Physical properties of new ordered bimetallic phases M0.25Cd0.75PS3 (M = ZnII, NiII, CoII, MnII)

Abstract

Four bimetallic phases of the thiophosphate family have been synthesized by the cationic exchange reaction using a freshly prepared K_0.5Cd_0.75PS₃ precursor phase and methanolic solutions of nitrates of the divalent cations Zn^II, Ni^II, Co^II, and Mn^II. All the materials were characterized by FTIR, PXRD, SEM-EDXS and (in the case of the diamagnetic compounds) by solid state NMR. For the K_0.5Cd_0.75PS₃ precursor, the X-ray powder diffraction data suggest a modification of the structure, while solid state NMR results confirm that this phase possesses an ordered arrangement of Cd vacancies. The cationic exchange reaction achieves a complete removal of potassium ions (no potassium detected by SEM-EDXS) and re-occupation of the vacancies by divalent cations. Therefore, the obtained compounds have an average composition of M_0.25Cd_0.75PS₃ (M = Zn^II, Ni^II, Co^II, Mn^II) and possess an ordered distribution of the substituent cations. Even with the paramagnetic substitution level of 25%, antiferromagnetic behaviour is present in the phases with Mn^II, Co^II and Ni^II, as evidenced by dc susceptibility and in the case of the Mn^II substituted phase by EPR. The cooperative magnetic interactions confirm the conclusion that the paramagnetic ions adopt an ordered arrangement. The analysis by broad band impedance spectroscopy allows to attribute the conductivity in these materials to charge movements in the layers due to the difference in electronegativity of the metal ions. Zn_0.25Cd_0.75PS₃ is the phase that shows the highest conductivity values. Finally, the band gap energies of the bimetallic phases tend to be lower than those of the single-metal phases, probably due to an overlap of the band structures.