Polymorphism and hydrogen discharge from holmium borohydride, Ho(BH4)3, and KHo(BH4)4

Holmium borohydride, Ho(BH4)3, its composites with LiBH4, and a mixed-cation derivative, K[Ho(BH4)4], have been prepared via mechanochemical reaction between HoCl3, LiBH4 (and also KBH4 in case of K[Ho(BH4)4]). These compounds are isostructural to the related rare earth borohydrides, adopting α-Y(BH4)3, β-Y(BH4)3, and Na[Sc(BH4)4]-type structures, respectively. The relative amount of α-Ho(BH4)3 and β-Ho(BH4)3 can be controlled by the composition of reagents. While β-Ho(BH4)3 has not been obtained from stoichiometric mixtures of HoCl3 and LiBH4, the excess of LiBH4 favours it as the main product. Thermal decomposition of α-Ho(BH4)3, as well as K[Ho(BH4)4] commences above 170 °C, with the fastest rate within 250-260 °C, which is slightly lower than for the corresponding borohydrides of yttrium. LiBH4 is destabilised thermally in the composites with β-Ho(BH4)3, which leads to desorption of >40% total amount of H2 below 450 °C, while in case of pure LiBH4 only <20% total amount of H2 is released in these conditions. The catalytic mechanism is as yet unknown.