First and Second Order Phase Transitions in Glyphosate at High Pressure

A recent high pressure Raman study indicates that glyphosate undergoes two phase transitions at ~1 GPa and between ~4 GPa. Single crystal X-ray diffraction finds no discontinuities in the lattice or structural parameters. These contradictory findings were resolved by partitioning the cell volume into contributions from occupied and void space to reveal two small events at 0.9 and 3.8 GPa in the occupied volume, suggesting that the transitions are second order and correspond to changes in the mechanism by which pressure is accommodated. This neutron time will resolve an unusual oscillation in the position of one H-atom, required for the packing energy calculations that partly define the order of the transition. Glyphosate becomes polycrystalline at ~6 GPa after a first order phase transition; we will also collect data for complete structural characterisation of this new phase.