Structural disorder and symmetry of nickel oxalate dihydrate affected by synthesis conditions

Nickel oxalate dihydrate α-NiC₂O₄∙2H₂O (space group C2/c) belongs to a family of oxalates with general formula M²⁺(C₂O₄)∙2H₂O (M = Fe, Mn, Mg, Zn, Ni, Co), known as humboldtine group minerals. To elucidate patterns of structural disorder we have synthesized Ni-oxalate dihydrate from water solution in a wide range of conditions (pH = 1.5–9.5, Ni/C₂O₄ = 0.17–4, T = 25–200 °C) and explored the synthesis products using powder X-ray diffraction including Rietveld refinement and scanning electron microscopy with EDX-spectroscopy. It was revealed that structural disorder of NiC₂O₄∙2H₂O affects the diffraction patterns and unit cell parameters. We have found an entire series of monoclinic structures, different by their degree of disorder. Excess of oxalate ions, weak-alkaline pH and especially temperature of crystallization medium are positively affecting the ordering of monoclinic nickel oxalate structure. There is an inverse correlation between structure disorder and its deviation from orthorhombic symmetry. Structural mechanisms of transitions between space groups Cccm, Fddd and C2/c in Ni(C₂O₄)∙2H₂O oxalates (polymorphism, desymmetrization) have been suggested. The study contributes to our understanding of crystal chemistry of humboldtine group oxalates and could be used in biotechnologies in future.