Solid state and combustion synthesis of Mn2+-doped scheelites – Their optical and magnetic properties

Abstract Pure and manganese doped calcium molybdates (Ca 1−x Mn x MoO 4 ) as well as molybdato-tungstates (Ca 1−x Mn x (MoO 4 ) 0.50 (WO 4 ) 0.50 ) for 0 x ≤ max. 0.15 were synthesized via high-temperature annealing and citrate-nitrate combustion method. The X-ray diffraction and scanning electron microscopy results confirm the formation of single, tetragonal scheelite-type phases (space group I 4 1 /a ) with an average crystallite size in the range of ~ 100 nm – max. 1 µm (combustion synthesis) or ~ 10 – ~ 40 µm (solid state reaction route). The melting point of each Mn-doped ceramic material is lower than the melting point of adequate scheelite-type matrix. IR analysis exhibited Mo-O or Mo-O as well as W-O stretching and bending bands of MoO 4 or MoO 4 /WO 4 tetrahedra, which related to a scheelite-type structure. The diffused reflectance spectra were used to calculate the optical direct band gap ( E g ) of pure and Mn-doped materials. Their E g value was estimated in the range of 3.65–4.34 eV. EPR measurements confirmed, that Mn 2+ ions exist in the structure as isolated magnetic centres if Mn doping is low. With increasing of manganese concentration the exchange and dipolar interactions lead to developing of the magnetic system, where antiferromagnetic interactions dominate among Mn 2+ ions. Strength of interactions is higher with higher manganese concentration.