Two new manganese(II) selenite polymorphs with formula Mn(SeO3)·H2O have been synthesized by slow evaporation from an aqueous solution. The crystal structure of both compounds (1) and (2) have been solved from X-ray diffraction data. The structure of (1) was determined from single-crystal X-ray diffraction techniques. The compound crystallizes in the Ama2 space group, with a=5.817(1), b=13.449(3), c=4.8765(9) A and Z=4. The structure of (2) has been solved from X-ray powder diffraction data. This phase crystallizes in the P21/n space group with unit-cell parameters of a=4.921(3), b=13.121(7), c=5.816(1) A, β=90.03(2) and Z=4. Both polymorphs exhibit a layered structure formed by isolated sheets of MnO6 octahedra and (SeO3)2− trigonal pyramids in the (010) plane. These layers, which contain one manganese and selenium atom crystallographically independent, are formed by octahedra linked between them through the selenite oxoanions. The difference of both compounds consists in the stacking of the layers along the b-axis. The IR spectra show the characteristic bands of the selenite anion. Studies of luminescence performed at 6 K and diffuse reflectance spectroscopy have been carried out for both phases. The Dq and Racah (B and C) parameters, from luminescence and diffuse reflectance spectroscopy, are Dq=705, B=750, C=3325 cm-1 for (1) and Dq=720, B=745, C=3350 cm-1 for (2). The ESR spectra of both compounds are isotropic with g-values of 1.99(1). Magnetic measurements indicate the presence of antiferromagnetic couplings in both phases. The J-exchange parameters have been estimated by fitting the experimental magnetic data to a model for square-planar lattice. The values obtained are J/k=-0.83, −0.91 K and J′/k=-0.97, −1.20 K, for polymorphs (1) and (2), respectively.< Leer menos