NEUTRON POWDER DIFFRACTION STUDY OF THE HIGH PRESSURE JAHN-TELLER SWITCH IN (ND₄)₂[Cu(D₂O)₆](SO₄). A. J. Schultz, IPNS, Argonne National Laboratory, Argonne, IL 60439, USA; H. Stratemeier, M. A. Hitchman, Chemistry Department, University of Tasmania, Hobart, Tasmania 7001, Australia; C. J. Simmons, Mathematics and Science Division, Brigham Young University - Hawaii, Laie, Hawaii 96762, USA.

The Jahn-Teller distortion switch in the deuterated ammonium copper(II) Tutton salt (ND₄)₂[Cu(D₂O)₆](SO₄) was observed to occur at room temperature at about 200 bar when increasing pressure, and at about 100 bar when decreasing pressure. This salt is especially interesting because the long axis of the distorted hydrogenated Cu(H₂O)₆²⁺ complex switches to a different pair of water molecules upon deuteration [1], and then switches back to the structure of the hydrogenous compound upon raising the pressure, as first shown from the single crystal study of (ND₄)₂[Cu(D₂O)₆](SO₄) using the IPNS SCD [2]. In the current study, the pressure-temperature phase diagram was examined in greater detail by powder neutron diffraction and the switching is shown to occur at very mild pressure at room temperature with a hysteresis of about 100 bar. The role of hydrogen bonding, lattice strain, and the pressure dependence of the EPR spectrum [3] will be discussed in light of these results.

1. B. J. Hathaway and A. W. Hewat, J. Solid State Chem. 51, 364 (1984).

2. C. J. Simmons, M. A. Hitchman, H. Stratemeier and A. J. Schultz, J. Am. Chem. Soc. 115, 11304 (1993).

3. W. Rauw, H. Ahsbahs, M. A. Hitchman, S. Lukin, D. Reinen, A. J. Schultz, C. J. Simmons and H. Stratemeier, Inorg. Chem., in press (1996).

Work at Argonne supported by the U.S. Department of Energy, BES-Materials Science, under Contract No. W-31-109-ENG-38.