Synthesis, thermal, spectroscopic and magnetic studies of the Mn(SeO3)·2H2O and Fe2(SeO3)3·3H2O selenites

Mn(SeO3)·2H2O (1) and Fe2(SeO3)3·3H2O (2) have been synthesized by slow evaporation from an aqueous solution in the case of (1) and using mild hydrothermal conditions for (2). The crystal structures of both phases have been refined by the Rietveld method. The compounds crystallize in different spatial groups, the P21/n monoclinic one with parameters a = 6.649(1) Å, b = 6.542(1) Å, c = 10.890(1) Å and β = 103.85(1)° being Z = 4 for (1) and the R3c trigonal space group with parameters a = 9.361(1) Å, c = 20.276(1) Å and Z = 6 for (2). The crystal structure of compound (1) consists of a three-dimensional framework formed by MnO6 octahedra and (SeO3)2− oxoanions with trigonal pyramidal geometry, which gives rise to Mn2O10 dimers of edge-sharing octahedra. The crystal structure of phase (2) can be described as a three-dimensional framework formed by MnO6 octahedra and (SeO3)2− oxoanions with trigonal pyramidal geometry. In this phase the octahedral entities are linked along the three crystallographic axes through the selenite anions. Diffuse reflectance spectrum and luminescent measurements for (1) indicate the existence of Mn2+ cations in a slightly distorted octahedral environment. Diffuse reflectance spectrum and Mössbauer spectroscopy, in the paramagnetic region, for (2) show the existence of Fe3+ cations in slightly distorted octahedral symmetry. ESR spectra of both compounds are isotropic with a g-value of 1.99(1) and 2.00(1), respectively. Magnetic measurements of both phases indicate an antiferromagnetic behavior. For phase (2), both, the ESR and magnetic measurements suggest a spin change from Fe3+ (S = 5/2) to Fe2+ (S = 2) at low temperatures.