Switching the anisotropy barrier of a single-ion magnet by symmetry change from quasi-D _5h to quasi-O _h

A reversible single-crystal-to-single-crystal transformation is used to dramatically change the relaxation behavior of a single-ion magnet. The coordination geometry of the Dy^III site of a [Zn-Dy-Zn] complex changes from pentagonal-bipyramid (quasi-D _5h) to octahedron (quasi-O _h), inducing an energy barrier change from 305 cm⁻¹ (439 K) to a negligible value, respectively. Ab initio calculations reveal that the ideal D _5h-Dy^III is of perfect axiality with a substantial energy barrier in accordance with the experimental result.