Abstract
The results of an X-ray diffraction investigation of powdered H3N.BH3 are reported. A first-order rotational order-disorder phase transition has been observed at 225K. In the high-temperature tetragonal phase the space group permits twelvefold reorientations of GH3 and NH3 groups. The cell parameters are linear functions of temperature in this phase. The low-temperature structure is orthorhombic. The length of the c axis is independent of temperature while the a axis decreases non-linearly in length as the temperature is increased. The length of the b axis increases non-linearly with increasing temperature. With the exception of the c axis which changes very little in length at the phase transition, all cell dimensions as well as the volume of the unit cell show drastic changes at the phase transition. The temperature dependences of the cell parameters are explained qualitatively in terms of the reorientations and relative orientations of the BH3 and NH3 groups.