Synthesis, structure and properties of blödite-type solid solutions, Na2Co1−xCux(SO4)2·4H2O (0 < x ≤ 0.18), and crystal structure of synthetic kröhnkite, Na2Cu(SO4)2·2H2O

Abstract

Based on different experimental methods—crystallization processes in aqueous solutions, infrared spectroscopy, single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) and TG–DTA–DSC measurements—it has been established that copper ions are included in sodium cobalt sulfate up to about 18 mol%, thus forming limited solid solutions Na2Co1−xCux(SO4)2·4H2O (0 < x ≤ 0.18) with a blodite-type structure. In contrast, cobalt ions are not able to accept the coordination environment of the copper ions in the strongly distorted Cu(H2O)2O4 octahedra, thus resulting in the crystallization of Co-free krohnkite. The solid solutions were characterized by vibrational and EPR spectroscopy. DSC measurements reveal that the copper concentration increase leads to increasing values of the enthalpy of dehydration (ΔHdeh) and decreasing values of the enthalpy of formation (ΔHf). The crystal structures of synthetic krohnkite, Na2Cu(SO4)2·2H2O, as well as of three Cu2+-bearing mixed crystals of Co-blodite, Na2Co1−xCux(SO4)2·4H2O with x(Cu) ranging from 0.03 to 0.15, have been investigated from single-crystal X-ray diffraction data. The new data for the structure of synthetic krohnkite facilitated to clarify structural discrepancies found in the literature for natural krohnkite samples, traced back to a mix-up of lattice parameters. The crystal structures of Co-dominant Na2Co1−xCux(SO4)2·4H2O solid solutions reveal a comparatively weak influence of the Jahn–Teller-affected Cu2+ guest cations up to the maximum content of x(Cu) = 0.15. The response of the MO2(H2O)4 octahedral shape by increased bond-length distortion with Cu content is clear cut (but limited), mainly concerning the M–OH2 bond lengths, whereas other structural units are hardly affected. However, the specific type of imposed distortion seems to play an important role impeding higher Cu/Co replacement ratios.