  We investigated the crystal and magnetic structures of the spin-1/2
frustrated antiferromagnet Cu3Mo2O9 in which the spin system consists of
antiferromagnetic chains and dimers. The space group at room temperature has
been reported to be orthorhombic Pnma (No. 62). We infer that the space group
above TN = 7.9 K is monoclinic P2_1/m (No. 11) from the observation of
reflections forbidden in Pnma in x-ray powder diffraction experiments at room
temperature. We determined the magnetic structure of Cu3Mo2O9 in neutron powder
diffraction experiments. Magnetic moments on dimer sites lie in the ac planes.
The magnitudes are 0.50 - 0.74 mu_B. Moments on chain sites may exist but the
magnitudes are very small. The magnetic structure indicates that a partial
disordered state is realized. We consider the origin of the magnetic structure,
weak ferromagnetism, and electric polarization.
