Synthesis, characterization and electrical properties of Na6M(SO4)4 (M = Co, Ni, Cu) vanthoffite materials

Highlights: • Vanthoffite materials were synthesized by the solid state reaction method. • Conductivity studies were done in the [633–713 K] temperature range. • Conduction is due to a migration of cations in a 2D directions or by jumping in vacancy sites. -- Abstract: In this work we synthesized pure phases of Na₆M (SO₄)₄ (M = Co, Ni, Cu) vanthoffite ceramics denoted as NMS using the solid-state reaction method. The obtained materials crystallized in the monoclinic system with space group P2₁/c. The infrared (IR) and visible (Vis) spectroscopies revealed the vibrational mode of sulfate groups and verified the absorption band of the bivalent metal. Electrical conductivity was studied using impedance spectroscopy in the 633–713 K temperature range. Results show that the materials present anionic semiconductor behavior and that conductivity increases with temperature. Ionic conduction within these materials arises from the migration of cations in the crystalline system in a 2D direction or from jumping into the vacancy sites. The activation energy values confirm that the materials represent candidates that may be used as electrodes in sodium-ion batteries.