We propose to study the crystal structures of RbMnBr3, RbFeBr3 and RbNiBr3 between 300 and 0.3 K. RbMnBr3 under goes two structural phase transition by reducing Rb coordination. In this process the linear chains of BX3- octahedral shift along the c-axis. The resulting unit cell is aV3 enlarged in the basal plane. Finally the accompanied disorder in the structure is gone in the lowest temperature phase. which enlarges by a factor 4a.For RbFeBr3, one expect on the basis of dielectric measurement to observe a similar sequence. This has so far not been observed . For RbNiBr3 no low temperature data are available. The structural phase transition have a distinct effect on the magnetic ordering going from incommensurate ordering volume on SXD we will be able to determine the crystal and magnetic ordering in detail.