Simon R. Johnson

Tuning the Decomposition Temperature in Complex Hydrides: Synthesis of a Mixed Alkali Metal Borohydride

Metal borohydrides continue to attract considerable interest as potential hydrogen storage materials owing to their very large gravimetric hydrogen densities. In terms of the equally important performance parameter, the thermal decomposition temperature Tdec, it has emerged [1–3] that the degree of charge transfer between the metal cation and the BH4 anion is a key component for any ultimate materials design. Although manipulation of Tdec in single-cation borohydrides is clearly limited by the intrinsic properties of the individual metal (its characteristic electronegativity, for example), double and multiple cation substitution allows more extensive and precise control of Tdec. Although this approach is supported by bulk thermochemical studies, there are few X-ray structural investigations of double cation substitutions in these important materials. Such structural information is key to assessing whether genuine new multinary compounds are formed, or whether microscopic segregation of constituent phases takes place. Herein we report the first synthesis and crystal structure determination of a mixed alkali metal borohydride, LiK(BH4)2. Importantly in this new material, the observed decomposition temperature lies between that of the constituent phases. This finding of a genuine, dual-cation single-phase material offers the real prospect of chemical control of Tdec by the manipulation of multication combinations. X-ray diffraction data of thirteen samples with varying initial ratio LiBH4:KBH4 were collected. LiK(BH4)2 (see Figure 1) was identified from the data as having the space group Pnma and approximate lattice parameters a= 7.9134 4, b= 4.4907 4, and c= 13.8440 4. The b-axis lattice parameter is very similar to that of orthorhombic LiBH4 (4.43686(2) 4), suggesting a degree of structural similarity between the phases. The BH4 units in LiK(BH4)2 form an approximately tetrahedral coordination around the lithium ion, which is similar to that found in orthorhombic LiBH4. The Li B bonds are greater in LiK(BH4)2 than in LiBH4 but with a narrower range of angles (see Supporting Information). The larger Li···B separations observed in the new phase may originate from the presence of potassium cations in the structure, which are considerably larger than their lithium counterparts (Li ionic radius 0.59 4, K ionic radius 1.38 4 in tetrahedral coordination). The arrangement of the BH4 units in LiK(BH4)2 and KBH4 differs considerably. KBH4 has an octahedral arrangement of BH4 units, whereas those in LiK(BH4)2 might be best described as monocapped trigonal prisms (see Supporting Information). The K···B distances in KBH4 are 3.364 4, [8] whereas in LiK(BH4)2 the (seven) distances are 3.404(3)(18) 4 (twice), 3.409(3) 4 (twice), 3.431(3) 4 (twice), and 3.475(3) 4 (once). It is thought that these larger separations arise because of the greater number of BH4 units present around the potassium cation. The BH4 units in LiK(BH4)2 appear to be distorted in a similar manner to those reported in the orthorhombic structure of LiBH4 by Soulie et al. [6] (see Supporting Information). Specifically, while KBH4 has all equivalent B H bonds, those in orthorhombic LiBH4 [6] and LiK(BH4)2 are separated into two equivalent pairs. LiK(BH4)2 was found to have a narrower range of B H bond lengths than LiBH4 Figure 1. A schematic diagram of a) the proposed LiK(BH4)2 structure and b) that of orthorhombic LiBH4. K crimson, Li yellow, B green, H gray.

The Monoammoniate of Lithium Borohydride, Li(NH3)BH4: An Effective Ammonia Storage Compound

Lithium borohydride absorbs anhydrous ammonia to form four stable ammoniates; Li(NH(3))(n)BH(4), mono-, di-, tri-, and tertraammoniate. This paper focuses on the monoammoniate, Li(NH(3))BH(4), which is readily formed on exposure of LiBH(4) to ammonia at room temperature and pressure. Ammonia loss from Li(NH(3))BH(4) commences around 40 degrees C and the compound transforms directly to LiBH(4). The crystal structure of Li(NH(3))BH(4) is reported here for the first time. Its close structural relationship with LiBH(4) provides a clear insight into the facile nature and mechanism of ammonia uptake and loss. These materials not only represent an excellent high weight-percent ammonia system but are also potentially important hydrogen stores.

Synthesis and crystal structure of Li4BH4(NH2)3

The solid solution, (LiNH2)x(LiBH4)(1-x), formed through the reaction of the two potential hydrogen storage materials, LiNH2 and LiBH4, is dominated by a compound that has an ideal stoichiometry of Li4BN3H10 and forms a body-centred cubic structure with a lattice constant of ca. 10.66 A.

Microwave enhanced reaction of carbohydrates with amino-derivatised labels and glass surfaces

The reaction between carbohydrates and amino-derivatised labels has been improved through microwave heating. We show it proceeds principally via solvent mediated heating rather than either direct microwave heating of the reagents or microwave influenced changes in the rates of mutarotation in the sugars beyond those obtained by conventional thermal heating. The method is applied to the attachment of sugars to an aminosilane-derivatised glass surface suitable for the construction of carbohydrate microarrays.

The synthesis and structural investigation of mixed lithium/sodium amides

The solid state reaction of lithium amide (LiNH2) and sodium amide (NaNH2) leads to the synthesis of the mixed metal amides, Li3Na(NH2)4 and LiNa2(NH2)3; the former being the end member of a solid solution, Li4−yNay(NH2)4 (0 ≤ y ≤ 1.0). The compound Li3Na(NH2)4 is found to be non-stoichiometric between the limits given whereas LiNa2(NH2)3 is stoichiometric; the difference in these two cases is attributed to the Li ion mobility of both compounds.

The size-induced metal-insulator transition in mesoscopic conductors

We discuss the possibility — and experimental realisation — of a Size-Induced-Metal-Insulator-Transition (SIMIT) occurring entirely within individual particles of mesoscopic conductors. Thus, nanoscale particles of indium and gold exhibit electrical conductivities a factor of ca. 107 below that of the corresponding bulk metal. In this size regime, individual mesoscopic particles of these prototypical metallic elements of the Periodic Table behave as low conductivity insulators or non-metals. In addition to its fundamental importance in condensed matter science, an understanding of the SIMIT in isolated mesoscopic conductors may allow for an unprecedented degree of control and design of microelectronic device materials based on mesoscopic conductors. For instance, by changing the physical size of a mesoscopic conductor any value of electrical conductivity — and associated complex dielectric function — can be adjusted between a metal and an insulator.