Hysteresis of the pressure-induced Jahn-Teller switch in deuterated ammonium copper(II) Tutton salt, (ND4)(2)[Cu(D2O)(6)](SO4)(2)

A pressure-induced phase change in the title compound involving a Jahn-Teller switch is observed at an applied pressure of similar to 200-250 bar at room temperature by both neutron diffraction and EPR spectroscopy. The transition reverses upon lowering the pressure with a hysteresis of about 100 bar. The single-crystal EPR spectra exhibit signals due to individual molecules in the low-pressure phase which become exchange-averaged in the high-pressure phase up to at least 600 bar in the case of the single-crystal measurements, and diverge at about 1 kbar as shown in previous powder measurements.