The low-temperature phase of morpholinium tetra­fluoro­borate

The crystal structure of the low-temperature form of the title compound, C(4)H(10)NO(+)·BF(4) (-), was determined at 80 K. Two reversible phase transitions, at 158/158 and 124/126 K (heating/cooling), were detected by differential scanning calorimetry for this compound, and the sequence of phase transitions was subsequently confirmed by single-crystal X-ray diffraction experiments. The asymmetric unit at 80 K consists of three BF(4) (-) tetra-hedral anions and three morpholinium cations (Z' = 3). Hydrogen-bonded morpholinium cations form chains along the [100] direction. The BF(4) (-) anions are connected to these chains by N-H⋯F hydrogen bonds. In the crystal structure, two different layers perpendicular to the [001] direction can be distinguished, which differ in the geometry of the hydrogen bonds between cationic and anionic species.