Pascal Schouwink

Structure and properties of complex hydride perovskite materials

Perovskite materials host an incredible variety of functionalities. Although the lightest element, hydrogen, is rarely encountered in oxide perovskite lattices, it was recently observed as the hydride anion H(-), substituting for the oxide anion in BaTiO3. Here we present a series of 30 new complex hydride perovskite-type materials, based on the non-spherical tetrahydroborate anion BH4(-) and new synthesis protocols involving rare-earth elements. Photophysical, electronic and hydrogen storage properties are discussed, along with counterintuitive trends in structural behaviour. The electronic structure is investigated theoretically with density functional theory solid-state calculations. BH4-specific anion dynamics are introduced to perovskites, mediating mechanisms that freeze lattice instabilities and generate supercells of up to 16 × the unit cell volume in AB(BH4)3. In this view, homopolar hydridic di-hydrogen contacts arise as a potential tool with which to tailor crystal symmetries, thus merging concepts of molecular chemistry with ceramic-like host lattices. Furthermore, anion mixing BH4(-)←X(-) (X(-)=Cl(-), Br(-), I(-)) provides a link to the known ABX3 halides.

Trimetallic borohydride Li3MZn5(BH4)15 (M = Mg, Mn) containing two weakly interconnected frameworks.

The compounds, Li3MZn5(BH4)15, M = Mg and Mn, represent the first trimetallic borohydrides and are also new cationic solid solutions. These materials were prepared by mechanochemical synthesis from LiBH4, MCl2 or M(BH4)2, and ZnCl2. The compounds are isostructural, and their crystal structure was characterized by in situ synchrotron radiation powder X-ray and neutron diffraction and DFT calculations. While diffraction provides an average view of the structure as hexagonal (a = 15.371(3), c = 8.586(2) Å, space group P63/mcm for Mg-compound at room temperature), the DFT optimization of locally ordered models suggests a related ortho-hexagonal cell. Ordered models that maximize Mg-Mg separation have the lowest DFT energy, suggesting that the hexagonal structure seen by diffraction is a superposition of three such orthorhombic structures in three orientations along the hexagonal c-axis. No conclusion about the coherent length of the orthorhombic structure can be however done. The framework in Li3MZn5(BH4)15 is of a new type. It contains channels built from face-sharing (BH4)6 octahedra. While X-ray and neutron powder diffraction preferentially localize lithium in the center of the octahedra, thus resulting in two weakly interconnected frameworks of a new type, the DFT calculations clearly favor lithium inside the shared triangular faces, leading to two interpenetrated mco-nets (mco-c type) with the basic tile being built from three tfa tiles, which is the framework type of the related bimetallic LiZn2(BH4)5. The new borohydrides Li3MZn5(BH4)15 are potentially interesting as solid-state electrolytes, if the lithium mobility within the octahedral channels is improved by disordering the site via heterovalent substitution. From a hydrogen storage point of view, their application seems to be limited as the compounds decompose to three known metal borohydrides.

The crystal chemistry of inorganic metal borohydrides and their relation to metal oxides.

The crystal structures of inorganic homoleptic metal borohydrides are analysed with respect to their structural prototypes found amongst metal oxides in the inorganic databases such as Pearsons Crystal Data [Villars & Cenzual (2015). Pearsons Crystal Data. Crystal Structure Database for Inorganic Compounds, Release 2014/2015, ASM International, Materials Park, Ohio, USA]. The coordination polyhedra around the cations and the borohydride anion are determined, and constitute the basis of the structural systematics underlying metal borohydride chemistry in various frameworks and variants of ionic packing, including complex anions and the packing of neutral molecules in the crystal. Underlying nets are determined by topology analysis using the program TOPOS [Blatov (2006). IUCr CompComm. Newsl. 7, 4-38]. It is found that the Pauling rules for ionic crystals apply to all non-molecular borohydride crystal structures, and that the latter can often be derived by simple deformation of the close-packed anionic lattices c.c.p. and h.c.p., by partially removing anions and filling tetrahedral or octahedral sites. The deviation from an ideal close packing is facilitated in metal borohydrides with respect to the oxide due to geometrical and electronic considerations of the BH4(-) anion (tetrahedral shape, polarizability). This review on crystal chemistry of borohydrides and their similarity to oxides is a contribution which should serve materials engineers as a roadmap to design new materials, synthetic chemists in their search for promising compounds to be prepared, and materials scientists in understanding the properties of novel materials.

Borohydrides: from sheet to framework topologies

The five novel compounds ALiM(BH4)4 (A = K or Rb; M = Mg or Mn) and K3Li2Mg2(BH4)9 crystallizing in the space groups Aba2 and P2/c, respectively, represent the first two-dimensional topologies amongst homoleptic borohydrides. The crystal structures have been solved, refined and characterized by synchrotron X-ray powder diffraction, neutron powder diffraction and solid-state DFT calculations. Minimal energies of ordered models corroborate crystal symmetries retrieved from diffraction data. The layered Li-Mg substructure forms negatively charged uninodal 4-connected networks. It is shown that this connectivity cannot generate the long sought-after, bimetallic Li-Mg borohydrides without countercations when assuming preferred coordination polyhedra as found in Mg(BH4)2 and LiBH4. The general properties of the trimetallic compound series are analogous with the anhydrous aluminosilicates. Additionally, a relationship with zeolites is suggested, which are built from three-dimensional Al-Si-O networks with a negative charge on them. The ternary metal borohydride systems are of interest due to their potential as novel hydridic frameworks and will allow exploration of the structural chemistry of light-metal systems otherwise subject to eutectic melting.

Modified Anion Packing of Na2B12H12 in Close to Room Temperature Superionic Conductors

Three different types of anion packing, i.e., hexagonal close packed (hcp), cubic close packed (ccp), and body centered cubic (bcc), are investigated experimentally and with ab initio calculations in the model system Na2B12H12. Solvent free and water assisted mechanical grinding provide polycrystalline samples for temperature-dependent synchrotron radiation X-ray powder diffraction and electrochemical impedance spectroscopy. It is shown that among the common close packed lattices, the hcp anionic backbone creates very favorable conditions for three-dimensional ionic conduction pathways, comprised of O-O, T-T, and T-O-T (O for octahedral, T for tetrahedral) cation hops. The hcp lattice is stable with respect to ccp and bcc lattices only at higher volumes per formula unit, which is achieved either by cationic substitution with larger cations or partial substitution of hydrogen by iodine on the closo-anion. It is found that the partial cationic substitution of sodium with lithium, potassium, or cesium does not lead to enhanced conductivity due to the obstruction of the conduction pathway by the larger cation located on the octahedral site. Substitution on the closo-anion itself shows remarkable positive effects, the ionic conductivity of Na2B12H12-xIx reaching values of close to 10-1 S cm-1 at a rather low temperature of 360 K. While the absolute value of σ is comparable to that of NaCB11H12, the temperature at which it is attained is approximately 20 K lower. The activation energy of 140 meV is determined from the Arrhenius relation and among the lowest ever reported for a Na-conducting solid.

Di-hydrogen contact induced lattice instabilities and structural dynamics in complex hydride perovskites

The structural phase transitions occurring in a series of perovskite-type complex hydrides based on the tetrahydroborate anion BH4(-) are investigated by means of in situ synchrotron x-ray powder diffraction, vibrational spectroscopy, thermal methods and ab initio calculations in the solid state. Structural dynamics of the BH4 anion are followed with quasi-elastic neutron scattering. We show that unexpected temperature-induced lattice instabilities in perovskite-type ACa(BH4)3 (A = K, Rb, Cs) have their origin in close hydridic di-hydrogen contacts. The rich lattice dynamics lead to coupling between internal B-H vibrations and phonons, resulting in distortions in the high-temperature polymorph that are identical in symmetry to well-known instabilities in oxide perovskites, generally condensing at lower temperatures. It is found that anion-substitution BH4(-) <-> (X = Halide) can relax distortions in ACa(BH4)3 by eliminating coulomb repulsive H(-)···H(-) effects. The interesting nature of phase transition in ACa(BH4)3 enters an unexplored field of weak interactions in ceramic-like host lattices and is the principal motivation for this study. Close di-hydrogen contacts suggest new concepts to tailor crystal symmetries in complex hydride perovskites in the future.

Role of the Li+ node in the Li‐BH4 substructure of double‐cation tetrahydroborates

The phase diagram LiBH4-ABH4 (A = Rb,Cs) has been screened and revealed ten new compounds LiiAj(BH4)i+j (A = Rb, Cs), with i, j ranging between 1 and 3, representing eight new structure types amongst homoleptic borohydrides. An approach based on synchrotron X-ray powder diffraction to solve crystal structures and solid-state first principles calculations to refine atomic positions allows characterizing multi-phase ball-milled samples. The Li-BH4 substructure adopts various topologies as a function of the compounds Li content, ranging from one-dimensional isolated chains to three-dimensional networks. It is revealed that the Li(+) ion has potential as a surprisingly versatile cation participating in framework building with the tetrahydroborate anion BH4 as a linker, if the framework is stabilized by large electropositive counter-cations. This utility can be of interest when designing novel hydridic frameworks based on alkaline metals and will be of use when exploring the structural and coordination chemistry of light-metal systems otherwise subject to eutectic melting.

Flux-assisted single crystal growth and heteroepitaxy of perovskite-type mixed-metal borohydrides

Structural investigations on mixed-metal borohydrides have been the subject of powder diffraction since the discovery that hydrogen-release temperatures can be tailored by using more electronegative metals. The lack of producing suitable samples for single crystal X-ray diffraction has defined powder diffraction as the method of choice which, however, is less sensitive to the structural details of the compounds in question. Here we show how to overcome this limitation by developing a flux-assisted single crystal growth procedure to lower the melting point of mixed-metal compounds that are thermally unstable and usually decompose before melting, or are unstable in the melt. We prove the validity of this principle on a member of the recently reported perovskite-type class of borohydrides and show that the defined approach is easily generalized. Interesting structural details are revealed that stand in contrast to the results obtained from samples produced by mechano-chemistry. The differences in lattice instabilities are discussed and put into context with the discovered epitaxial relationships between rocksalt-type ABH4 and perovskite-type ACa(BH4)3 (A = alkaline metal). In this context, the preliminary results provide a valuable scheme that can be made use of when physical deposition of metal borohydrides reaches its working stage.

Structural design principles for close packed Na and Li solid-electrolytes built from mixed anion borane lattices

Energy storage solutions on a large scale have long been identified as a primary target within renewable energy research. While various complex hydrides, such as the salts built from the [BH4] anion, have shown high Li+ ionic conductivity, the compound family of the metal boranes are more promising contenders when it comes to Na-superionics and all-solid battery concepts. Very recently, the complex anion landscape of complex hydrides has been extended to include the carborane (CB) [CB11H12] -. When used to template close-packed ionic conductors, they show the highest reported Li+ and Na+ ionic conductivities of all reported complex hydrides1 and are amongst the highest for Na solid electrolytes in general. Due to their quasi-spherical shape, this molecule easily forms the cubic close packed structure (ccp), an hard backbone to tailor vacancy concentration achieving high ionic mobility2, which is boosted additionally by well-known “paddle wheel effect” 3. We present different approaches to design in particular Na-conducting solid electrolytes with ever lower operating temperatures. On the one hand, it is conceptually straightforward to rebuild benchmark conductors from the literature. In this sense, the ionic conductor RbAg4I5 was taken as a template to reproduce its CB analogues, resulting in double cation phases containing alkali metals, and with an ideal composition of CsA4(CB11H12)5 (A = Li +, Na+) in the specific case of the parent phase RbAg4I5 The ionic mobility of the resulting new phases will be presented and related to the crystal structure. On the other hand, we show how the close packing of the anion lattice may be controlled by anion-mixing of CB [CB11H12] and dodeca-boranes [B12H12] 2-, both their Na-endmembers known to have superionic ht-phases. Making use of the knowledge of preferred coordination polyhedra in higher boranes, this approach allows us to control the occupancy of tetrahedral (T) and octahedral (O) vacancies by modifying the carborane – dodeca-borane ratio, made possible due to the different charges of the polyanions. A control of site occupancies in packed lattices is known to be the key point to achieve high ionic conductivities at a suitable temperature and therefore highly promising approach to tailor metal boranes for battery application. References: 1. Tang et al Energy. Envrion. Sci., 2015, 8, 3637 2. Wang et al Nat. Mat. 2015, 14, 1026 3. Jansen Angew. Chem. Int. Ed. 1991, 30, 1547

Novel solvates M(BH₄)₃S(CH₃)₂ and properties of halide-free M(BH₄)₃ (M = Y or Gd).

Rare earth metal borohydrides have been proposed as materials for solid-state hydrogen storage because of their reasonably low temperature of decomposition. New synthesis methods, which provide halide-free yttrium and gadolinium borohydride, are presented using dimethyl sulfide and new solvates as intermediates. The solvates M(BH4)3S(CH3)2 (M = Y or Gd) are transformed to α-Y(BH4)3 or Gd(BH4)3 at ~140 °C as verified by thermal analysis. The monoclinic structure of Y(BH4)3S(CH3)2, space group P2₁/c, a = 5.52621(8), b = 22.3255(3), c = 8.0626(1) Å and β = 100.408(1)°, is solved from synchrotron radiation powder X-ray diffraction data and consists of buckled layers of slightly distorted octahedrons of yttrium atoms coordinated to five borohydride groups and one dimethyl sulfide group. Significant hydrogen loss is observed from Y(BH4)3 below 300 °C and rehydrogenation at 300 °C and p(H2) = 1550 bar does not result in the reformation of Y(BH4)3, but instead yields YH3. Moreover, composites systems Y(BH4)3-LiBH4 1 : 1 and Y(BH4)3-LiCl 1 : 1 prepared from as-synthesised Y(BH4)3 are shown to melt at 190 and 220 °C, respectively.