Crystallization of SrAl12O19 Nanocrystals from Amorphous Submicrometer Particles.

Advanced instrumentation and modern analysis tools such as transmission electron microscopy (TEM) have led to phenomenal progress in understanding crystallization, in particular from solution, which is a prerequisite for the design-based preparation of a target crystal. Nevertheless, little has been understood about the crystallization pathway under high-temperature annealing (HTA) conditions. Metal oxide crystals are prominent materials that are usually obtained via HTA. Despite the widespread application of hydro-/solvothermal methods on the laboratory scale, HTA is the preferred method in many industries for the mass production of metal oxide crystals. However, poor control over the morphology and grain sizes of these crystals under extreme HTA conditions limits their applications. Here, applying ex-situ TEM, the transformation of a single amorphous spherical submicrometer precursor particle of SrAl12O19 (SA6) at 1150 °C toward a nanosized thermodynamically favored hexagonal crystal is explored. It is illustrated in real space, step by step, how both kinetic and thermodynamic factors contribute to this faceting and morphology evolution. These results demonstrate a nonclassical nucleation and growth process consisting of densification, crystallite domain formation, oriented attachment, surface nucleation, 2-dimensional (2D) growth, and surface diffusion of the atoms to eventually result in the formation of a hexagonal platelet crystal. The TEM images further delineate a parent crystal driving the crystal lattice and morphological orientation of a network of interconnected platelets.

Correction: Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications.

Correction for 'Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications' by Romain Moury et al., Dalton Trans., 2020, 49, 12168-12173, https://doi.org/10.1039/D0DT02170A.

Study of the Temperature- and Pressure-Dependent Structural Properties of Alkali Hydrido-closo-borate Compounds.

In this work, we report on the structural properties of alkali hydrido-closo-(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature-dependent X-ray diffraction experiments combined with density functional theory (DFT) calculations. The mechanical properties are determined via pressure-dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds, revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermodiffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure-induced phase transition for K2B12H12 from Fm3̅ to Pnnm symmetry around 2 GPa. A temperature-induced phase transition for Li2B10H10 was also observed for the first time by thermodiffraction, and the crystal structure determined by combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high-pressure and high-temperature phases) may be related via a group-subgroup relationship, with the notable exception of the room-temperature phase of Li2B10H10.

Fe4(OAc)10[EMIM]2: Novel Iron-Based Acetate EMIM Ionic Compound.

We synthesized and characterized a novel iron(II) aceto EMIM coordination compound, which has a simplified empirical formula Fe4(OAc)10[EMIM]2, in two different hydration forms: as anhydrous monoclinic compound and triclinic dihydrate Fe4(OAc)10[EMIM]2·2H2O. The dihydrate compound is isostructural with recently reported Mn4(OAc)10[EMIM]2·2H2O, while the anhydrate is a superstructure of the Mn counterpart, suggesting the existence of solid solutions. Both new Fe compounds contain chains of Fe2+ octahedrally coordinated exclusively by acetate groups. The EMIM moieties do not interact directly with the Fe2+ and contribute to the structural framework of the compound through van der Waals forces and C-H···O hydrogen bonds with the acetate anions. The compounds have a melting temperature of ∼94 °C; therefore, they can be considered metal-containing ionic liquids. Differential thermal analysis indicates three endothermic transitions associated with melting, structural rearrangement in the molten state at about 157 °C, and finally, thermal decomposition of the Fe4(OAc)10[EMIM]2. Thermogravimetric analyses indicate an ∼72 wt % mass loss during the decomposition at 280-325 °C. The Fe4(OAc)10[EMIM]2 compounds have higher thermal stability than their Mn counterparts and [EMIM][OAc] but lower compared to iron(II) acetate. Temperature-programmed desorption coupled with mass spectrometry shows that the decomposition pathway of the Fe4(OAc)10[EMIM]2 involves four distinct regimes with peak temperatures at 88, 200, 267, and 345 °C. The main species observed in the decomposition of the compound are CH3, H2O, N2, CO, OC-CH3, OH-CO, H3C-CO-CH3, and H3C-O-CO-CH3. Variable-temperature infrared vibrational spectroscopy indicates that the phase transition at 160-180 °C is associated with a reorientation of the acetate ions, which may lead to a lower interaction with the [EMIM]+ before the decomposition of the Fe4(OAc)10[EMIM]2 upon further heating. The Fe4(OAc)10[EMIM]2 compounds are porous, plausibly capable of accommodating other types of molecules.

Boron Hydrogen Compounds: Hydrogen Storage and Battery Applications.

About 25 years ago, Bogdanovic and Schwickardi (B. Bogdanovic, M. Schwickardi: J. Alloys Compd. 1-9, 253 (1997) discovered the catalyzed release of hydrogen from NaAlH4. This discovery stimulated a vast research effort on light hydrides as hydrogen storage materials, in particular boron hydrogen compounds. Mg(BH4)2, with a hydrogen content of 14.9 wt %, has been extensively studied, and recent results shed new light on intermediate species formed during dehydrogenation. The chemistry of B3H8-, which is an important intermediate between BH4- and B12H122-, is presented in detail. The discovery of high ionic conductivity in the high-temperature phases of LiBH4 and Na2B12H12 opened a new research direction. The high chemical and electrochemical stability of closo-hydroborates has stimulated new research for their applications in batteries. Very recently, an all-solid-state 4 V Na battery prototype using a Na4(CB11H12)2(B12H12) solid electrolyte has been demonstrated. In this review, we present the current knowledge of possible reaction pathways involved in the successive hydrogen release reactions from BH4- to B12H122-, and a discussion of relevant necessary properties for high-ionic-conduction materials.

Thermal and Electrochemical Interface Compatibility of a Hydroborate Solid Electrolyte with 3 V-Class Cathodes for All-Solid-State Sodium Batteries.

Thermal stability of solid electrolytes and their compatibility with battery electrodes are key factors to ensure stable cycling and high operational safety of all-solid-state batteries. Here, we study the compatibility of a hydroborate solid electrolyte Na4(B12H12)(B10H10) with 3 V-class cathode active materials: NaCrO2, NaMnO2, and NaFeO2. Among these layered sodium transition metal oxide cathodes, NaCrO2 shows the highest thermal compatibility in contact with the hydroborate solid electrolyte up to 525 °C in the discharged state. Furthermore, the electrolyte remains intact upon the internal thermal decomposition of the charged, that is, desodiated, cathode (Na0.5CrO2) above 250 °C, demonstrating the potential for highly safe hydroborate-based all-solid-state batteries with a wide operating temperature range. The experimentally determined onset temperatures of thermal decomposition of Na4(B12H12)(B10H10) in contact with 3 V-class cathodes surpass those of sulfide and selenide solid electrolytes, exceeding previous thermodynamic calculations. Our results also highlight the need to identify relevant decomposition pathways of hydroborates to enable more valid theoretical predictions.

Thermal Conversion of Unsolvated Mg(B3H8)2 to BH4 - in the Presence of MgH2.

In the search for energy storage materials, metal octahydrotriborates, M(B3H8) n , n = 1 and 2, are promising candidates for applications such as stationary hydrogen storage and all-solid-state batteries. Therefore, we studied the thermal conversion of unsolvated Mg(B3H8)2 to BH4 - as-synthesized and in the presence of MgH2. The conversion of our unsolvated Mg(B3H8)2 starts at ∼100 °C and yields ∼22 wt % of BH4 - along with the formation of (closo-hydro)borates and volatile boranes. This loss of boron (B) is a sign of poor cyclability of the system. However, the addition of activated MgH2 to unsolvated Mg(B3H8)2 drastically increases the thermal conversion to 85-88 wt % of BH4 - while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH2 substantially decreases the formation of (closo-hydro)borates and provides the necessary H2 for the B3H8-to-BH4 conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH4 - under moderate conditions of temperature (200 °C) in less than 1 h, making the MgB3H8-MgH2 system very promising for energy storage applications.

Experimental investigation of Mg(B3H8)2 dimensionality, materials for energy storage applications.

Mg(B3H8)2 is a crucial reaction intermediate in the thermal decomposition of the hydrogen storage material Mg(BH4)2 and is discussed as a potential solid-state Mg-ion conductor. We successfully synthesized unsolvated Mg(B3H8)2 and highlight that Mg(B3H8)2 exists mainly as a low-dimensional solid. In addition, the Mg2+ conductivity was evaluated to be 1.4.10-4 S cm-1 at 80 °C.

Synthesis, Characterization, and Crystal Structures of Two New Manganese Aceto EMIM Ionic Compounds with Chains of Mn2+ Ions Coordinated Exclusively by Acetate.

We synthesized and determined crystal structures of two manganese(II) aceto EMIM coordination compounds with simplified empirical formulas Mn4(OAc)10[EMIM]2 and Mn4(OAc)10[EMIM]2·2H2O. Both compounds feature extended chains of Mn2+ octahedrally coordinated exclusively by acetate anions, which has been observed for the first time. The EMIM moieties and water molecules participate in hydrogen bonding with acetate anions but do not directly interact with the metal cation. Both compounds have melting temperatures around 120 °C and can be considered as (non-room-temperature) ionic liquids. The structural arrangement represented by the two title compounds is robust in terms of accommodating other types of cations and allows for tuning of physical properties of the ionic liquid by means of cation substitution. Thermal analysis results obtained using TGA-DSC and VT IR suggest melting phase transitions around 120 °C, followed by structural rearrangement in the molten state taking place around 140-160 °C. Compounds I and II have a higher thermal stability range compared to [EMIM][OAc] ionic liquid, with an onset decomposition temperature above 260 °C.

Modified Density Functional Dispersion Correction for Inorganic Layered MFX Compounds (M = Ca, Sr, Ba, Pb and X = Cl, Br, I).

MFX (M = Ca, Ba, Sr, Pb and X = Cl, Br, I) compounds have received considerable attention due to their technological application as X-ray detectors, pressure sensors, and optical data storage materials, when doped with rare-earth ions. MFX compounds belong to the class of layered materials with a tetragonal Matlockite crystal structure, characterized by weakly stacked double-halide layers along the crystallographic c-axis. These layers predominantly determine phase transitions, elastic, and mechanical properties. However, the correct description of the lattice parameter c is a challenge for most standard DFT functionals, which tend to overestimate the lattice parameter c. Because of the weak interactions between the halide layers, dispersion-corrected functionals seem to be a better choice. We investigated 11 different inorganic layered MFX compounds for which experimental data are available, with standard and dispersion-corrected functionals to assess their performance in reproducing the lattice parameter c, structural, and vibrational properties of the MFX compounds. Our results revealed that these functionals do not describe the weak interactions between the halide layers in a balanced way. Therefore, we modified Grimme's popular DFT-D2 dispersion correction scheme in two different ways by (i) replacing the dispersion coefficients and van der Waals radii with those of noble gas atoms or (ii) increasing the van der Waals radii of the MFX atoms up to 40%. Comparison with the available experimental data revealed that the latter approach applied to the PBE (Perdew-Burke-Ernzerhof)-D2 functional with 30% increased van der Waals radii, which we coined PBE-D2* (Srvdw 1.30) is best suited to fine-tune the description of the weak interlayer interactions in MFX compounds, thus significantly improving the description of their structural, vibrational, and mechanical properties. Work is in progress applying this new, computationally inexpensive scheme to other inorganic layered compounds and periodic systems with weakly stacked layers.

Boron Hydrogen Compounds for Hydrogen Storage and as Solid Ionic Conductors.

Metal borohydrides have been studied since the beginning of this century as potential hydrogen storage materials due to their high gravimetric hydrogen content. Many new compounds have been synthesized and characterized, however to date the main problem are the kinetics of dehydrogenation and rehydrogenation. In this review we address thermodynamical and chemical properties of boron hydrogen compounds which come into play for hydrogen storage and which must be considered in the search for efficient catalysts. More recently, closo and nido hydridoborate and related closo hydridocarborate compounds have been identified as good ionic conductors for all-solid-state lithium or sodium batteries. The properties of these fascinating and very promising compounds for battery applications are illustrated with recent literature results.

The influence of silica surface groups on the Li-ion conductivity of LiBH4/SiO2 nanocomposites.

Lithium borohydride is a promising lithium ion conductor for all-solid-state batteries. However, the compound only exhibits high ionic conductivity at elevated temperatures, typically above 110 °C. It was shown that the addition of oxides such as silica or alumina increases the room temperature ionic conductivity by 3 orders of magnitude. The origin of this remarkable effect is not yet well understood. Here, we investigate the influence of oxide surface groups on the ionic conductivity of LiBH4/SiO2 nanocomposites. We systematically varied the density and nature of the surface groups of mesoporous silica by heat treatment at different temperatures, or surface functionalization, and subsequently prepared LiBH4/SiO2 nanocomposites by melt infiltration. The ionic conductivity is strongly influenced by the heat treatment temperature, hence the density of the free surface silanol groups. Replacing some of the silanol groups with hydrophobic surface groups resulted in an order of magnitude reduction of the room temperature ionic conductivity, suggesting that their presence is crucial to obtain high ionic conductivity in the nanocomposites. This systematic study and insight provide a basis for further exploration of the impact of surface groups, and for the rational design of novel solid-state nanocomposite electrolytes via interface engineering.

Direct Solution-Based Synthesis of Na4 (B12 H12 )(B10 H10 ) Solid Electrolyte.

All-solid-state batteries (ASSBs) promise higher power and energy density than batteries based on liquid electrolytes. Recently, a stable 3 V ASSB based on the super ionic conductor (1 mS cm-1 near room temperature) Na4 (B12 H12 )(B10 H10 ) has demonstrated excellent cycling stability. This study concerns the development of a five-step, scalable, and solution-based synthesis of Na4 (B12 H12 )(B10 H10 ). The use of a wet chemistry approach allows solution processing with high throughput and addresses the main drawbacks for this technology, specifically, the limited electrode-electrolyte contact and high cost. Moreover, a cost-efficient synthesis of the expensive precursors Na2 B10 H10 and Na2 B12 H12 is also achieved through the same process. The mechanism of the reactions is investigated and two key parameters to tune the kinetics and selectivity are highlighted: the choice of counter cation (tetraethylammonium) and solvent.

Accurate Computational Thermodynamics Using Anharmonic Density Functional Theory Calculations: The Case Study of B-H Species.

The thermal decomposition of boron-hydrogen compounds is complex and multistep and involves the formation of various intermediates. An accurate description of the thermodynamics of the reactants, products, and intermediates is required for an in-depth understanding of their reactivity. In this respect, we have proceeded to the accurate determination of the key thermodynamic functions (ΔH(T), S(T), and C P (T)) of 44 isolated B-H molecular species involved in the decomposition of B-H solids, with the inclusion of anharmonic effects. An excellent agreement is observed with available experimental data. We report the analytic expressions of these functions obtained by fitting them with NASA functions in the 200-900 K temperature range. Because the vibrational spectra of these species are their fingerprints, we also report the predicted IR and Raman spectra. The calculated anharmonic spectra show an excellent agreement with experiments and allow for a clear-cut identification of fundamentals, combinations, and overtones.

Quantitative Assessment of B-B-B, B-Hb -B, and B-Ht Bonds: From BH3 to B12 H122.

We report the thermodynamic stabilities and the intrinsic strengths of three-center-two-electron B-B-B and B-Hb -B bonds ( Hb : bridging hydrogen), and two-center-two-electron B-Ht bonds ( Ht : terminal hydrogen) which can be served as a new, effective tool to determine the decisive role of the intermediates of hydrogenation/dehydrogenation reactions of borohydride. The calculated heats of formation were obtained with the G4 composite method and the intrinsic strengths of B-B-B, B-Hb -B, and B-Ht bonds were derived from local stretching force constants obtained at the B3LYP-D2/cc-pVTZ level of theory for 21 boron-hydrogen compounds, including 19 intermediates. The Quantum Theory of Atoms in Molecules (QTAIM) was used to deepen the inside into the nature of B-B-B, B-Hb -B, and B-Ht bonds. We found that all of the experimentally identified intermediates hindering the reversibility of the decomposition reactions are thermodynamically stable and possess strong B-B-B, B-Hb -B, and B-Ht bonds. This proves that thermodynamic data and intrinsic B-B-B, B-Hb -B, and B-Ht bond strengths form a new, effective tool to characterize new (potential) intermediates and to predict their role for the reversibility of the hydrogenation/dehydrogenation reactions.

New Insights into the Influence of the 4f55d1 State in the 4f6 Electronic Configuration of Sm2+ in Crystal Hosts.

The use of the Sm2+ luminescence properties in numerous applications appeals for a better understanding of its electronic structure. This work compares luminescence data and crystal field parameters from 31 Sm2+-containing compounds to assess the effects of the crystal field on its energy levels. In particular, the relationship between the 5D0-7F0 and 5D0-7F1 transition energies is analyzed and compared with previously published data for the isoelectronic Eu3+. It appears that for Sm2+, in contrast to Eu3+, the energy of the 5D0 state cannot be considered to be constant and implies the involvement of an extra state (presumably the 4f55d1 level) in the mixing of the 4f6 states. On the other side, the total crystal field strength is correlated to the splitting of the 7F1 states for both Sm2+ and Eu3+ in lower symmetry environments. The plot of the 5D0-7F0 energy as a function of the 7F1 splitting clearly evidences the mixing of the 4f6 state with the environment-sensitive 4f55d1 state for Sm2+, which is finally confirmed by the discrepancy of the ratio between the 5D1 and 7F1 splittings from its theoretical value in the absence of any mixing with the 4f55d1 state.

Theoretical Study of Halogenated B12H nX(12- n)2- (X = F, Cl, Br).

The closoborane and their derivatives have attracted high interest due to their superionic conductivity. Very recently, high ionic conductivities have been reported for compounds containing the closoborane ion B12H122-. In this work, we address halogen-substituted ions B12H nX(12- n)2- ( n = 0-3, 6, 9-12 and X = F, Cl, Br) using DFT calculations to probe the structures, the chemical stability, and the electrochemical stability, as well as spectroscopic properties in view of potential future applications. Considering the theoretical reaction n/12 B12H122- + (12- n)/12 B12X122- → B12H nX(12- n)2-, it appears that for X = Cl and Br the compounds with n = 6 are stabilized by about 100 kJ/mol. The calculation of the vertical detachment energy (which is indirectly related to the electrochemical stability) shows an increasing stability with increasing halogen content. These results suggest that, for practical applications, it is likely that a partially halogenated ion offers the best compromise. The calculations of vibrational properties and NMR chemical shifts also reveal several systematic trends, which are discussed and compared to available literature values.

Identification and optical features of the Pb4Ln2O7 series (Ln = La, Gd, Sm, Nd); genuine 2D-van der Waals oxides.

We report on the identification and survey of the Pb4Ln2O7 series (Ln = La, Gd, Sm and Nd) which turn out to be real van der Waals 2D oxides. In the neutral layers, strong covalent Pb-O bonds together with external stereoactive Pb2+ lone pairs, which act as sensitizers, lead to an ideal matrix for enhanced and tunable luminescence by lanthanide emitters, tested here for Sm3+ and Eu3+ doping. DFT calculations and preliminary ex-solution experiments validate the weak bonding between the layers separated by 3.5 Å and suggest a indirect to direct crossover realized by isolating the layers.

Pressure-induced phase transitions in Na2B12H12, structural investigation on a candidate for solid-state electrolyte.

closo-Borates, such as Na2B12H12, are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na2B12H12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.