Alkali metal – yttrium borohydrides: The link between coordination of small and large rare-earth

•The system Li-A-Y-BH4 (A=K, Rb, Cs) contains nine compounds in total.•Y3+ forms octahedral frameworks and tetrahedral complex anions.•Bi- and trimetallic double-perovskites crystallize in most systems.•Various AY(BH4)4 crystallize with structure types analogous to metal oxides.•Double-perovskites decompose and form a novel borohydride-closo-borane.

The system Li–A–Y–BH4 (A=K, Rb, Cs) is found to contain five new compounds and four further ones known from previous work on the homoleptic borohydrides. Crystal structures have been solved and refined from synchrotron X-ray powder diffraction, thermal stability of new compounds have been investigated and ionic conductivity measured for selected samples. Significant coordination flexibility for Y3+ is revealed, which allows the formation of both octahedral frameworks and tetrahedral complex anions with the tetrahydroborate anion BH4 both as a linker and terminal ligand. Bi- and trimetallic cubic double-perovskites c-A3Y(BH4)6 or c-A2LiY(BH4)6 (A=Rb, Cs) form in all the investigated systems, with the exception of the Li–K–Y system. The compounds with the stoichiometry AY(BH4)4 crystallize in all investigated systems with a great variety of structure types which find their analog amongst metal oxides. In-situ formation of a new borohydride – closo-borane is observed during decomposition of all double perovskites.

Graphical abstractThe system Li–A–Y–BH4 (A=K, Rb, Cs) is found to contain five novel compounds and four further ones previously reported. Significant coordination flexibility of Y3+ is revealed, which can be employed to form both octahedral frameworks and tetrahedral complex anions, very different structural topologies. Versatility is also manifested in three different simultaneously occurring coordination modes of borohydrides for one metal cation, as proposed by DFT optimization of the monoclinic KY(BH4)4 structural model observed by powder diffraction.Figure optionsDownload full-size imageDownload as PowerPoint slide