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

Four bimetallic phases of the thiophosphate family have been synthesized by the cationic exchange reaction using a freshly prepared K0.5Cd0.75PS3 precursor phase and methanolic solutions of nitrates of the divalent cations ZnII, NiII, CoII, and MnII. All the materials were characterized by FTIR, PXRD, SEM-EDXS and (in the case of the diamagnetic compounds) by solid state NMR. For the K0.5Cd0.75PS3 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 M0.25Cd0.75PS3 (M = ZnII, NiII, CoII, MnII) 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 MnII, CoII and NiII, as evidenced by dc susceptibility and in the case of the MnII 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. Zn0.25Cd0.75PS3 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.

Magnetic behaviour of bimetallic layered phases M'0.2Mn0.8PS3·0.25 H2O (M' = ZnII, CuII, NiII, CoII)

In this work the magnetic properties of bimetallic phases M'_0.2Mn_0.8PS₃·0.25H₂O (M' = Co^II, Ni^II, Cu^II or Zn^II) have been explored and compared with those of the pristine phase MnPS₃. Magnetic susceptibility, high field magnetization and electron paramagnetic resonance (EPR) studies reveal that the transition temperature between the antiferromagnetic and paramagnetic order for the pristine phase is shifted to lower values in the bimetallic phases. From magnetization measurements the critical field of the spin-flop transition is found to be dependent on the nature of the added secondary transition metal ion. EPR spectra of all compounds in the temperature range of 8-300 K present a single resonance line shape. Temperature dependence of the EPR parameters, like line width, g values and double integrated area (I_DIN), are obtained from the spectra and present a scenario compatible with the magnetization results. The temperature dependence of the first derivative of the product (I_DINT) shows two maxima for all samples, with exception of the Co^II phase, indicating two critical temperatures, while these critical temperatures could not be clearly determined by dc susceptibility.