Displacive components of the low-temperature phase transitions in lawsonite

The phase transitions in deuterated lawsonite were investigated with high-r esolution, time-of-flight neutron diffraction between 2 and 500 K. From the analysis of spontaneous strain data, the thermodynamics of the phase trans ition at 273 K are not changed by the deuteration process. Shifts in atomic positions with temperature indicate continuous changes for a framework oxy gen and for one of the deuterium atoms, whereas for the other deuterium ato m, a more discontinuous behavior was observed in the average structure. Com parison of O . . .D and O . . .O bond lengths with IR data from a non-deute rated lawsonite permits a detailed analysis of assignments of O-H stretchin g modes.