A series of dynamic single crystals of [M II (en) 3 ]SO 4 (M = Ni, Mn, and Cd) shows tunable dielectric properties and anisotropic thermal expansion.

A series of dynamic single crystals with a chemical formula of [M II (en) 3 ]SO 4 (en = ethylene and M II = Ni II , Mn II , and Cd II ) was synthesized. As the temperature decreases, these materials exhibit dielectric switching in the vicinity of the phase transition point accompanied by anisotropic thermal expansion in the cell parameters as a consequence of the order-disorder structural change of SO2-4 in a cavity surrounded by five [M II (en) 3 ] 2+ complex cations. Because the variation of metal centers with different ionic radii changes the shape of the complex cation, which affects the distribution of hydrogen-bond interactions around the SO2-4, the dynamic motion of SO2-4 is substantially tuned. Correspondingly, the dielectric properties and anisotropic thermal expansion of materials were largely shifted, especially in the single crystals of [Mn II (en) 3 ]SO 4 , whose structural change is distinctly different from the crystals of Ni(II) and Cd(II). The detailed structural mechanism accounting for the different physical properties of these materials was discussed.