On the formation of solid solutions with blödite- and kröhnkite-type structures

Abstract

In this contribution, we provide experimental results on the synthesis and some properties (structural, spectroscopic and thermal) of limited solid solutions Na2Ni1−xCux(SO4)2·4H2O (0 < x ≤ 0.17). Based on different experimental methods—vibrational spectroscopy (infrared and Raman), X-ray powder diffraction at ambient and elevated temperature, electron paramagnetic resonance spectroscopy (EPR) and TG–DTA–DSC methods, it has been established that the copper ions are included in the nickel blodite-type structure up to about 17 mol%. However, the nickel ions are not able to accept the coordination environment of the copper ions in the strongly distorted [Cu(H2O)2O4] clusters due to the Jahn–Teller effect, thus resulting in the crystallization of a krohnkite free from nickel ions. The X-ray powder diffraction data show that the included copper ions do not influence considerably on the lattice parameters of the blodite matrix owing probably to the buffer effect of the sodium ions and hydrogen bonds. The vibrational spectra are interpreted in the light of their crystal structures. The TG and DTA methods combined with X-ray powder diffraction at elevated temperature reveal that the dehydration of Na2Ni(SO4)2·4H2O and Na2Ni1−xCux(SO4)2·4H2O solid solutions occurs stepwise, thus forming dihydrates and anhydrous compounds—Na2Ni(SO4)2 and Na2Ni1−xCux(SO4)2 of the same structures. It has been found that the increased amount of copper ions causes increasing in the values of ΔH deh and decreasing in the values of ΔH f of the solid solutions. EPR spectra of the solid solutions are presented and compared to that of Na2Zn0.86Cu0.14(SO4)2·4H2O.