Pressure tuning between NH•••N hydrogen-bonded ice analogue and NH•••Br polar dabcoHBr complexes

At normal conditions 1,4-diazabicyclo[2.2.2] octane hydrobromide [C6H13N2](+). Br- forms centrosymmetric crystals, space group P (6) over bar hm2, NH+center dot center dot center dot N hydrogen-bonded linear polycationic chains with disordered protons in the structure. As in H2O ice Ih, the protons in [C6H13N2](+). Br- crystals remain disordered at low temperatures. Above 0.4 GPa the [C6H13N2](+). Br- crystals transform into a new polar NH+center dot center dot center dot Br- hydrogen bonded complex, space group Cmc2. It has been crystallized in-situ in a diamond anvil cell and its structure determined by X-rays. The low-pressure triggering of this transformation indicates that it is a possible source of defects in the real structure at normal conditions, where, along with disproportionation defects, they can be responsible for anomalous dielectric properties, including relaxor-like behavior of NH center dot center dot center dot N hydrogen-bonded compounds.