Fluoride substitution in LiBH₄ is studied by investigation of LiBH₄-LiBF₄ mixtures (9:1 and 3:1). Decomposition was followed by in-situ synchrotron radiation X-ray diffraction (in-situ SR-PXD), thermogravimetric analysis and differential scanning calorimetry with gas analysis (TGA/DSC-MS) and in-situ infrared spectroscopy (in-situ FTIR). Upon heating, fluoride substituted LiBH₄ forms (LiBH_4-xF_x) and decomposition occurs, releasing diborane and solid decomposition products. The decomposition temperature is reduced more than fourfold relative to the individual constituents, with decomposition commencing at T / °C = 80 °C. The degree of fluoride substitution is quantified by sequential Rietveld refinement and shows a selective manner of substitution. In-situ FTIR experiments reveal formation of bands originating from LiBH_4-xF_x. Formation of LiF and observation of diborane release implies that the decomposing materials have a composition that facilitates formation of diborane and LiF, i.e. LiBH_4-xF_x (LiBH₃F). An alternative approach for fluoride substitution was performed, by addition of Et₃N∙3HF to LiBH₄, yielding extremely unstable products. Spontaneous decomposition indicates fluoride substitution to have occurred. From our point of view, this is the most significant destabilization effect seen for borohydride materials so far.

Borohydrides have attained high interest in the past few years due to their high volumetric and gravimetric hydrogen content. Synthesis of di/trimetallic borohydride is a way to alter the thermodynamics of hydrogen release from borohydrides. Previously reported preparations of M(BH₄)₂ involved chloride containing species such as SrCl₂. The presence of residual chloride (or other halide) ions in borohydrides may change their thermodynamic behavior and their decomposition pathway. Pure monometallic borohydrides are needed to study decomposition products without interference from halide impurities. They can also be used as precursors for synthesizing di/trimetallic borohydrides. In this paper we present a way to synthesize halide free alkaline earth metal (Sr, Ba) and europium borohydrides starting with the respective hydrides as precursors. Two novel high temperature polymorphs of Sr and Eu borohydrides and four polymorphs of Ba borohydride have been characterized by synchrotron X-ray powder diffraction, thermal analysis, and Raman and infrared spectroscopy and supported by periodic DFT calculations. The decomposition routes of these borohydrides have also been investigated. In the case of the decomposition of strontium and europium borohydrides, the metal borohydride hydride (M(BH₄)H₃, M = Sr, Eu) is observed and characterized. Periodic DFT calculations performed on room temperature Ba(BH₄)₂ revealed the presence of bidentate and tridentate borohydrides.

Two reactive hydride composite systems, Ca(BH₄)₂–NaNH₂ and Mg(BH₄)₂–NaNH₂, were systematically studied by in situ synchrotron radiation powder diffraction, in situ Fourier transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry. Metathesis reactions between the amides and borohydrides take place in both systems between 100°C and 150°C yielding amorphous materials with the proposed composition M(BH₄)(NH₂). Simultaneously, a fraction of NaNH₂ decomposes to Na₃N and ammonia via a complex pathway. The main gas released under 300°C is ammonia for both systems, while significant amounts of hydrogen are released only above 350°C.

B₁₂H₁₂^2− can be formed during the thermal decomposition of metal borohydrides (M(BH₄)_x). Halogen ions such as fluoride or chloride can contribute to destabilize the BH₄^− ions. Hydride and fluoride mixed species like B₁₂H_nF_(12−n)^2− will be probable products after hydrogen release from mixed boro-hydride-fluoride (BH_xF_(4−x)^−) or borohydride-borofluoride systems (BH₄^−, BF₄^−). Various number of isomers are possible for B₁₂H_nF_(12−n)^2− (n = 2–11). DFT calculations were performed on isolated ions of all the possible isomers for (n = 0–3, 9–12), using B3LYP functionals and 6-31G(d,p) basis set. Relative stability, vibrational and NMR spectroscopy of these isomers are discussed and compared with available experimental data.

A bimetallic dodecaborate LiNaB₁₂H₁₂ has been successfully synthesized for the first time, through a sintering process of LiBH₄, NaBH₄ and B₁₀H₁₄. LiNaB₁₂H₁₂ has a cubic Pa-3 space group symmetry at room temperature, and transforms into a high temperature phase with Fm-3m symmetry at 488 K, which is lower than that of Li₂B₁₂H₁₂ and Na₂B₁₂H₁₂. The ionic conductivity at 550 K reaches 0.79 S/cm, which is approximately 8 times higher than that of Na₂B₁₂H₁₂ and 11 times higher than that of Li₂B₁₂H₁₂. The Li/Na compositional and thus an induced positional disorder in LiNaB₁₂H₁₂ are suggested to be responsible for the reduced phase transition temperature and the improved super ionic conductivity compared to its monometallic counterparts.

Borohydrides are actively considered as potential hydrogen storage materials. In this context fundamental understanding of breaking and forming B-H bond is essential. Isotope exchange reactions allow isolating some parts of this reaction without introducing major structural or chemical changes. Experiments were performed on Ca(BH₄)₂and Ca(BD₄)₂ as a function of temperature and pressure. A complete exchange can be realized in about 9h at 200 °C using a deuterium pressure of 20 bar. The activation energy, estimated using first order kinetics, for the forward reaction (Ca(BH₄)₂ → Ca(BD₄)₂) was found to be 82.1 ± 2.7 kJ/mol (P = 35 bar) and the one for the backward reaction (Ca(BD₄)₂ → Ca(BH₄)₂) was found to be 98.5 ± 8.3 kJ/mol (P = 35 bar). Pressure dependent study shows that the reaction rate increases with increasing pressure up to 35 bar. This behavior is consistent with first adsorption step prior to diffusion into the solid and isotope exchange according to the scheme described below.

Inorganic compounds with BH₄^- ions are the subject of many recent investigations in the context of potential hydrogen storage materials. In this work, Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectra of a series of reference and research compounds (including deuterated samples) are collected and made available to the research community.