Bimetallic Borohydrides in the System M(BH₄)₂–KBH₄ (M = Mg, Mn): On the Structural Diversity

Abstract

Four novel bimetallic borohydrides have been discovered, K2M(BH4)4 (M = Mg or Mn), K3Mg(BH4)5, and KMn(BH4)3, and are carefully investigated structurally as well as regarding their decomposition reaction mechanism by means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), vibrational spectroscopies (Raman and IR), thermal analysis (TGA and DTA), and ab initio density functional theory (DFT) calculations. Mechano-chemical synthesis (ball-milling) using the reactants KBH4, α-Mg(BH4)2, and α-Mn(BH4)2 ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K2M(BH4)4, most importantly concerning the extent to which the complex anion [M(BH4)4]2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K3Mg(BH4)5 is ascribed to the motion of BH4 groups inducing hydrogen repulsive effects, and the dynamics of K3Mg(BH4)5 are investigated. Decomposition in the manganese system proceeds via the formation of KMn(BH4)3, the first perovkite type borohydride reported to date

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

“…Borohydride precursors ABH 4 ( = A K, Rb, Cs) were purchased at Katchem and used as received. Ca(BH 4 ) 2 was purchased at Sigma-Aldrich and Mn(BH 4 ) 2 prepared according to a similar procedure as used for Mg(BH 4 ) 2 [9]. The synthesis of KYb(BH 4 …”

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

“…However, only recently the first perovskite-type metal borohydride, KMn(BH 4 ) 3 , was reported, though followed up by extensive research reporting multiple new perovskite-type metal borohydrides containing the complex anion BH 4 − , see also [116,[122][123][124]. Structurally, the metal borohydride perovskites differ from regular oxide and halide ABX 3 perovskites as their polymorphic transitions are not always intuitively towards higher symmetry with increasing temperature.…”