Mixed sodium nickel-manganese sulfates: Crystal structure relationships between hydrates and anhydrous salts

Abstract

The present contribution provides new structural and spectroscopic data on the formation of solid solutions between hydrated and dehydrated sulfate salts of sodium-nickel and sodium-manganese in a whole concentration range: Na2Ni1−xMnx(SO4)2·yH2O, 0≤ x≤1.0. Using powder XRD, electron paramagnetic resonance spectroscopy (EPR), IR and Raman spectroscopy it has been found that double sodium-nickel and sodium-manganese salts form solid solutions Na2Ni1−xMnx(SO4)2·4H2O with a blödite-type of structure within a broad concentration range of 0≤x≤0.49, while the manganese rich compositions Na2Ni1−xMnx(SO4)2·2H2O (0.97≤x≤1.0) crystallize in the kröhnkite-type of structure. The Ni-based blödites Na2Ni1−xMnx(SO4)2·4H2O dehydrate between 140 and 260°C into anhydrous salts Na2Ni1−xMnx(SO4)2, 0≤ x≤0.44, with a structure where Ni1−xMnxO6 octahedra are bridged into pairs by edge- and corner sharing SO42− groups. Both TEM and EPR methods show that the Ni2+ and Mn2+ ions are homogenously distributed over three crystallographic positions of the large monoclinic cell. The dehydration of the kröhnkite phase Na2Ni1−xMnx(SO4)2·2H2O yields the alluaudite phase Na2+δMn2-δ/2(SO4)3, where the Na-to-Mn ratio decreases and all Ni2+ dopants are released from the structure. The process of the dehydration is discussed in terms of structural aspects taking into account the distortion degree of the Ni,MnO6 and SO4 polyhedra.