Borohydride as Magnetic Superexchange Pathway in Late Lanthanide Borohydrides

The magnetic characteristics of a series of borohydride-based systems, including binary -/-Ln(BH4)(3) phases (Ln = Gd, Tb, Dy, Ho, Er, and Tm) with direct Ln-HBH-Ln bridges and selected mixed-metal systems, LiYb(BH4)(4), NaYb(BH4)(4), KHo(BH4)(4), RbTm(BH4)(4), with isolated Ln(3+) ions embedded in [Ln(BH4)(4)](-) anions are described for the first time using SQUID magnetometry and DFT+U calculations. Crystal field effects as well as the nature and strength of magnetic superexchange interactions via BH4- ligands are established based on a ligand field theory approach. We find Auzel's scalar crystal field strength parameter (Nv, the weighted quadratic mean of the ligand field parameters B-k(q)) for these systems to be substantial, in particular for -Ho(BH4)(3), indicating a significant covalent component in the HoH bonding. The BH4- anion is capable of mediating both weakly ferro- or antiferromagnetic superexchange. Quantum mechanical DFT+U calculations, even when including spin-orbit coupling effects, do not reliably describe the sign and the size of the magnetic superexchange constants in these systems.