X-ray and Synchrotron FTIR Studies of Partially Decomposed Magnesium Borohydride

Abstract

Magnesium borohydride (Mg(BH₄)₂) is an attractive compound for solid-state hydrogen storage due to its lucratively high hydrogen densities and theoretically low operational temperature. Hydrogen release from Mg(BH₄)₂ occurs through several steps. The reaction intermediates formed at these steps have been extensively studied for a decade. In this work, we apply spectroscopic methods that have rarely been used in such studies to provide alternative insights into the nature of the reaction intermediates. The commercially obtained sample was decomposed in argon flow during thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC) to differentiate between the H₂-desorption reaction steps. The reaction products were analyzed by powder X-ray diffraction (PXRD), near edge soft X-ray absorption spectroscopy at boron K-edge (NEXAFS), and synchrotron infrared (IR) spectroscopy in mid- and far-IR ranges (SR-FTIR). Up to 12 wt% of H₂ desorption was observed in the gravimetric measurements. PXRD showed no crystalline decomposition products when heated at 260–280 °C, the formation of MgH₂ above 300 °C, and Mg above 320 °C. The qualitative analysis of the NEXAFS data showed the presence of boron in lower oxidation states than in (BH₄)⁻. The NEXAFS data also indicated the presence of amorphous boron at and above 340 °C. This study provides additional insights into the decomposition reaction of Mg(BH₄)₂.

1. Introduction

Hydrogen is the energy that fuels the sun and other stars. With the potential of low-carbon production from renewable energies, and with the largest energy content by weight among chemical fuels, hydrogen is considered to be one of the most likely pillars for future economies [1,2]. At the same time, being the lightest and the least dense amongst the fuels, hydrogen gas requires significant compressing for storage and transportation. Currently, molecular hydrogen is mostly stored in liquid and pressurized forms that brings about issues of energy losses, safety risks, and expensive composite storage tanks. The idea of solid-state hydrogen storage is a lucrative alternative to overcome the challenges [3,4,5,6,7,8]. In this storage method, hydrogen is chemically incorporated in a lattice of a compound with the volumetric hydrogen densities often higher than those in liquid molecular hydrogen (71 g∙L⁻¹). Most of these compounds are stable at room temperature (RT) and above, thus bypassing the need for cryocooling. Moreover, they can be formed at pressures lower than those currently utilized in the hydrogen storage tanks (350–700 bar).

In this context, magnesium borohydride (Mg(BH₄)₂) is recognized as an attractive hydrogen storage compound due to its high hydrogen content (14.9 wt%, 82–147 g_H2/L), and relatively low H₂ release temperature that has the potential to be further reduced. In particular, experiments show that the decomposition onset of Mg(BH₄)₂ requires temperatures of at least 200 °C in order to break the rather strong B-H bonds [9,10]. However, H-D isotopic exchange studies have shown that the B-H bond in Mg(BH₄)₂ can be perturbed at temperatures as low as around 100 °C and pressure of 3 bar H₂ [11,12].

The decomposition reaction of Mg(BH₄)₂ suggests multiple steps with amorphous intermediate reaction products [13,14,15,16,17,18,19]. Mg(BH₄)₂ exists in multiple phase polymorphs. If the decomposition starts from the γ-phase, for example, as in the present work, two phase transitions between 145–215 °C are observed prior to decomposition [20]. At temperatures between 215–330 °C, the first hydrogen desorption is observed with the production of amorphous B-H species. In the next stage, the formation of crystalline MgH₂ takes place between 340–355 °C, which is then followed by the formation of (i) amorphous boron (B) at a temperature between 355–372 °C and (ii) MgB₂ from 372–420 °C. As suggested by Guo et al. [13], the undesirable by-product diborane can form at about 280 °C along with various amorphous B-H species and desorbed H₂ that can interact with each other in multiple steps during the decomposition reaction. Most studies indicate an approximately correct theoretical weight loss for the completely decomposed Mg(BH₄)₂, except for decomposition in a high dynamic vacuum [18], that rules out the stochiometric release of undesirable borane gaseous products. Some researchers suggest the formation of MgB₁₂H₁₂ [21] and similar higher borane compounds that would hinder rehydrogenation and reversibility, although these have not been observed consistently. The formation of Mg(B₃H₈)₂ was also suggested [9,22]. However, for most alkali- and alkaline-earth borohydrides, the triborane, B₃H₈⁻, decomposes to borohydride, BH₄⁻, and not vice versa [23]. It was found that the presence of an additional hydrogen source, such as MgH₂, is essential for the selective BH₄ → B₃H₈ reaction. In the absence of MgH₂, Mg(B₃H₈)₂ does form, but in lower amounts. A recent theoretical study accessed thermodynamic stabilities and the intrinsic strengths of various intermediates in the decomposition of borohydrides, emphasizing their decisive role in hydrogenation and dehydrogenation reactions [24]. Thus, the decomposition pathway of Mg(BH₄)₂ has been extensively studied through the past decade, adding knowledge of its hydrogen storage properties. The consensus on the decomposition pathway and the nature of the decomposition products has not been reached.

Rehydrogenation of completely decomposed Mg(BH₄)₂ requires high temperature (<400 °C), high H₂ pressure (950 bars), and long reaction time (72 h) and yields > 11 wt% of H₂ [25]. However, the decomposed Mg(BH₄)₂ at the step of the amorphous intermediates can be rehydrogenated at comparatively lower temperatures between 250 and 285 °C, pressure of 120 bar H₂, and shorter reaction times. In this case, about 3 wt% can be cycled for at least three cycles [19]. This amount is lower than the weight loss observed for the decomposition step in this temperature range. One of the reasons behind the incomplete rehydrogenation at this step might be the formation of several decomposition products, only some of them being reversible back to Mg(BH₄)₂. The formation of MgO at the expense of Mg(BH₄)₂ might also be responsible for the reduced reversibility [26]. Therefore, to enhance the release of H₂ on reversibility, it is important to understand the nature of the intermediate amorphous reaction products.

Decomposition and rehydrogenation reactions in hydrides have been typically studied with diffraction techniques in combination with thermogravimetric and calorimetric studies (TGA-DSC). The first method is an excellent approach that allows us to determine the phase composition and crystal structure of the crystalline phases in the samples. In situ studies, in particular at synchrotron radiation facilities, allow for the following changes with sub-minute time resolution in wide temperature and hydrogen pressure ranges. The TGA-DSC methods allow us to differentiate between various thermally induced events, for example, phase transitions, melting, and decomposition steps, and to evaluate the amount of released gaseous products as well as to assess the thermodynamic and kinetic aspects of the reactions. This combination of the two is, however, not usually sufficient to characterize the reactions with amorphous intermediates. Thus, other techniques, in particular various spectroscopies combined with DFT calculations, have been applied for additional structural characterization of the decomposition of Mg(BH₄)₂ and similar compounds. Vibrational spectroscopies, such as Raman scattering, Fourier-transformed infrared (FTIR) and inelastic neutron scattering (INS), identify molecular bonds and lattice vibrations. The majority of the FTIR studies on Mg(BH₄)₂ report the data in the mid-infrared (mid-IR) region as summarised by us earlier [10]. Vitillo et al. also presented far-infrared (far-IR) data at room temperature [18]. Pure Mg(BH₄)₂ exhibits well-defined and described IR spectra. The ex situ mid-infrared measurements of the amorphous decomposition products showed broad features in the B-H bending and stretching regions partially leading to inconclusive results [18,19,26]. Measurements at lower temperatures might reduce thermal effects and narrow the absorption bands. Our attempts to access these phases with in situ (and ex situ) Raman spectroscopy revealed strong fluorescence of the samples with 532 nm [20], but also with 457, 633, and 785 nm laser wavelengths (unpublished results). X-ray absorption spectroscopy (XAS) is also a suitable tool to study the local order around a chosen element. XAS has, at times, been applied to follow the local structure of additive, catalysts and dopants in Mg(BH₄)₂ upon H₂ desorption and absorption reactions [19,27]. In this case, the elements of interest are mostly transition metals, and hard X-rays are employed with rather simple experimental set-ups and good quality results. XAS measurements at B K-edge of the decomposition phases of Mg(BH₄)₂ and other borohydrides are scarce, although they could shed more light onto the local structure of boron anions in hydrogenation and dehydrogenation reaction products. The reason for this is the use of soft X-rays and an ultra-high vacuum for the most typical experimental setups, posing notable experimental challenges. In addition, Mg(BH₄)₂ is air-sensitive and special care must be taken in order to not introduce contamination during the sample preparation and transfer or into the measurements. Ray et al. have studied the H₂ adsorption reaction in MgB₂ with B K-edge XAS but found only direct formation of BH₄⁻ anions, which is certainly not the case of the reverse dehydrogenation reaction [28]. In the subsequent work, the team investigated the decomposition of Mg(BH₄)₂ with B K-edge XAS and concluded that γ-Mg(BH₄)₂ followed multiple reaction pathways in both the dehydrogenation and rehydrogenation processes [29]. Sahle et al. [30] employed inelastic X-ray scattering at the B and Mg edges for in situ studies of Mg(BH₄)₂ decomposition. They observed the formation of reaction intermediate(s) at 300 °C upon hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB₁₂H₁₂. Higher boranes and MgH₂ were found between 300 °C and 400 °C with the boron atoms in similar electronic states to that in B₁₂H₁₂ anions, with subsequent decomposition into the constituting elements afterwards.

The described knowledge gaps in the nature of boron-based amorphous intermediates prompted the current study. In this work, the reaction products formed at different reaction steps during the partial decomposition of γ-Mg(BH₄)₂ in the temperature range 240–400 °C have been studied by using combined thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), powder X-ray diffraction (PXRD), and synchrotron-based in situ FTIR (SR-FTIR) and X-ray absorption spectroscopy (XAS), in order to identify the intermediate reaction products in the decomposition reaction of γ-Mg(BH₄)₂. We measured these samples with SR-FTIR in the mid-IR and far-IR regions at cryogenic temperatures with the aim of solving the problem of peak broadening in the RT FTIR spectra and enable comparison of the results with the theoretically predicted spectral modes of various B_xH_y clusters [18,24]. The far-IR spectra of H₃BO₃ (boric acid) are reported here, to the best of our knowledge, for the first time and can be used as a reference for future studies in the related compounds. X-ray absorption spectra are expected to shed more light on the local structure of boron atoms in the intermediate decomposition phases of Mg(BH₄)₂.

2. Materials and Methods

Commercial gamma magnesium borohydride (γ-Mg(BH₄)₂) (Sigma-Aldrich, Sigma-Aldrich, St. Louis, MO, USA, 95%) was used to prepare the samples.

Amorphous boron powder (B) (purity >95%, sigma-Aldrich), boric anhydride (B₂O₃,) (>99.98, Sigma-Aldrich), magnesium boride powder (MgB₂) (>99%, boric acid (H₃BO₃) (ACS reagent, >99.5%, Sigma-Aldrich), and lithium borohydride (LiBH₄) (>90%, Sigma-Aldrich) were used as references.

Thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC): partially decomposed samples of Mg(BH₄)₂ were obtained by heating the as received compound at different decomposition temperatures between 240–400 °C in the instrument for thermogravimetric analysis combined with differential scanning calorimetry TGA/DSC 3+ from Mettler Toledo. The samples were heated at 5 °C/min up to the required temperature and then left to decompose isothermally until no weight loss was observed. The experiments were carried out in Ar flow of 200 mL per minute in alumina (Al₂O₃) crucibles with pierced lids. The sample was filled into crucibles inside a glove box and transferred into the TGA-DSC that was placed outside the glove box within a few seconds.

Powder X-ray diffraction (PXRD) data were collected at a Bruker D8 Advance diffractometer with copper X-ray source of wavelength (λ) 1.5418 Å. The samples were sealed in glass capillaries whilst inside an Ar atmosphere, and measured in the range 2ϴ = 11–60° with a constant rotation rate of 120 rpm. Data were collected in steps of 0.01° with 0.2 s per step.

Synchrotron X-ray absorption spectroscopy (XAS) data were collected at the dipole magnet RGBL beamline at the Helmholtz-Zentrum Berlin für Materialien und Energie synchrotron radiation facility (Berlin, Germany). The samples were loaded onto stainless-steel sample holders covered by indium tape inside the Ar-filled glove box. Then, they were transferred into the measurement chamber without contact with air. The measurements were performed in an ultra-high vacuum, in total electron yield (TEY) mode. The samples were measured at 45° to the X-ray beam, in 160–260 eV range with 0.4–0.5 eV resolution and 10 s per step.

The experimental values were obtained as the total electron yield (TEY). Incident flux, I₀, was obtained from the measurements of reference Au foil: (i) the measured TEY from the Au foil was normalized to the ring current, (ii) the TEY Au signal was normalized to the absorption cross-section of gold:

$$flux\left(I_0\right)=\frac{TEY\left(Au\right)}{\left\{ring current\right\}·{\sigma }_a^{Au}}$$

Once $\left(I_0\right)$ was obtained, the total absorption cross-section for the sample $({\sigma }_a^{sample})$ was calculated by dividing the normalized TEY of the sample to the ring current by the incident $flux\left(I_0\right)$ as given below:

$${\sigma }_a^{sample}=\frac{TEY\left(Sample\right)}{\left\{ring current sample\right\}·flux\left(I_0\right)}$$

Next, the absorption cross-section for each sample was modified by subtracting the user-defined background by taking some points from the pre-edge and the post-edge region of the spectrum. The intensity of the spectrum thus obtained was then normalized between [0, 1] to get the final spectrum.

Synchrotron Fourier-transformed infrared spectroscopy (SR-FTIR) were performed at IRIS beamline of the the Helmholtz-Zentrum Berlin für Materialien und Energie synchrotron radiation facility (Berlin, Germany). A first set of mid-IR and far-IR spectra was recorded in the temperature range RT-10 K on: (i) two reference materials: H₃BO₃ and Mg(BH₄)₂; and (ii) Mg(BH₄)₂ decomposed at 260 °C and 280 °C. The samples were diluted in CsI for the mid-IR measurements and in wax for the far-IR measurements. The mid-IR spectra were obtained with the globular source of a Bruker Vertex instrument whereas a synchrotron beam was used to record the far-IR spectra. Every spectrum was obtained with a spectral resolution of 4 cm⁻¹ averaging 1024 scans. All the pellets have been prepared in a MBraun glove box where the level of water and O₂ were kept <0.5 ppm and <1.5 ppm, respectively, during use but exposed to air for a short time while transferring them to the cryostat.

3. Results

3.1. TGA-DSC Analysis of Decomposition of γ-Mg(BH₄)₂

The TGA-DSC profiles of the γ-Mg(BH₄)₂ decomposition in Ar flow are presented in Figure 1 and indicate the following:

The weight losses distinctively indicate several reaction steps in the decomposition reactions (Figure 1a).

• Some weight loss, ~0.5 wt%, occurred between RT and the phase-transition temperature region (T_ons = 152 °C).
• ~0.8 wt% occurred during the two phase transitions at 163 and 196 °C.
• The number of the reaction steps is the same for the 240 °C and 260 °C isotherms. The DSC profiles of these reactions show the onset of the first decomposition reaction peak (I). This peak is more pronounced in the 280 °C isotherm and is complete in the 300 °C isotherm. Thus, similar reaction products can be expected after the decomposition at 240 and 260 °C, and slightly different at 280 °C. These reaction products decompose as shown by the strong exothermic peak at 305 °C. Moreover, since a larger weight loss was observed at 260 °C than that at 240 °C, we could expect the presence of Mg(BH₄)₂ in the latter sample due to the incomplete decomposition reaction at this step. The final weight loss for reaction step I was, thus, observed as 8.8% after decomposition at 300 °C for 5 h when the step was complete.
• The DSC profile of the isotherm obtained at 320 °C shows the onset of yet another peak (II). The weight loss at this temperature was larger (9.9 vs. 8.8% than that at 300 °C) Thus, the reaction products are expected to be somewhat different after the 320 °C isotherm.
• Between 320 and 340 °C, the reaction step II was completed, and two new reactions corresponding to the thermal events III and IV occurred where the latter one was incomplete. The reaction step IV was completed during the decomposition at 400 °C. Thus, yet another and different reaction products can be expected after the decomposition after 340 and 400 °C.
• The most significant weight losses occurred during the decomposition at 300 and 340 °C. These should correspond to the formation and decomposition of MgH₂.
• The decomposition at 400 °C was not complete as it resulted in 12 wt% weight loss compared to the 14.9 wt% of the theoretical content of H₂. A significant release of borane gases is, thus, not expected because it would correspond to larger weight losses with respect to the theoretical value. As all MgH₂ was decomposed at this stage, the remaining hydrogen might be trapped in higher borane intermediates.
• Some isotherms showed weight gain at elevated temperatures. This could likely be related to oxygen contamination and formation of MgO. For these isotherms, the weight loss values (summarized in Table 5) refer to the maximum observed values.
• The TGA-DSC results are discussed in Section 4 after the analysis of the decomposition phases obtained at each of the steps.

3.2. PXRD Analysis of the Reaction Products

The powder X-ray diffraction patterns of the starting and decomposed samples, recorded ex situ, are shown in Figure 2. The results reveal the absolute prevalence of γ-Mg(BH₄)₂ in the starting material, presence of ht-Mg(BH₄)₂ after the 240 °C isotherm [20], but no crystalline phases after decomposition at 260 °C. MgH₂ appeared at 300 °C, though traces can be distinguished already after 280 °C, and decomposed into Mg after 320 and 340 °C. In addition, MgO was observed in the patterns of the samples treated at and above 320 °C. A broad background that might be related to amorphous phases was observed in some of the patterns.

3.3. Synchrotron FTIR Analysis of the Decomposition Reaction Products

The samples measured with SR-FTIR included pristine Mg(BH₄)₂, the borohydride decomposed at 260 °C and 280 °C, and H₃BO₃ as the reference for oxygen-related impurity phases. Mid-IR and far-IR spectra, recorded at 10 K, of pristine Mg(BH₄)₂ and after its decomposition at 260 °C and 280 °C along with the spectrum of H₃BO₃ (blue curve) are reported in Figure 3.

The spectra of H₃BO₃ were recorded as a reference for possible impurities. In addition, upon contamination with atmospheric water, Mg(BH₄)₂ can hydrolyze to Mg(BO₂)₂ as [31]:

Mg(BH₄)₂ + 4H₂O → Mg(BO₂)₂ + 8H₂

(1)

In H₃BO₃, boron atoms are surrounded by three oxygen atoms in planar coordination (three coordinated boron, B_tri). In magnesium diborate Mg(BO₂)₂, boron bonds with four oxygen atoms in tetrahedral coordination, B_tetra. The anion consists of two BO₄ tetrahedrons with an oxygen bridge. In tri-, tetra-, penta-, and hexaborates both B_tri and B_tetra are present in various combinations [32].

Table 1. Vibrational frequencies for the normal modes of isolated planar BO₃ (D_3h) and tetrahedral BO₄ (T_d) groups in crystalline state [33]. All of the frequencies are in cm⁻¹.

Group	ν1	ν2	ν3	ν4
[BO3]3−	1060 (R) 1	668, 648 (IR)	1490–1428 (R, IR)	545 (R, IR)
[BO4]5−	880 (R)	371 (R)	886 (R, IR)	627 (R, IR)

¹ R—Raman active, IR—infrared active.

lists the normal modes of these two anions whereas

Table 2. FTIR features of H₃BO₃ (planar B_tri-O groups) and Mg(BO₂)₂ · 3H₂O (tetrahedral B_tetra-O groups) [32] in comparison with our data for H₃BO₃. All of the frequencies are in cm⁻¹.

H3BO3	Mg(BO2)2 · 3H2O	H3BO3
Ref. [32]	Ref. [32]	This work
	3571, vs 1
	3491, vs
3202, br	3179, vs	3160 vs, br
2515, m	2531, m	2550 m, br
2362, m		2388 m
2262, s		2283 m
2030, w		2046–2010 w
1423 (ν3) 2, s, br		1448 vs, br
	1297, vs	1245 w
	1221, m
1193 (ν3?), vs	1163, vs	1212 vs
	1042, vs
	953, vs
883, m	873 (ν3), vs	886 m
811, m	802, w	815 m, br
644 (ν2), s
546 (ν4), vs	586 (ν4), vs	550, s
n/a		79, m

¹ vs—very strong, s—strong, m—medium, w—weak, br—broad, sh—shoulder; ² ν_s,_a—symmetric, asymmetric stretching; δ—bending; data are from ref. [32].

shows the list of the peaks in experimentally observed FTIR spectra of the two compounds.

The most prominent differences between the H₃BO₃ and Mg(BO₂)₂ · 3H₂O are the peaks at 1423 (s) and 1193 (vs) cm⁻¹ (for H₃BO₃) and 1297 (vs) and 1042 (vs) cm⁻¹ (Mg(BO₂)₂ · 3H₂O). These features are taken here as the fingerprint for the presence of such groups in the samples. The peak positions observed in the spectra of H₃BO₃ are blue-shifted with respect to the literature data. This is likely associated with the difference in the diluting matrix, CsI here and KBr in Ref. [32]. In the far-IR spectrum of H₃BO₃, only one peak is observed at 79 cm⁻¹ that, due to its low energy, could be assigned to the translational motions of the whole borate unit.

The mid-IR spectrum of Mg(BH₄)₂ is dominated by the characteristic peaks, due to stretching and bending of B-H bonds in BH₄⁻ ions with distorted tetrahedral local symmetry [10]. These peaks are listed in

Table 3. FTIR features of γ-Mg(BH₄)₂ from literature and this work (orange curve on Figure 3). All of the frequencies are in cm⁻¹.

Observed Peak Refs. [10,11]	Mode	Assignment 2	This Work
n/a	ν1	νs (B-H)	−
1370, m, br 1	ν2	δs (H-B-H)	−
2270 ± 2, vs	ν3	νa (B-H)	2300 s, br
1260, s	ν4	δa (H-B-H)	1276 s, br
1120 ± 10, m	ν4	δa (H-B-H)	1144 s, br
2660, m	ν2 + ν4	overtone	2672 m
434, m		Mg-B stretching	455 m, br
409, m		Mg-B stretching	425 m, br
257, 231, 208, m		B-Mg-B bending	240 w, 210 m

¹ vs—very strong, s—strong, m—medium, w—weak, br—broad, sh—shoulder; ² ν_s,_a—symmetric, asymmetric stretching; δ_s,a—symmetric, asymmetric bending; data and assignment are from refs. [10] (mid-IR), and [18] (far-IR).

. The symmetric stretching ν₁ is probably too weak, even though it can be observed theoretically, whereas the asymmetric stretching ν₄ is visibly split due to the symmetry distortion of the BH₄⁻ tetrahedra in Mg(BH₄)₂.

In the far-IR spectrum of Mg(BH₄)₂ (Figure 3b, orange curve), only the weak feature at 79 cm⁻¹ is similar to the one in H₃BO₃. Other peaks are similar to those reported earlier [18]. It can be mentioned that most of the measurements on borohydrides and similar compounds reported in the literature have been performed using attenuated total reflection (ATR) sampling, a method where no sample dilution is required. In this study, the samples were measured in the transmission mode, diluted in CsI or wax, and thus, the effect from the matrix can be expected in the spectra in addition to a slight shift of the peaks and slightly different intensity ratios between higher-energy and lower-energy modes [34]. For example, the broad absorptions at 3600–3300 with 1620 cm⁻¹ (water) and the weak peaks at ca. 2900 cm⁻¹ (C-H bonds of organics) are most probably related to the impurities in the matrix.

In the decomposed samples (Figure 3), the peaks due to BH₄⁻ disappeared to the benefit of B-H vibrations of “higher” boron hydride compounds. The peaks after decomposition at 280 °C and 260 °C are very similar and are listed in

Table 4. Experimentally observed peaks in the FTIR spectra of γ-Mg(BH₄)₂ decomposed at 260 °C and 280 °C. All of the frequencies are in cm⁻¹.

Peak No.	Energy, cm−1	Tentative Assignment
1	2510 s, br	ν(B-H)
2	2410, s	ν(B-H)
3	2375, m	ν(B-H)
4	2320, m	ν(B-H), /ν(Mg-H-B)
5	2140, w	ν(B-H-B)
6	2075, w	ν(B-H-B)
7	1620, m	δ(H-O-H)/δ(B-H)
8	1460, s, br	νa(Btri-O)/δ(B-H)
9	ca. 1230, br	νs(Btri-O)/ν(Btetra-O)/δ(B-H)
10	~1030, s, br	ν(Btetra-O)/δ(B-H)
11	930, sh	ν(B-B)/δ(B-H)
13	79, m	T (Btri-O)

. The features of H₃BO₃ might be partially responsible for the observed peaks 8–10, however the spectra obviously contain contributions from other species. Notably, the weak features at 2130 and 2075 cm⁻¹ not present in the pristine Mg(BH₄)₂ or H₃BO₃ / Mg(BO₂)₂ are suggesting that the boranes with bridged hydrogen atoms in the structure are formed at this stage. These could be (B₂H₇)⁻, (B₃H₇)²⁻ [22], (B₃H₈)⁻ [22,35,36], (B₄H₁₀)²⁻ [18], etc. These peaks are accompanied by the peaks at > 2300 cm⁻¹, which are characteristic for the stretching of terminal hydrogen atoms of such boron hydride anions and Mg-H-B stretching [37].

There are several high-frequency peaks above and below 2400 cm⁻¹ (1–4) corresponding to the stretching modes of bonds involving boron and terminal hydrogen atoms, B-H_term. This was observed in case of (B₃H₈)⁻ ions that comprise different types of boron atoms: two equivalent at the base (B1) and one in the third corner (B2) of an isosceles triangle [23]. All of the three atoms have two terminal hydrogens, but the bonds have slightly different energy roughly divided in the regions above and below 2400 cm⁻¹. In CsB₃H₈, the B1-H_term stretching was found at 2360–2325 cm⁻¹, and the stretching of B2-H_term at 2405–2467 cm⁻¹ [23] with 2405 cm⁻¹ being the most prominent feature. The stretching of the bridged hydrogens (B-H-B) was observed at 2120 and 2080 cm⁻¹. In η²-Mg(B₃H₈)₂(THF)₂, the respective absorptions were recorded at 2383, 2432 for the terminal, and 2115 and 2069 cm⁻¹ for the bridged units [37]. In addition, the 2280 and 2335 cm⁻¹ peaks were assigned to bridged Mg–H–B stretching. Neither work reported vibrations above 2435 cm⁻¹ for the (B₃H₈)⁻. The observed absorptions correspond well with the modes 1–6 in Table 4, but we are left with 2510 cm⁻¹ unaccounted for and an absence of a feature responsible for the Mg-H-B interactions. In addition, the lower frequency modes of (B₃H₈)⁻ might be referred from the broad absorptions in Figure 3a. However, also in this region, a few unaccounted-for modes remain. In particular, H-B-H bending modes were observed at 1168, 1000, 802, and 782–732 cm⁻¹ in ref. [23] with no peaks at 1460, or 930 cm⁻¹, as in our study. (B₂H₇)⁻ ions have distinct features at 2405, 2050 (broad), and 1030 cm⁻¹ (sharp) [38] but not in the other peaks observed here. In addition, this anion is very unstable and has only one hydrogen bridge, hence one corresponding vibration. Some boranes and borane anions, such as B₅H_x (x = 9, 11) have strong IR absorptions in the region 1620–1400 cm⁻¹ assigned to B-H bending modes [39,40,41]. At the same time, the stretching modes of terminal B-H in the pentaboranes are shifted to higher frequencies with respect to the modes 1,2 in Table 4 (>2548, and 2460 cm⁻¹), one of the ν(B-H-B) is located below 2000 cm⁻¹ (clearly absent in Figure 3a); a few prominent lower frequency modes do not correspond well with features 9–11 from Table 4 (1164, 1043, 951, 899 cm⁻¹ for B₅H₁₁ [39] and 1045–1035, 900 cm⁻¹ for B₅H₉ [40]). In B₄H₁₀, the most prominent structural feature is the dual pair of bridges at 2120 cm⁻¹ holding the molecule together [39]. The intensity of these modes is as strong as ν(B-H), which is certainly not the case for the weak features 5,6. The stretching of terminal B-H₂ is found at 2550 and 2455 in tetraborane, the prominent low-frequency absorptions include 1380 (s), 1145 (s), 1067 (m), 967 (s) cm⁻¹, and a group at 850 cm⁻¹. (B₄H₁₀)²⁻ [18] is proposed as a plausible intermediate. It does not have bridged hydrogens, but its other theoretically calculated modes might correspond to the spectra observed here. The data in Table 4 does not correspond to the FTIR features of the B₆H₉⁻ anion [41].

The broad absorption in the 2440–2630 cm⁻¹ region can include stretching of B_nH_n²⁻ (n = 6, 8, 9, 10, 11, 12) anions [42]. B₉H₉⁻: 2529, 2450, and 2415 cm⁻¹, B₁₂H₁₂⁻: 2485 (strong, s), 1071 (m), and 719 (m) cm⁻¹ [43]; B₁₀H₁₀²⁻ salts have a very strong absorption at 2470 cm⁻¹ (equatorial B-H stretch) with a shoulder at 2530 cm⁻¹ (apical B-H stretch); 1015 cm⁻¹ and 1070 cm⁻¹ were attributed to cage deformational (ibid). B₆H₆⁻ has features at 2432, 1051, and 731 cm⁻¹ [43]. In summary, closo-boranes B_nH_n²⁻ (n = 6, 8, 9, 10, 11, 12) do not have peaks in the region 1200–2500 cm⁻¹ [42], and at ca. 900 cm⁻¹.

In summary, the FTIR spectra of the Mg(BH₄)₂ decomposed at 260 °C and 280 °C indicate: (i) formation of the same species at both temperatures; (ii) formation of several anionic species with the most probable candidates being B₃, B₅ borohydrogens with bridged H-B-H groups; (iii) the assignment is hindered by the presence of borate impurities.

We observed two different behaviors for the mid-IR and far-IR spectra upon decreasing the temperature. In fact, while the position and FWHM of the mid-IR bands remain almost unchanged in the whole temperature range (not shown), all the far-IR signals showed a decrease in their FWHM (see Figure 4 where the effect of the temperature on the band at 70 cm⁻¹ is shown).

Unfortunately, the extinction coefficient of the species originating from the decomposition of Mg(BH₄)₂ (besides borates) was so low in the far-IR region that their signal remained very broad and not so clearly distinguishable from the background also at 10 K, although we have used the highest concentration possible based on sample availability. It would be important to repeat the experiment using pure samples without any diluent.

3.4. Soft X-ray Absorption Spectroscopy at the B K-Edge of the Decomposition Products

XAS measurements were carried out with a TEY approach that is based on measuring the sample current produced by Auger and photoelectrons. For low Z-number elements and concentrated samples, the TEY mode is favorable compared to the total fluorescence yield mode (TFY) as the fluorescence becomes greater than the Auger yield of elements with Z > 30. Additionally, the TFY spectra can be distorted due to self-absorption in concentrated samples. On the other hand, the TFY method is more sensitive to bulk properties as it probes about 200 nm–5 μm Å [28], whereas the TEY method is determined primarily by the penetration ranges of Auger electrons that have ranges of tens to hundreds of nm [44]. The probed sample thickness, R_p, can be approximately determined through the relation:

R_p ≈ 1000·E^1.4/ρ

where E is an initial electron energy in keV, ρ is the material’s density in g/cm³ and R_p is given in nm. According to this relation, the samples probed at the B K-edge energy range can be estimated as 14–28 nm for porous γ-Mg(BH₄)₂, 10–20 nm for the denser phases of Mg(BH₄)₂ and perhaps for the MgB_xH_y intermediates, and 5–11 nm for H₃BO₃, etc., i.e., in the range from a few to a few tens of nanometers.

For the qualitative data analysis, spectra of Mg(BH₄)₂ decomposed at various temperatures were compared with each other and the spectra of the reference compounds: boron (B), boric acid (H₃BO₃), boron oxide (B₂O₃), fresh gamma magnesium borohydride (γ-Mg(BH₄)₂), lithium borohydride (LiBH₄) and magnesium boride (MgB₂). Figure 5a,b show the experimental NEXAFS B K-edge spectra for the references and the samples, respectively. The XAS spectra of boric acid and boron oxide were analyzed as the reference for potential impurities in the spectra of Mg(BH₄)₂ and MgB_xH_y. In many compounds with boron in trigonal coordination and an empty 2p_z orbital, a strong, sharp and low energy peak has been observed due to the 1s → 2p_z transition [45]. On the other hand, in high symmetry environments, such as icosahedra of elemental boron, polyhedra of borane and carborane complexes, and tetrahedral coordination, where 2p_z hybridization occurs, such a feature is not usually found, and the spectra are mostly characterised by a broad band at higher energy. Blau et al. [46] have proposed that NEXAFS spectra of boron with triply degenerate 2p orbitals should exhibit only one broad peak, whereas two peaks can be expected where 2p orbitals would split and 1s → 2p_z transition is possible. The latter transistion should be expected as a sharp feature at a lower energy.

Thus, in B_tri-O planar anions, the X-ray absorption at the boron K-edge promotes 1s → 2p_z (π*) electronic transitions characterized by a lower energy sharp prominent peak at 194 eV. In addition, a weak peak is observed at 197.5 eV stemming from a dipole-forbidden transition 1s → a₁’ σ* MO formed by B_3s, O_2s and O_2p. The forbidden transition activates due to the distortion of BO₃ from the perfect planar coordination as the O-B-O angles can vary in the 119.6–200.4° region. The second allowed broad feature around 198–205 eV consists of at least three distinct peaks at 200, 202 and 204 eV. They can be assigned either to the transitions of B 1s electrons to unoccupied B-O sigma antibonding (σ*) orbitals of e’ (B_2px + B_2py + O_2s + O_2p) symmetry [47] or to B_3p and B_4p rather than 2p interactions [48]. The NEXAFS spectra of B_tetra-O anions are characterized by the peaks at 198 eV (strong, B_1s → *σ_t2 (B_2p + O_2p) transition) and 200 eV (B_1s → a₁ (B_2s + O_2p) transition). The broad feature at 201.5 eV may be assigned to B_1s → *σ_t2 and B_1s → *σ_a1 where the distortion can lift the t₂ degeneracy and cause further splitting or broadening of the peak [47]. It has been noted that even the compounds where only B_tetra-O should nominally be present, the features of B_tri-O can be observed, as this coordination is more favorable near the surface and at defects [47,49]. The spectra of H₃BO₃ and B₂O₃ shown in Figure 5a predominantly include the features of B_tri-O.

Magnesium boride, MgB₂, has a hexagonal layered structure (space group P₆/mmm) with boron honeycomb sheets interlaying with the Mg triangular sheets. Thus, boron atoms are covalently bound to three neighboring B in-plane and have a formal oxidation state −1. Hybridization of p_x and p_y orbitals of B atoms results in the formation of two strong covalent σ bonds. In addition, B p_z orbitals form a π-bond with the 3s states of Mg [50]. Theory predicts an edge at 186.8 eV as a result of 2p_xy unoccupied final states above the Fermi level of MgB₂ [28], but a very weak peak was observed here (Figure 5a, orange curve). According to Ray et al., this can be explained by oxidation of the sample. Indeed, the spectrum of MgB₂ is dominated by the peak at 194 eV [28]. The peak at 186.8 eV, however, was considered a fingerprint for the formation of MgB₂.

The spectrum of the amorphous boron closely resembles that of the crystalline β-B₁₀₅ phase [51] reflecting a similar local structure of B₁₂ icosahedra where each boron atom is coordinated to five neighbors. The sharp feature at 194 eV stems from contamination with B_tri-O species as the spectra reported in literature do not contain such a peak [46]. The edge energy of 188 eV is lower than that in oxides, as expected, due to the lower oxidation state of boron with no formal charge. The peaks at 191 and 193 eV are characteristic of this structure.

The spectrum of LiBH₄ was measured along with Mg(BH₄)₂ to investigate the common features of BH₄⁻ ions in NEXAFS. In LiBH₄ and Mg(BH₄)₂, boron in sp³ hybridization is tetrahedrally coordinated to four H atoms. Interestingly, both the borohydrides have a peak at ca. 192 eV. However, the peaks have a complex and different structure, evidencing variations in the local environment of boron atoms. For example, a lower-energy shift in LiBH₄ may suggest less positive charge of boron atoms. The spectral structure of γ-Mg(BH₄)₂ (Figure 5a, red spectrum) with the first peak at about 192 eV and the sharp peak at 193 eV showed tetrahedral B coordination in BH₄⁻ as earlier studied by Jeong et al. [29].

Figure 5b shows NEXAFS spectra of decomposed Mg(BH₄)₂. One can immediately note that all the spectra demonstrate peaks due to boron oxides with planar anions evidencing sample oxidation.

At the same time, the region at 187–197 eV contains the information on other boron species. The spectra for the samples decomposed at 240 °C and 260 °C both showed the peak at 192 eV for the tetrahedral coordination of boron in BH₄⁻ suggesting Mg(BH₄)₂ residuals. The spectrum of the sample decomposed at 280 °C also showed a weak feature in that region. Other than that, the spectrum is of a rather poor quality, making it difficult to extract further conclusions. In the next spectrum (300 °C) we can observe a rather strong complex peak at ca. 192 eV, at the energies between those of amorphous boron and Mg(BH₄)₂. This suggests the presence of boron species in a lower oxidation state than in the (BH₄)⁻ and closer to zero. That feature disappears in the 320 °C sample though it is also possible that this sample is oxidised to a larger extent. The spectra of the 340 °C and 400 °C samples demonstrate, again, a strong broad peak at 191.6 eV of a slightly different shape. Together with the spectrum profile above 195 eV, these spectra are very similar to those of the amorphous boron with some difference in the structure of the 191.6 eV peak. MgB₂ was not found in the studied samples. However, this might also be due to the fact that the species was not present in the oxidised surface layer probed by TEY.

It can be noted that the different quality of the spectra might evidence different electronic properties of the samples as the data quality depends on the sample’s conductivity. Among those, the phases obtained at 280–320 °C have the worst quality and perhaps lowest conductivity. For higher decomposition temperatures, the noise decreases again. On the other hand, the noise can also be associated with the larger heterogeneity of the species in the 280–320 °C range.

4. Discussion and Conclusions

With a multi-technique experimental approach, we have analyzed different reaction phases in the partial decomposition of γ-Mg(BH₄)₂ in an Ar atmosphere. A maximum of 12% weight loss at 400 °C was measured by thermogravimetry, with the production of amorphous B-H species. Although no analysis of the desorbed gaseous species was carried out, the overall weight loss suggests that the main released gas was hydrogen since the desorption of boron species would correspond to larger weight losses. PXRD confirmed the absence of crystalline compounds in the partially decomposed samples at 260–280 °C temperatures, and formation of MgH₂ after decomposition at 300 °C with the subsequent incomplete decomposition to Mg until 400 °C. Formation of MgB₂ was not observed with PXRD and NEXAFS. SR-FTIR in transmission mode indicated the formation of bridged B-H-B species in the partially decomposed samples at 260–280 °C as well as the presence of several amorphous phases. Some contamination of the sample was observed, evidenced by the peaks of MgO in the PXRD patterns after 320 °C and boron oxide species in the NEXAFS and FTIR spectra. In the far-FTIR spectra, the peaks at the positions similar to that of H₃BO₃ were weak in the pure Mg(BH₄)₂ and more intense in the samples decomposed at 260–280 °C. To the best of our knowledge, we reported, for the first time in situ, (300–10 K) far-FTIR spectra of H₃BO₃ and Mg(BH₄)₂. Combining the results from TGA-DSC, NEXAFS spectrum at boron K-edge, and the PXRD report, the sample when decomposed from 240–300 °C comprises the tetrahedral coordination of B probably in terms of BH₄⁻ ions and B_4-xH_x species. The sample decomposed at 300 °C shows boron in oxidation states between those in (BH₄)⁻ and amorphous boron. Summarizing, our combined experimental approach added more insights to the decomposition reaction of γ-Mg(BH₄)₂ in Ar for the equilibrium reaction products as collected in

Table 5. Summary of the study of decomposition products of γ-Mg(BH₄)₂ in Ar.

Tdec (°C)	Maximum Weight Loss, wt% (TGA)	Released No. of H2 Molecules	Nominal Composition after H2 Desorption	Reaction Step (TGA-DSC)	Crystalline Reaction Products (PXRD)	FTIR	NEXAFS
240	2.1	1.1	MgB2H7	I	ht-Mg(BH4)2	n/a	Btri-O, (BH4)−
260	4.5	2.4	MgB2H5.6	I	No	- several BxHy phases- B-H-B species - B3-B5 clusters	Btri-O, (BH4)−
280	5.2	2.8	MgB2H5.2	I	No		Btri-O
300	8.8	4.7	MgB2H3.3	I	MgH2	n/a	Btri-O, boron species between (BH4)− and B0
320	9.9	5.3	MgB2H2.8	II	Mg, MgO	n/a	Btri-O
340	11.6	6.2	MgB2H1.8	III, IV	Mg, MgO	n/a	Btri-O, B0
400	12	6.5	MgB2H1.5	IV	Mg, MgO	n/a	Btri-O, B0

:

This constitutes a further step in the knowledge of the very complex decomposition mechanism of this attractive hydrogen storage material.