Abstract
The fast ionic, high temperature (HT) phase of LiBH4 can be stabilized by Br¯ substitution. Lithium borohydride bromide compounds, Li(BH4)1–xBrx, have been synthesized mechanochemically, with and without thermal treatment and the resulting phase behavior determined as a function of composition. Single phase materials exist for 0.29 ≤ x ≤ 0.50 with conductivity 2 orders of magnitude higher than LiBH4 at 313 K. Powder neutron diffraction has been used to resolve the details of the crystal structure of one such compound. These demonstrate that 7Li(11BD4)0.667Br0.333 retains the HT structure (hexagonal space group P63mc, a ≈ 4.2 Å, c ≈ 6.7 Å) from 293 to 573 K. The borohydride bromide exhibits considerable static and dynamic disorder, the latter invoking complex rotational motion of the (BH4)¯ anions.
Highlights
The decomposition and release of hydrogen from a number of complex hydrides such as lithium borohydride is linked to a transition to a fast lithium ion conducting state.[1,2] At room temperature LiBH4 crystallizes with an orthorhombic (LT) structure and at 383 K undergoes a phase transition to a hexagonal (HT) phase that exhibits fast lithium ion conductivity, with values of up to 10−2 S cm−1.1 The structure of the LT phase has been thoroughly examined and can be described as a distorted variant of the wurtzite structure with the Li+ cations occupying the four-coordinate tetrahedral sites between hexagonally stacked layers of BH4−.3 The distortion to orthorhombic symmetry allows buckling of the anion layers and ordering of the slightly distorted tetrahedral anions
Partial anion replacement effected by mechanical and thermal reaction with appropriate halides leads to the formation of the hexagonal structure analogous to the high temperature (HT) phase of LiBH4.9−11 the HT form has never been isolated at room temperature as a single phase with bromide
We have used these data to compile a phase diagram for the full compositional series Li(BH4)1−xBrx, and we report these results together with a detailed structural model of the orientational disorder of the BH4 anion in the fast Li+ conducting hexagonal phase
Summary
The decomposition and release of hydrogen from a number of complex hydrides such as lithium borohydride is linked to a transition to a fast lithium ion conducting state.[1,2] At room temperature LiBH4 crystallizes with an orthorhombic (LT) structure and at 383 K undergoes a phase transition to a hexagonal (HT) phase that exhibits fast lithium ion conductivity, with values of up to 10−2 S cm−1.1 The structure of the LT phase has been thoroughly examined and can be described as a distorted variant of the wurtzite structure with the Li+ cations occupying the four-coordinate tetrahedral sites between hexagonally stacked layers of BH4−.3 The distortion to orthorhombic symmetry allows buckling of the anion layers and ordering of the slightly distorted tetrahedral anions. Partial anion replacement effected by mechanical and thermal reaction with appropriate halides leads to the formation of the hexagonal structure analogous to the HT phase of LiBH4.9−11 the HT form has never been isolated at room temperature as a single phase with bromide. The resultant materials have been characterized by both powder X-ray and powder neutron diffraction, thermal analysis, transport measurements, and electron microscopy. We have used these data to compile a phase diagram for the full compositional series Li(BH4)1−xBrx, and we report these results together with a detailed structural model of the orientational disorder of the BH4 anion in the fast Li+ conducting hexagonal phase
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