Pressure-induced switch of the direction of the unique Jahn-Teller axis of the chromium(II) hexaqua cation in the deuterated ammonium chromium Tutton salt

Inelastic neutron scattering (INS) spectra are presented for chromium(II) Tutton salts, as a function of the temperature and pressure. Transitions are observed between the levels of the 5Ag (Ci) ground term and the data modeled with a conventional S = 2 spin Hamiltonian. At 10 K and ambient pressure, the zero-field-splitting parameters of the ammonium salt, (ND4)2Cr(D2O)6(SO4)2, are determined as D = -2.431(4) cm(-1) and E = 0.091(4) cm(-1), evolving to D = -2.517(4) cm(-1) and E = 0.127(5) cm(-1) upon application of 7.5(1.0) kbar of quasi-hydrostatic pressure. By contrast, the change in the INS spectrum of the rubidium salt in this pressure range is comparitively minor. The results are interpreted using a 5Ee vibronic-coupling Hamiltonian, in which low-symmetry strain, perturbing the adiabatic potential-energy surface, is pressure-dependent. It is argued that, for the ammonium salt, the change with pressure of the anisotropic strain impinging upon the Cr(D2O)6]2+ cation is sufficient to cause a switch of the long and intermediate Cr-OD2 bonds, with respect to the crystallographic axes.