*

We present a systematic investigation of the crystal structure and magnetism of MnBi_2nTe_3n+1 (n = 1, 2, 3, 4) which have been recently found to be intrinsic magnetic topological insulators. Neutron diffraction unveils that considerable Bi atoms occupy at the Mn sites while no detectable magnetic Mn occupies at non-magnetic sites in all the cases. The occupancy of Mn monotonically decreases with the increase of n. Polarized neutron diffraction on MnBi₄Te₇ reveals that its magnetization density is obviously accumulated at the Mn site, corroborating the distribution of the chemical defects. We suggest that the site defects but not site mixings are inherent and should be a common mark to this family. Our work provides material-specified structural parameters that may be useful for band structure calculations to understand the observed topological surface states and for designing quantum magnetic materials through chemical doping.