The effects of Mn substitution on the structure and magnetic properties of the SeMO3 (M = Cu1-xMnx) solid solution have been studied. Rietveld refinements of the x-ray diffraction patterns of these samples indicate that the manganese ions occupy copper sites. This replacement induces significant changes in the M-O bond lengths that give rise to abrupt decreases of the [M-O-6] octahedral distortion. In contrast, the M-O(1)-M and M-O(2)-M bond angles remain essentially constant. The magnetic behaviour of this solid solution was studied in the temperature range of 2 K < T < 300 K. The temperature dependence of the inverse magnetic susceptibility is well described by the Curie-Weiss law at high temperatures, in the composition range studied. We found that the substitution of Mn for Cu induces a sharp drop in the saturation moment of SeCuO3. At about 10% of Mn there is a change from positive to negative Weiss constant theta(W) that is mainly due to the [M-O-6] octahedral distortion introduced by the substitution of the larger Mn ions in the structure. The M-H isotherms show a weak ferromagnetism at about 52 K in SeMnO3
