Solvent Dependent Phase Transition between Two Polymorphic Phases of Manganese–Tungstate: From Rigid to Hollow Microsphere

Crystallization of manganese (Mn2+) and tungstate (WO42–) ions in the presence of citric acid under different water/ethanol mixtures has been systematically investigated under solvothermal conditions. A unique phase transition between two polymorphic phases, formulated as MnWO4, manganese tungstate, and Mn8W12O42(OH)4·8H2O, manganese heteropolytungstate (Mn-HPT), was observed along with a striking morphological alteration from rigid to hollow microsphere. The effective coordination of citrate ion to tungstate (tungstate–citrate 1:1 complex) in aqueous solution before the hydrothermal treatment drives the system to nucleate the less symmetric, monoclinic (space group: P2/c) MnWO4 phase, which is the thermodynamically preferred polymorph. However, formation of the tungstate–citrate complex can be tuned by changing the dielectric constant of the solvent or by decreasing the citric acid to tungstate molar ratio. Results show that both conditions assist in the formation of the kinetically stable, more symmetric, cubic Mn-HPT (space group: Im3̅) phase at the same reaction temperature and time. The formation of the Mn-HPT phase is mediated by a kinetically advantageous crystallization process from an amorphous precursor, while later on it gradually converted into more stable MnWO4 phase according to “Ostwald rule of successive phase transformation”. Optimum reaction conditions for the synthesis and plausible growth mechanisms of both microspheres were proposed on the basis of solvent, reaction time, temperature, and the presence of citric acid. Magnetic properties of both samples were investigated in order to illuminate the nature of magnetic interaction within the crystal lattice.