Influence of the NiII/MnII ratio on the physical properties of heterometallic Ni2xMn(2−2x)P2S6 phases and potassium intercalates K0.8Ni2xMn(1.6−2x)P2S6·2H2O

Abstract

Bimetallic Ni_2xMn_2−2xP₂S₆ phases, where 2X = 0.4 (Ni0.4), 0.8 (Ni0.8) and 1.2 (Ni1.2), were synthesized by a microwave assisted method, starting from the corresponding potassium intercalates K_0.8Mn_1.6P₂S₆·2H₂O, K_0.8Ni_0.4Mn_1.2P₂S₆·2H₂O, and K_0.8Ni_0.8Mn_0.8P₂S₆·2H₂O. The magnetic, optical, and electrical properties were recorded, in order to visualize the influence of the secondary nickel(II) ions on the physical properties of the pristine Mn₂P₂S₆ phase. Both dc susceptibility and EPR data show that the antiferromagnetic interactions are attenuated in the layers of the bimetallic phases, while the critical temperature remains constant (ca. 80 K) for all the bimetallic phases, similar to that of Mn₂P₂S₆. Conversely, the absorption edge of the solid state UV-visible spectra shifted to lower energies as the percentage of nickel(II) increased, tending to that of the band gap value of the pristine Ni₂P₂S₆ phase. Moreover, the electric conductivity of Ni0.4 was similar to that of the pristine Mn₂P₂S₆ phase, while that of Ni0.8 and Ni1.2 approached the values of the electric conductivity of the pristine Ni₂P₂S₆ phase. On the other hand, the potassium intercalates K_0.8Mn_1.6P₂S₆·2H₂O and K_0.8Ni_0.4Mn_1.2P₂S₆·2H₂O showed magnetization at 16 K, the latter being much weaker as compared to the first. The detrimental effect of the secondary nickel(II) ions in the layers of the bimetallic phases became evident for K_0.8Ni_0.8Mn_0.8P₂S₆·2H₂O, where the magnetization at low temperatures was not observed. The absorption edge shifted to higher energy values for K_0.8Mn_1.6P₂S₆·2H₂O and K_0.8Ni_0.4Mn_1.2P₂S₆·2H₂O, as compared to Mn₂P₂S₆ and Ni0.4, respectively, but the value remained close to that of Ni₂P₂S₆ for K_0.8Ni_0.8Mn_0.8P₂S₆·2H₂O. In Ni0.8 and Ni0.4, the electrical conductivity at low frequencies stems from polarization processes such as Maxwell/Wagner/Sillars electrode polarization. Moreover, an additional polarization process is observed in the intercalates because K⁺ ions in the interlayer act as charge transport carriers. Consequently, the interacalates present higher electrical conductivity compared with the pristine Mn₂P₂S₆ phase, by a factor of ca. 10².