A series of dynamic single crystals of [M-II(en)(3)]SO4 (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)]SO4 (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 SO42- 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 SO42-, the dynamic motion of SO42- 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)]SO4, 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.