Borohydride as Magnetic Superexchange Pathway in Late Lanthanide Borohydrides

Abstract

The magnetic characteristics of a series of borohydride‐based systems, including binary α‐/β‐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 Ln3+ 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 Bkq) for these systems to be substantial, in particular for α‐Ho(BH4)3, indicating a significant covalent component in the Ho···H 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.