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 200-25 0 bar at room temperature by both neutron diffraction and EPR spectros copy. The transition reverses upon lowering the pressure with a hyster esis of about 100 bar. The single-crystal EPR spectra exhibit signals due to individual molecules in the low-pressure phase which become exc hange-averaged in the high-pressure phase up to at least 600 bar in th e case of the single-crystal measurements, and diverge at about 1 kbar as shown in previous powder measurements.