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

Four novel bimetallic borohydrides have been discovered, K 2 M (BH 4 ) 4 ( M = Mg or Mn), K 3 Mg(BH 4 ) 5, and KMn(BH 4 ) 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 KBH 4, α-Mg(BH 4 ) 2, and α-Mn(BH 4 ) 2 ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K 2 M (BH 4 ) 4, most importantly concerning the extent to which the complex anion [ M (BH 4 ) 4 ] 2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K 3 Mg(BH 4 ) 5 is ascribed to the motion of BH 4 groups inducing hydrogen repulsive effects, and the dynamics of K 3 Mg(BH 4 ) 5 are investigated. Decomposition in the manganese system proceeds via the formation of KMn(BH 4 ) 3, the first perovkite type borohydride reported to date.