated as the eigenvalues obtained by diagonalizing the mass-weighted Hessian matrix at the Γpoint. The numerical Hessian matrix was computed by finite differences of the analytical gradient vector: the displacement for each atomic coordinate was set to 3 103 Å, reducing the convergence of the SCF cycle to 1011 Hartree.47,48 The IR intensities were computed through a Berry phase approach.49,50

’ RESULTS AND DISCUSSION 1. Structural Optimization by ab Initio Calculations. Each MBX4 (M = Li, Na, K; X = H, F) crystalline structure was fully optimized starting from the corresponding experimental structural data, and is displayed in Figure 1af. Table 1 lists the most relevant geometrical information, considering both the unit cell parameters and interatomic bond distances and angles. Concerning lithium, the optimized borohydride (orthorhombic, Pnma space group, Figure 1a, Table 1) is characterized by a systematic enlargement of the BH distances by approximately 0.16 Å with respect to the experimental geometry. Nevertheless, the unit cell volume is underestimated only by about 0.5% of the experimental value. The same behavior is observed for sodium borohydride (cubic, experimental space group Fm3m, computed F43m, Figure 1c, Table 1), though the volume variation due to the optimization process is smaller (0.24%). As for the potassium borohydride (same symmetry as sodium, Figure 1e, Table 1), the optimized unit cell volume is almost unchanged with respect to the experimental one (overestimated by 0.05%). In all of the calculated crystalline structures, [BX4] units retain almost ideal tetragonal symmetry with XBX angle varying in the 109.1109.5° range and the B-X distance varying within 00.022 Å. Data of Table 1 reveal a systematic overestimation of BH and BF bond lengths. This effect is probably due to both the deficiency of the chosen functional and the lack of flexibility in the basis set. 2. Spectroscopy of [BX4] (X = H, F) in the Solid State. 2.1. Site Symmetry and Crystal Field Effects. Free [BH4] and [BF4] species belong to Td symmetry group with four normal

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modes of vibration. All of them (~v1(A1), ~v2(E), ~v3(F2), and ~v4(F2)) are Raman active, whereas the ~v3 and ~v4 are only IR active. The BH stretching modes (~v1, ~v3) of [BH4] fall in the 25002100 cm1 region, and the HBH bending vibrations (~v2, ~v4), in the 1200900 cm1 region. The vibrational features of the heavier [BF4] group are downward shifted, the ~v1, ~v3 being at 1100700 cm1 and the ~v2, ~v4 at 600300 cm1. For the wide range of tetrafluorborates and other [XF4] compounds (X = C, Si, Bc, Al, Ge, N, P, etc.), the position of the normal modes follows the trend: ~v3 > ~v1 > ~v4 > ~v2, whereas in borohydrides and other [XH4]-type molecules or ions the trend ~v1 > ~v3 > ~v2 > ~v4 is observed.56 In crystals the symmetry of the [BX4] tetrahedra changes due to the site symmetry and crystal field effects, as it is summarized in Table 2 and discussed in more details in the Supporting Information (Chapter S1). Due to this degenerate modes usually split. In the experimental spectra, however, the splitting effect can be hidden due to superposition of the bands, which can become isolated only at low temperature20 or with polarized Raman measurements on single crystal.35 In this paper we assign the experimental spectra in terms of factor group splitting wherever possible. In case of lower resolution of experimental peaks (as for the stretching modes of MBF4, for example) the site symmetry or even molecular symmetry assignment is performed. 2.2. Vibrational Properties of NaBH4 and KBH4. 2.2.1. [BH4] Internal Vibrations. In the cubic cells of NaBH4 and KBH4 the borohydride tetrahedra retain the Td molecular symmetry, for which only two modes are allowed in IR and four in Raman (see Table 2). Therefore the vibrational features of [BH4] tetrahedra in the environment of Na and K cations are remarkably similar, (Figures 2 (IR), and 3 (Raman)). The computed and experimental frequencies are listed in the Tables 3 and 4. For the NaBH4 and KBH4, as well as for the rest of the crystals, if other not specified, the experimental IR modes are assigned to the closest calculated values taking into account also the calculated relative intensities. The Raman-active modes are assigned to the closest calculated values of the modes with the highest symmetry, since symmetric vibrations in most cases produce stronger Raman bands than do antisymmetric vibrations, and totally symmetric “breathing” vibrations produce the most intense Raman bands56 (in nonpolarized spectra of polycrystalline compounds). As anharmonicity is not accounted for by the calculations the BH stretching modes of NaBH4 and KBH4 are, not surprisingly, overestimated (+79, ..., +115 cm1; see Tables 3 and 4) whereas the computed bending modes are in better agreement with experiment (22, ..., +9 cm1), as anharmonicity plays a less important role. It should be noted that the best agreement between the calculated and experimental frequencies is achieved for the experimental data obtained at low temperature. Since we acquire our spectra at room temperature, the experimental modes are expected to be red-shifted, as also was demonstrated by Hagemann et al.17 The strongest peaks in experimental spectra (Figures 2 and 3) are assigned to the fundamental vibrations as they are listed in the Tables 3 and 4. The low intensity peaks, which are absent in calculated spectra are assigned to the overtones and combination modes. It is noteworthy that the overtone 2~v4 gains additional IR intensity due to Fermi resonance with the ~v3 fundamental.58 Splitting of the 2~v4 overtone into several peaks in the Raman spectra of NaBH4 and KBH4 (Figure 3) appear due to the fact that the triply degenerate mode ~v4 gives at least three Ramanactive overtones (of A1, E, and F2 symmetry). Another feature 18893


Correlation table is based on factor group analysis and correlation method described in refs 56 and 57 and references therein. b Number of molecules in the primitive unit cell. c The structure F43m was adopted rather than Fm3m as explained in the text. d A0 mode further splitting under the factor group D2h. a

{B1g+B3u}

R: Ag, B1g, B2 g, B3g; IR: B1u, B2u, B3u {Ag+B2u} + {B3g+B1u} +

{B1g+B3u}

{Ag+B2u} + {Au+B2g} D2h (D17 2h), Z = 2

{Ag+B2u}

{Ag+B2u} + {B3g+B1u} +

R: A1, E; IR: A2, E 4Ag+2B1g+B2g+2B3g+Au +2B1u+4B2u+2B3u {A1+E}+2{A2+E} A1 + B1 + B2 NaBF4

2{A1+E} A1 + A2 D3 C2v (D43), Z = 3 Cmcm

{A1+E} A1

{A1+E}+2{A2+E} A1 + B1 + B2

R: A1, E, F2; IR: F2 5A1+ 4A2+9E A + 2B A + 2B 2A A

A1

Td C2

Td

Z=1 P3121 KBH4 LiBF4

F43mc NaBH4,

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(T2d),

R: Ag, B1g, B2g, B3g; IR: B1u, B2u, B3u

A1+ E + 2F2 {B1g+B3g+Au+B2u} F2 {B1g+B3g+Au+B2u} F2 {B1g+B3g+Au+ B2u} E

6Ag+3B1g+6B2g+3B3g+3Au+6B1u+3B2u+6B3u 2A + A

2{Ag+B2g+B1u+B3u}+ 2{Ag+B2g+B1u+B3u}+

2A + A A +A

{Ag+B2g+B1u+ B3u} + D2h KBF4

A Cs Pnma

(D16 2h), Z = 4

LiBH4,

{Ag+B2g+B1u+B3u}d

0 0 00 0 00 0

infrared active species factor group

crystal

structure, Zb solid

0

ν3 (F2) ν2 (E) ν1 (A1) [BX4] site symmetry and

normal modes of free [BX4] with Td symmetry

[BX4] internal vibrations

Table 2. Correlation Table a for the Internal Vibrations of [BX4] (X = H, F) Anions in the Solids

ν4 (F2)

splitting of normal modes of Td molecular symmetry due to site symmetry and factor group effects

irreducible representation for [BX4] internal vibrations (Γint), R, IR: Raman,


not predicted in calculated spectra can be seen in IR spectra at about 1400 cm1. This broad weak band (vide infra for discussion) has never been reported before, for sodium and potassium borohydrides, and it is also present in the experimental spectrum of LiBH4 (Figure 2). In the experimental IR spectrum of KBH4 (Figure 2) a peak labeled 2 10ν~4 due to the first overtone of the H10BH bending can be distinguished at 2229 cm1. The natural 10B: 11B ratio is ca. 1:4. The difference in BH stretching and bending vibrations for two isotopes is in the range of 315 cm1, the values for 10B being at higher frequencies.59 Therefore in real spectra one should expect either higher-frequency additional band/shoulder of about one forth of intensity (as in case of KBH4, RbBH4, and CsBH411) or broadening of the BH peaks if the spectral resolution is not sufficient. Our assignment is in agreement with literature data with only minor differences in the observed peak positions (up to 5 cm1).11,58,60 Here it can be noted that there is a small difference in the infrared peak position comparing to other works performed on pure samples,11 whereas the differences become larger (913 cm1) for the infrared spectra of borohydrides deposited on NaCl or KBr plates.18,21 This fact can indicate the interaction with the alkaline halide support, which should be taken into consideration while choosing a proper optical material for sample preparation in IR transmission experiments. 2.2.1. [BH4] Librations. Rotational modes of [BH4] have F1 symmetry (Raman and IR inactive). Their overtones and combinational modes, however, can gain IR and Raman activity. We have found the computed modes of F1 symmetry at 315 (NaBH4) and 287 (KBH4) cm1. These values for the [BH4] librational movements are in a good agreement with the values obtained in the INS and Raman studies (Table 5). [BH4] translational modes of F2 symmetry are both IR and Ramanactive. We did not observe these modes in Raman spectra probably because they are weak and broad at ambient temperature. The most intense calculated infrared modes of F2 symmetry were found at 150 and 173 cm1 for NaBH4 and KBH4, respectively. 2.3. Vibrational Properties of LiBH4. 2.3.1. Fundamental Internal Vibrations of [BH4]. The orthorhombic unit cell of LiBH4 belongs to Pnma space group. Since the site symmetry of [BH4] species is lowered to Cs, the degeneracy of normal modes is completely removed, giving rise to additional peaks in the spectra (Figures 2 and 3). Observed experimental modes are assigned, as it was mentioned previously, basing on the calculated values (Tables 3 and 4), and more discussion on peak assignment can be found in the Supporting Information (Chapter S2). It is worth mentioning that the proposed trend for the position of the fundamentals of the [XH4] group: ~v1 > ~v3 > ~v2 > ~v4 is found by our calculations for Na and K borohydrides. For LiBH4, on the contrary, ~v3 > ~v1, probably, because of the strong coupling between the modes caused by the similar Li and B masses. It is also noteworthy that the infrared frequencies reported here are in good agreement with the values calculated utilizing different DFT functionals within the plane-wave pseudopotential method.16 Our computed Raman frequencies, on the contrary, are in worse agreement with the experimental stretching values and in a better agreement for the bending frequencies compared to the DFT calculations reported in literature.20,63 2.3.2. Overtones and Combinations of [BH4] Internal Modes. Since ~v4 mode is split into two widely separated components, more peaks due to the overtones and combinational bands including ~v4 normal mode appear: ~v2 + ~v4(A0 ), at ca. 18894



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Figure 2. Experimental (ATR) and computed infrared spectra of MBX4 (M = Li, Na, K; X = H, F) solids. The intensity of the experimental spectra of tetrafluorborates in the 16001300 cm1 region expanded for clarity.

2540 cm1 (IR), ca. 2560 cm1 (R), ~v2 + ~v4 (A00 ), ca. 2375 cm1 (IR), 2~v4 (A0 ), at ca. 2465 cm1(IR), and 2480 cm1(Raman), 2~v4 (A00 ), at 2178 cm1(IR). The weak broad peak ca. 2170 cm1 in Raman spectrum of LiBH4 should comprise several Ramanactive combinations of ~v4 components. 2.3.3. Librational Modes of [BH4] and the Lattice Modes. A broad peak at ca. 1400 cm1 similar to the one of NaBH4 and KBH4, but more intense, appears in the spectrum of LiBH4. It is possible that this feature evidences the presence of boron oxide or hydrated boron oxide impurities (MxByOz 3 nH2O), which may be present in the samples. Boron oxide impurities can originate from metal borates (for example, NaBO2), used for the preparation of the borohydrides, and the hydrated boron oxides may be formed upon the contact of MBH4 with the atmospheric moisture. It should be mentioned here that the commercial samples of the highest available purity were used for the experiments and were not in contact with air in our laboratory (see the Experimental section). Therefore if borate species are present in the samples, they were introduced during synthesis or transportation. This is a relevant aspect when studying commercial samples, as the presence of such impurities can modify the thermodynamic properties of the compounds. In order to understand whether the 1400 cm1 peak originates from the impurities coming from the contact with moisture, we recorded the spectra of LiBH4 subjected to air and wetted KBH4. The spectra of these samples treated in vacuum at elevated temperatures were also recorded and are reported in the Figure S2 of the Supporting Information. In case of LiBH4, the intensity of the 1400 cm1 peak varies only slightly upon reaction with atmospheric moisture, however it does decrease upon treatment at 150 °C in dynamic vacuum. Since the spectrum after treatment was obtained at room temperature, there is no issue of phase transition impact on the spectrum. High temperature phase of LiBH4 is known to be stable at >111 °C only.51 It is worth mentioning here that upon contact with air, the spectrum of LiBH4 is strongly modified: the OH stretching and HOH bending modes of water appear in the 36003200 and 16601620 cm1 regions, respectively. Apart from that the strong peaks at 1160 and 1140 , 630, and 536503 cm1 appear.

The latter three peaks most probably can be assigned to the crystalline water librational modes. The first two peaks were erroneously interpreted as BF asymmetric stretching and taken for the evidence of formation of LiBHxF4-x species.6 However they should rather be assigned to the combinational modes of the 630500 cm1 peaks. More detailed assignment is out of the scope of this study, but it has become rather clear that contact with air distorts significantly the spectrum of LiBH4, and the spectra of such samples should be interpreted with care. As far as the spectra of KBH4 are concerned, they have shown little difference in the 1420 cm1 peak intensity for the wetted and treated at high temperature KBH4 (Figure S2b). It should be mentioned here that the XRD patterns revealed pure phase of KBH4. In the patterns of LiBH4 two additional peaks were present at 30.0° and 27.9°, but of a very weak intensity. Therefore we think that if the impurities are present in the KBH4 and LiBH4, they cannot be responsible for such strong infrared peaks, as those at 1400 cm1. The infrared BO stretching frequencies of various boron oxide species fall into the 1500900 cm1 and bending into the 1000500 cm1 regions.64,65 In case of hydrated borates the peaks due to OH stretching (32003500 cm1), HOH (ca.1630 cm1), and BOH (12001400 cm1) bending can be found.66 It is worth mentioning that in the spectra of asobtained LiBH4 and KBH4 very weak absorptions in the 30003500 cm1 region, and also at 950, 740, 640 cm1 (in the IR spectrum of LiBH4) and at 1195 and 885 cm1 (in the spectrum of KBH4) can be distinguished (Figure S2). The intensities of these peaks, however, are considerably smaller than those of the 1400 cm1 absorptions. At the same time, no extra peaks were found in the Raman spectra of the same materials, although the borates have very strong and readily distinguishable peaks at ca. 800 cm1.66 In this way, the assignment of the 1400 cm1 peaks to the impurities is rather doubtful. Alternatively, the peaks at 1400 cm1 can be assigned to the combinations of the fundamental modes and the librational modes of [BH4] tetrahedra. A similar feature was observed in the halide salts of ammonia and attributed to the combination band of [NH4]+ fundamental bending with the libration mode 18895



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Figure 3. Experimental Raman spectra of MBX4 (M = Li, Na, K; X = H, F) compounds.

Table 3. Calculated and Experimental Infrared-Active [BX4] Internal Modes a (cm1) of MBX4 Solids (M = Li, Na, K; X = H, F) LiBH4 as.b

mode

ν3, A0

B3u

2412 s

ν3, A0

B1u

2403 m

ν3, A0

B3u

2393 s

ν3, A0

B1u

2392 s

ν3, A00

B2u

2389 vs

ν1, A

0

ν1, A

0

calc.

NaBH4 exp.

mode

calc.

KBH4 exp.

mode

calc.

LiBF4 exp.

mode

calc.

NaBF4 exp.

exp.

mode

calc.

1114(0)

ν3, A1 B2u 1038vs

ν3, A0

B1u 1137(0)

ν3, B E

1011 vs

ν3, B2 B3u 1027vs

1010 ν3, A0

B3u 1059 vs

ν3, B E

1010 s

ν3, A0

B3u 1020 m

ν3, A0

B1u 1014 vs

2300

1020 ν3, B1 B1u 987vs

ν3, B A2 1009 m ν1, A E

763 (0) 794

B3u

2332 w

ν4, B E

528 w

ν2, A0

B3u

1316 m

ν2, A0

B1u

1315 vw

ν2, A00

B2u

1285 m

1285

ν4, A0

B3u

1246 m

1232 ν4, F2 F2 1109 1108 ν4,F2

ν4, A0

B1u

ν4, A0 ν4, A0

1307

c

ν1, A1 B2u 743 (0) 782

ν1, A0

B1u 738 vw

ν4, A1 B2u 526w

550

ν1, A0

B3u 736 vw

c

ν4, B A2 521 w

567

ν4, B2 B3u 498vw

526

ν4, A0

B3u 502 w

ν4, A E

538

ν4, B1 B1u 487vw

517

ν4, A0

B1u 502 (0)

514 w

exp.

1010

ν3, A00 B2u 976 vs

ν3, B A2 1003 s

2270

2334 w

ν4, A

calc.

ν3, F2 F2 2399 2284 ν3, F2 F2 2346 2267 ν3, A E

B1u

00

mode

KBF4

ν4, B A2 510 w F2 1101 1110 ν4, B E

506 w

1244 m

ν2, A E

377 s

o/r

B3u

1076 w

ν2, A E

352 s

o/r

B1u

1074 vw

B2u

1064 m

ν2, A1 B2u 358(0)

771 533

ν4, A0

B3u 499 (0)

ν4, A0

B1u 494 w

470

ν4, A0

B2u 490 w

n/r

ν2, A00 B3u 345 (0) o/r ν2, A00 B2u 338 (0) o/r ν2, A0

1089

B1u 338 (0) o/r

Relative intensities are given for calculated infrared modes: 0 zero or near zero intensity of IR-active mode, vw very weak, w-weak, s strong, vs very strong; other notations: o/r out of instrument range, n/r not resolved, sh shoulder. b Assignment. c Deduced from combinational modes. a

of ammonia tetrahedra in the crystal.6769 This feature was observed in F, Cl, and Br salts of ammonia, where the [NH4]+ does not rotate freely, but was not found in NH4I where [NH4]+ enjoys free rotation. By analogy to these findings we can assign the bands at ca. 1400 cm1 to the combination of a fundamental and the libration mode of [BH4] tetrahedra. The fundamental could be either ~v2 or ~v4. As it is shown in Table 5, the deduction of the ~v4 from the infrared experimental peak at 1400 cm1 gives the values of [BH4] librational movements peaks more close to those obtained in Raman, inelastic neutron scattering studies,61,62 and our calculations. In this way, the peaks at 1400 cm1 in the infrared spectra of LiBH4, and also NaBH4 and KBH4 can be attributed to the ~v4 + ~vL combinations. Computed librational and translatory frequencies, and the Raman spectrum of LiBH4 in 800160 cm1 region are reported

in the Supporting Information (Figure S1 and Table S1). At room temperature we observed only two broad Raman peaks, centered at 255((60) and 193((40) cm1. As also noted by Hagemann et al.,17 [BH4] librations in LiBH4 are strongly coupled with the translational modes, including those of Li. In order to separate the [BH4] librations from the Li translatory movements, we have performed the calculations on the LiBH4 crystal with the very heavy Li isotopes (M = 1000) (see column “1000Li freq.” in the Table S1). Among the resulted vibrational frequencies those at 438 (B1 g), 432 (B1u), and 392 (Au, silent mode) cm1 appeared to be the least affected (Δν = ν(6Li) ν (1000Li) = 5, ..., 7 cm1) by the substitution and can be assigned the librational movements. The most strongly affected modes in 202135 cm1 region (Δν = 120174 cm1) can be considered the Li translation modes. Two B3u modes, strongly shifted by 8284 cm1 after 1000Li 18896



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Table 4. Calculated and Experimental Raman-Active [BX4] Internal Modes (cm1) of the MBX4 Solids (M = Li,Na,K; X = H,F) LiBH4

KBH4

LiBF4

NaBF4

KBH4

as.a

mode

calc.

ν3, A0

B2g

2430

2382 2309 ν3, B E

1114

ν3, B2

B3g 1075 1120 ν3, A0

B2g 1085

ν3, A0

Ag

2411 2320

ν3, B E

1011

ν3, A1

Ag

1024 1060 ν3, A0

B2g 1063

ν3, A00

B1g

2392

ν3, A E

1010

ν3, B1

B1g 1011 1040 ν3, A0

Ag

1044 1097

ν3, A00

B3g

2390

ν3, A A1 1000 1050

ν3, A0

Ag

1019 1043

ν3, A0

B2g

2388

ν3, A00

B1g 1000

ν3, A0

Ag

2375 2275

ν3, A00

B3g 996

Ag

2332 2299 ν3, F2

ν1, A0

B2g

2330

ν2, A0

B2g

1325

528

ν4, A1

Ag

ν2, A0

Ag

1325 1320

ν4, A A1

520

541 ν4, B2

B3g

ν2, A00

B3g

1284 1290

ν4, A E

514

ν4, B1

B1g

ν2, A00

B1g

1274

ν4, B E

506

ν4, A0

Ag

B2g

1245

F2 1101 1123 ν2, A E

377

ν4, A00

B3g 499

Ag

1239 1240

ν2, A A1

363

ν4, A00

B1g 498

B2g

1082 1110

ν2, A A1

334

ν4, A00

B3g

1077

ν2, A E

352

ν4, A0

Ag

ν4, A00

B1g

ν1, A

ν4, A

0

0

ν4, A0 ν4, A

a

NaBH4

0

exp.

mode

calc.

ν1, A1 A

exp.

mode

calc.

2442 2335 ν1, A1 A

F2 2399

ν 3 , F2

exp.

mode

F2 2346 2285 ν1, A E ν1, A A1

ν2, E

ν4, F2

E

1269 1279 ν2, E

F2 1109 1122 ν4, F2

E

1242 1249 ν4, B E

calc.

exp.

ν1, A1

763 758

mode

calc.

Ag

747

exp.

mode

784 ν1, A0

Ag

calc.

739

ν1, A0

B2g 738

528

554 ν4, A0

B2g 505

504

532 ν4, A0

Ag

502

532 ν4, A0

B2g 501

797

504 499

351

344 ν2, A0

Ag

321

369 ν2, A00

B3g 335

1075 1097

ν2, A0

B2g 335

1070

ν2, A00

B1g 334

ν2 , A 1 A g 355 ν2,A2

B2g

336

exp.

775

534 n/r

360

Assignment. N/r not resolved.

Table 5. Librational Frequency of [BH4] Tetrahedra, ~vL, in cm1 in Li, Na, and K Borohydrides at 300 K, as Deduced from Combination with a Fundamental ~vf: ~v4 or ~v2 ~vL infrared (deduced)a compound

~vL + ~vf (IR) (observed)

(~vL + ~vf) ~v2

(~vL + ~vf) ~v4

calculationsa

INS b

Raman c

LiBH4

1435 ( 85

135 ( 85

200 ( 85 345 ( 85

440430

376

NaBH4

1400 ( 70

120 ( 70

290 ( 70

315

313

325 ( 15

KBH4

1430 ( 85

170 ( 85

330 ( 85

287

286

282 ( 15

This study. For LiBH4 two components of ~v4: at 1232 and 1089 cm1 are considered. For NaBH4 and KBH4 Raman-active ~v2 modes at 1279 cm1 and 1260 cm1 respectively are considered. b Reference 61. c Reference 62. a

substitution, initially appear as high as 340 and 238 cm1, evidencing the strong coupling of Li and [BH4] vibrations. It can be noted that the modes obtained here with local Gaussian basis set are, by large part, in a better agreement with the lowtemperature experimental Raman data obtained elsewhere,17,20 than the results obtained with the other theoretical approach.20,63 2.4. Vibrational Properties of LiBF4, NaBF4, and KBF4. 2.4.1. Fundamental Vibrations. The crystals of the three tetrafluorborates belong to different space groups so that the [BF4] site symmetry is different in all three solids (see Figure 1 and Table 2). The stretching ~v3 mode split in all the tetrafluorides. In the infrared experimental spectra (Figure 2) this region is poorly resolved and only one broad peak at ca. 1000 cm1 with shoulders appears. Notice that, as the bending modes fall around 500 cm1, the band at 1000 cm1 can also comprise the overtones of the ~v4 mode. In Raman spectra (Figure 3) the components of the ~v3 mode have very weak intensity in 10431120 cm1 region, and are attributed to the species of the highest symmetry (Ag or A1). In case of NaBF4 in addition to the peak at 1060 cm1, attributed to ~v3(Ag), to other very weak peaks at 1120 and 1040 cm1 are found and assigned to the

modes with the closest calculated values. It is noteworthy that our experimental Raman peaks are by 2040 cm1 different from the peaks observed in the polarized Raman single-crystal studies.35 Notice that the ~v3 BF stretching mode falls in the region of the HBH bending mode (~v4). Moreover, the peaks of hydrated LiBH4 occur in the same region (see Figure S1). Therefore the spectra of fluorine-substituted borohydrides should be examined carefully in the 12001000 cm1 range. Unlike in the spectra of borohydrides, the ~v3 and ~v1 modes in the spectra of tetrafluorborates are well-separated. The symmetrical stretching mode (~v1) is predicted to appear in the IR spectra of all tetrafluorborates, although its calculated intensity is zero or close to zero. In experimental spectra ~v1 is only observed for KBF4 (Figure 3). In Raman spectra (Figure 4) this mode is the most intense and is downward-shifted in three salts: LiBF4 (797 cm1), NaBF4 (784 cm1), and KBF4 (775 cm1). The same trend (with Δ~v of 11 and 8 cm1) is also predicted by calculations. The bending modes of tetrafluorborates fall into 600500 cm1 (~v4) and the 400300 cm1 (~v2) regions.70 Assignment of experimentally observed modes is reported in the Tables 3 and 4, 18897


The Journal of Physical Chemistry C and more discussion can be found in the Supporting Information (Chapter S3). Overall, the calculated vibrational frequencies of [BX4] in borofluorides are more close to the experimental ones compared to those of borohydrides. By contrast to the computed stretching modes of borohydrides, which resulted overestimated, those of tetrafluorborates are underestimated. Only a very little shift of [BF4] internal vibrations (depending on the cation surrounding) is observed by comparison to the borohydrides. These facts suggest lower degree of anharmonicity in tetrafluorborates (due to the restricted librational degrees of freedom of the heavier and larger [BF4] units) and lower perturbation of [BF4] by the surrounding. 2.4.2. Combinational Modes. The weak IR peaks at ca. 1300 cm1 (Figure 2), not present in the calculated spectra, are considered as combinational modes. Taking into account the positions of fundamentals we assigned these features to the ~v1+~v4 combinational bands as follows. For LiBF4 the broad peak at 1332 cm1 is assigned to ~v1(A)+~v4(A) combination; it gives the position of ~v1(A) at 794 cm1, which value is indicated in the Table 3. For NaBF4 the two peaks at 1332 and 1308 cm1 are assigned to the combination of ~v1(A1)+ ~v4(A1) and ~v1(A1) + ~v4(B2) respectively. In this case it will give the position of ~v1(A1) at 782 cm1. In the IR spectrum of KBF4 the ~v1 mode at 771 cm1 is clearly visible. It should be noted that the A0 and A00 components of the ~v4 mode of KBF4, according to calculations, are separated by no more than 12 cm1, by contrast to the LiBH4 case (which belongs to the same space group). If combined with different components of ~v1 mode, the ~v4 will give the vibrations at 1304 cm1 and 1292 cm1, which is exactly the 13041291 cm1 position of the experimentally observed peaks. 2.4.3. Librational and Lattice Modes. The region of librational and lattice modes is out of our infrared instrument range. In Raman no peaks lower than 300 cm1 were observed. In absence of the experimental data, only calculated frequencies are discussed here. In LiBF4 the rotational, translational, as well as the ~v2 FBF bending modes are strongly mixed due to the heavy weight of [BF4] anions and light cations. The list of calculated frequencies, presented in Table S2 of the Supporting Information, shows no clear difference between the low-region FBF bending modes and the lattice vibrations. Calculations on the 1000Li substituted crystal have also proved that the Lidependent modes are mixed with those not strongly affected by the isotopic exchange. For example, one of the modes, redshifted by 111 cm1 after isotopic exchange, was initially present at 459 cm1, even higher, than the FBF bending is expected (ca. 300 cm1). Considering that the [BF4] is significantly heavier than Li, we can attribute the lowest frequencies (16132 cm1) in the Table S2 to the [BF4] rotational and translational movements. They are also the least affected by Liisotope substitution. Those in the 363286 cm1, (red-shifted by 172195 cm1 by the Li-substitution) can then be associated with Li translations, although the number of these modes is larger than the number of Li translational modes, so that this region should also comprise some other modes. The character of representation for the ~v2 mode is denoted as 2A1+2E out of which only one E at 377 cm1 is moderately affected by the Liisotopic substitution. Due to this we assign ~v2, as it is shown in the Tables 3 and 4, even if some of the modes assigned to FBF bending are strongly affected by the Li substitution. The character of representation for the [BF4] librational modes in the NaBF4 (factor group D17 2h) is Au+B1g+B2g+B3g+B1u+B2u, and

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the only Au (silent) and B2g (IR) modes in the calculated lowfrequency region are those at 102 and 92 cm1 (see Table S3). It allows us to identify the region of [BF4] librations in NaBF4 as 111 - 80 cm1 and assign the rest of the modes in that region to the librations (apart from Ag). The higher-frequency region (197133 cm1) can then be attributed to Na translations and the rest of the peaks at the lower frequencies to [BF4] translations. In KBF4 the modes due to potassium and borofluorides are mixed due to the increased weight of cations. Indeed, the low frequency modes cannot be clearly grouped and attributed to particular type of vibrations (Table S4). It can be suggested that the modes in ca. 10570 cm1 region are due to the [BF4] librations (by comparison to NaBF4), whether those in 146110 cm1 region due to potassium, and those in 7040 cm1 are due to [BF4] translations.

’ CONCLUSIONS Vibrational properties of the [BX4] ions in Li, Na, and K borohydrides and tetrafluorborates were studied by infrared, Raman spectroscopies and by DFT PBE calculations. These properties depend strongly on the nature of surrounding lattice, so that the spectra of these compounds cannot be interpreted considering only the molecular Td symmetry since the crystal field effects are strong. It is particularly evident, for instance, in case of LiBH4, where different components of the ~v4 mode are separated by more than 170 cm1. Without considering these effects, an erroneous interpretation of the spectra can be given. This issue is particularly important in characterization of novel/modified/substituted borohydrides since the symmetry lowering effects can be misunderstood and considered as the evidence of solid solution formation or modification of borohydrides. The IR and Raman spectra and the peak assignment of borohydrides are mostly in agreement with literature data, although a new peak at ca. 1400 cm1, not reported before, is detected in the IR spectra of borohydrides. This peak is suggested to be the combinational band of the libration modes of [BH4] tetrahedra and the fundamental ~v4, although its attribution to the impurities might be possible. If the former is the case, modifications in the intensity of this peak, especially in case of LiBH4, where it is particularly intense, can evidence the alterations in the surrounding of the [BH4] tetrahedra and/or their interionic distances. The spectra of LiBF4 and KBF4 are interpreted for the first time in terms of factor group splitting. These results can be utilized in the studies of fluoride-containing electrolytes. DFT calculations reproduce with better accuracy the vibrational spectra of borofluorides with respect to those of borohydrides due to anharmonicity effects not taken into account in the theoretical analysis. On the basis of this combined study including the punctual comparison between experiment and computations, other research papers are in preparation to apply the same approach for investigating fluorine-substituted borohydrides. Indeed, experimental IR and Raman spectra of mixed compounds appear very complex to be interpreted: DFT calculations provide information on the position of the new BF stretching and HBF bending bands as well as on the changes in the vibrational properties of [BH4] tetrahedra caused by introduction of a whole [BF4] into the unit cell of MBH4. In that respect, the present results on pure compounds have a central role to allow for a detailed interpretation of the complexity of the mixed compound spectra. 18898



’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectra of LiBH4 in the 600150 cm1 region, ATR spectra of hydrated LiBH4 and KBH4, calculated frequencies of LiBH4, KBF4, and NaBF4 lattice vibrations, and all calculated frequencies for LiBF4, powder X-ray diffraction patterns of all samples, discussion on the assignment of the experimental frequencies This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*(O.Z.) E-mail: [email protected]. Tel.: +39 011-6707840. Fax: +39 011-6707855. (M.C.) E-mail: [email protected]. Tel.: +39 011-6704597. Fax: +39 011-2364596.

’ ACKNOWLEDGMENT Financial support from the European Union under FP7 (FLYHY project, grant agreement 226943) is thankfully acknowledged. ’ REFERENCES (1) Zuttel, A.; Borgschulte, A.; Orimo, S. I. Scripta Mater. 2007, 56, 823. (2) Yin, L.; Wang, P.; Fang, Z.; Huiming, C. Chem. Phys. Lett. 2008, 450, 318. (3) Corno, M.; Pinatel, E.; Ugliengo, P.; Baricco, M. J. Alloys Compd. 2010, doi:10.1016/j.jallcom.2010.10.005 (4) Au, M.; Spencer, W.; Jurgensen, A.; Zeigler, C. J. Alloys Compd. 2008, 462, 303. (5) Zhang, Y.; Zhang, W. S.; Fan, M. Q.; Liu, S. S.; Chu, H. L.; Zhang, Y. H.; Gao, X. Y.; Sun, L. X. J. Phys. Chem. C 2008, 112, 4005. (6) Gosalawit-Utke, R.; Bellosta von Colbe, J. M.; Dornheim, M.; Jensen, T. R.; Cerenius, Y.; Minella, C. B.; Peschke, M.; Bormann, R. J. Phys. Chem. C 2010, 114, 10291. (7) Lee, J. Y.; Lee, Y.-S.; Suh, J. Y.; Shim, J. H.; Cho, Y. W. J. Alloys Compd. 2010, 506, 721. (8) Eigen, N.; Bosenberg, U.; von Colbe, J. B.; Jensen, T. R.; Cerenius, Y.; Dornheim, M.; Klassen, T.; Bormann, R. J. Alloys Compd. 2009, 477, 76. (9) Gosalawit-Utke, R.; Suarez, K.; Bellosta von Colbe, J. M.; Bosenberg, U.; Jensen, C. T.; Cerenius, Y.; Minella, C. B.; Pistidda, C.; Barkhordarian, G.; Schulze, M.; Klassen, T.; Bormann, R.; Dornheim, M. J. Phys. Chem. C 2011, 115, 3762. (10) Zhang, B. J.; Liu, B. H. Int. J. Hydrogen Energy 2010, 35, 7288. (11) Renaudin, G.; Gomes, S.; Hagemann, H.; Keller, L.; Yvon, K. J. Alloys Compd. 2004, 375, 98. (12) Ketelaar, J. A. A.; Schutte, C. J. H. Spectrochim. Acta, Part A 1963, 17, 1240. (13) Gomes, S.; Hagemann, H.; Yvon, K. J. Alloys Compd. 2002, 346, 206. (14) Araujo, C. M.; Ahuja, R.; Talyzin, A. V.; Sundqvist, B. Phys. Rev. B: Condens. Matter 2005, 72. (15) Parker, S. F. Coord. Chem. Rev. 2010, 254, 215. (16) Andresen, E. R.; Gremaud, R.; Borgschulte, A.; RamirezCuesta, A. J.; Zuttel, A.; Hamm, P. J. Phys. Chem. A 2009, 113, 12838. (17) Hagemann, H.; Filinchuk, Y.; Chernyshov, D.; van Beek, W. Phase Transitions 2009, 82, 344. (18) Harvey, K. B.; McQuaker, N. R. Can. J. Chem. 1971, 49, 3272. (19) Harvey, K. B.; McQuaker, N. R. Can. J. Chem. 1971, 49, 3282. (20) Racu, A. M.; Schoenes, J.; Lodziana, Z.; Borgschulte, A.; Zuttel, A. J. Phys. Chem. A 2008, 112, 9716. (21) Schutte, C. J. H. Spectrochim. Acta, Part A 1960, 16, 1054.

ARTICLE

(22) Barkhordarian, G.; Jensen, T. R.; Doppiu, S.; Bosenberg, U.; Borgschulte, A.; Gremaud, R.; Cerenius, Y.; Dornheim, M.; Klassen, T.; Bormann, R. J. Phys. Chem. C 2008, 112, 2743. (23) Rongeat, C.; D’Anna, V.; Hagemann, H.; Borgschulte, A.; Zuttel, A.; Schultz, L.; Gutfleisch, O. J. Alloys Compd. 2010, 493, 281. (24) Au, M.; Jurgensen, A. J Phys Chem B 2006, 110, 7062. (25) Somer, M.; Acar, S.; Koz, C.; Kokal, I.; Hohn, P.; Cardoso-Gil, R.; Aydemir, U.; Akselrud, L. J. Alloys Compd. 2010, 491, 98. (26) Orimo, S.; Nakamori, Y.; Ohba, N.; Miwa, K.; Aoki, M.; Towata, S.; Zuttel, A. Appl. Phys. Lett. 2006, 021920. (27) Her, J. H.; Yousufuddin, M.; Zhou, W.; Jalisatgi, S. S.; Kulleck, J. G.; Zan, J. A.; Hwang, S. J.; Bowman, R. C.; Udovic, T. J. Inorg. Chem. 2008, 47, 9757. (28) Allis, D. G.; Hudson, B. S. Chem. Phys. Lett. 2004, 385, 166. (29) Remhof, A.; yodziana, Z.; Buchter, F.; Martelli, P.; Pendolino, F.; Friedrichs, O.; Zuttel, A.; Embs, J. P. J. Phys. Chem. C 2009, 113, 16834. (30) Demichelis, R.; Civalleri, B.; Ferrabone, M.; Dovesi, R. Int. J. Quantum Chem. 2010, 110, 406. (31) Advanced Inorganic Fluorides: Synthesis, Characterization, and emva, B., Tressaud, A., Eds.; Elsevier Application; 1st ed.; Nakajima, T., Z Science S.A.: Lausanne, Switzerland, 2000. (32) Cote, G. L.; Thompson, F. R. S. Proc. R. Soc. London Ser. A, Math. Phys. Sci. 1951, 210, 217. (33) Quist, A. S.; Bates, J. B.; Boyd, G. E. J. Chem. Phys. 1971, 55, 2836. (34) Quist, A. S.; Bates, J. B.; Boyd, G. E. J. Chem. Phys. 1971, 54, 4896. (35) Rajakumar, V.; Krishnamurthy, N.; Ramakrishnan, V.; Kumar, J. J. Raman Spectrosc. 1992, 23, 75. (36) Rajendran, S.; Uma, T. Bull. Mater. Sci. 2000, 23, 27. (37) Eriksson, T.; Andersson, A. M.; Bishop, A. G.; Gejke, C.; Gustafsson, T.; Thomas, J. O. J. Electrochem. Soc. 2002, 149, A69. (38) Jacob, M. M. E.; Arof, A. K. Electrochim. Acta 2000, 45, 1701. (39) Zhou, Y. A.; Zhang, X. Y.; Nian, H. E.; Pang, Y. N.; Wang, H. B. Acta Chimica Sinica 2010, 68, 1399. (40) Dovesi, R.; Orlando, R.; Civalleri, B.; Roetti, C.; Saunders, V. R.; Zicovich-Wilson, C. M. Z. Kristallogr. 2005, 220, 517. (41) Dovesi, R.; Saunders, V. R.; Roetti, C.; Zicovich-Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M.; University of Torino: 2009. (42) Orlando, R.; Dovesi, R.; Roetti, C. J. Phys.: Condens. Matter 1990, 38, 7769. (43) Gatti, C.; Saunders, V. R.; Roetti, C. J. Chem. Phys. 1994, 101, 10686. (44) Nada, R.; Catlow, C. R. A.; Pisani, C.; Orlando, R. Modell. Simul. Mater. Sci. Eng. 1993, 1, 165. (45) Merawa, M.; Labegurie, P.; Ugliengo, P.; Doll, K.; Dovesi, R. Chem. Phys. Lett. 2004, 387, 453. (46) Dovesi, R.; Roetti, C.; Freyria Fava, M.; Prencipe, M.; Saunders, V. R. Chem. Phys. 1991, 156, 11. (47) Pascale, F.; Zicovich-Wilson, C. M.; Gejo, F. L.; Civalleri, B.; Orlando, R.; Dovesi, R. J. Comput. Chem. 2004, 25, 888. (48) Zicovich-Wilson, C. M.; Torres, M. R.; Pascale, F.; Valenzano, L.; Orlando, R.; Dovesi, R. J. Comput. Chem. 2008, 29, 2268. (49) Dall’Olio, S.; Dovesi, R.; Resta, R. Phys. Rev. B: Condens. Matter 1997, 56, 10105. (50) Resta, R. Rev. Mod. Phys. 1994, 66, 809. (51) Hartman, M. R.; Rush, J. J.; Udovic, T. J.; Bowman, R. C.; Hwang, S. J. J. Solid State Chem. 2007, 180, 1298. (52) Filinchuk, Y.; Hagemann, H. Eur. J. Inorg. Chem. 2008, 20, 3127. (53) Matsumoto, K.; Hagiwara, R.; Mazej, Z.; Goreshnik, E.; Zemva, B. J. Phys. Chem. B 2006, 110, 2138. (54) Brunton, G. Acta Crystallogr., Sect. B: Struct. Sci. 1968, 24, 1703. (55) Brunton, G. Acta Crystallogr., Sect. B: Struct. Sci. 1969, B25, 2161. (56) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part A: Theory and Applications in Inorganic Chemistry, 6th ed.; Wiley: NJ, 2009; Vol. A. 18899



ARTICLE

(57) Fateley, W. G.; McDevitt, N. T.; Bentley, F. F. Appl. Spectrosc. 1971, 25, 155. (58) Carbonniere, P.; Hagemann, H. J. Phys. Chem. A 2006, 110, 9927. (59) Emery, A. R.; Taylor, R. C. J. Chem. Phys. 1958, 28, 1029. (60) Orimo, S.; Nakamori, Y.; Zuttel, A. Mater. Sci. Eng., B 2004, 108, 51. (61) Tomkinson, J.; Waddington, T. C. J. Chem. Soc., Faraday Trans. 1976, 272, 528. (62) Hagemann, H.; Gomes, S.; Renaudin, G.; Yvon, K. J. Alloys Compd. 2004, 363, 126. (63) Miwa, K.; Ohba, N.; Towata, S.; Nakamori, Y.; Orimo, S. Phys. Rev. B: Condens. Matter 2004, 69. (64) Padmaja, G.; Kistaiah, P. J. Phys. Chem. A 2009, 113, 2397. (65) Figen, A. K.; Piskin, S. Adv. Powder Technol. 2010, 21, 513. (66) Jun, L.; Shuping, X.; Shiuang, G. Spectrochim. Acta, Part A 1995, 51A, 519. (67) Wagner, E. L.; Hornig, D. F. J. Chem. Phys. 1950, 18, 305. (68) Plumb, R. C.; Hornig, D. F. J. Chem. Phys. 1953, 21, 366. (69) Vedder, W.; Hornig, D. F. J. Chem. Phys. 1961, 35, 1560. (70) Bates, J. B.; Quist, A. S.; Boyd, G. E. J. Chem. Phys. 1971, 54, 124.

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Vibrational Properties of MBH4 and MBF4 Crystals (M= Li, Na, K): A

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