Trimetallic Borohydride Li₃MZn₅(BH₄)₁₅ (M = Mg, Mn) Containing Two Weakly Interconnected Frameworks

Abstract

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.

Complex and liquid hydrides for energy storage

“…Solvent-based methods can provide a solvent-free monometallic borohydride, often by using a metal borohydride solvate as an intermediate, such as M(BH 4 ) x , M = Mg, Mn, Y, Eu, Gd [27][28][29][30][31]. This approach has also led to the first tri-metallic borohydrides [32][33][34].…”

Characterization of Gas‐Solid Reactions using In Situ Powder X‐ray Diffraction

“…[47] A wide range of other bi-and trimetallic challenging and fascinating structures has also been characterized by SR-PXD. [48][49][50][51][52] 4Ϫ with a distorted cubane Ce 4 Cl 4 core discovered in the crystal structure of LiCe(BH 4 ) 3 Cl. [25] …”

Complex Metal Hydrides for Hydrogen, Thermal and Electrochemical Energy Storage

“…The first trimetallic borohydrides, Li 15 , M = Mg and Mn, are isostructural with cation disorder in the hexagonal (P6 3 /mcm) structure related to bimetallic LiZn 2 (BH 4 ) 5 [126]. The five trimetallic compounds ALiM(BH 4 ) 4 (A = K or Rb; M = Mg or Mn) and K 3 Li 2 Mg 2 (BH 4 ) 9 have negatively charged Li-Mg/Mn layered structures [127].…”