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

Abstract

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.

Jahn–Teller distortions in transition metal compounds, and their importance in functional molecular and inorganic materials

“…behave normally under pressure. 44,46,47 The other example is catena-[CuF 2 (OH 2 ) 2 (m-pyrazine)], which crystallises as a 1D coordination polymer, with the co-parallel chains being linked into three dimensions by O-HÁ Á ÁF hydrogen bonding (Fig. 5).…”

A perspective on applications of ligand-field analysis: inspiration from electron paramagnetic resonance spectroscopy of coordination complexes of transition metal ions

“…The configuration d 6 is best represented by the important ion Fe II and the reader is directed to the recent work by Tregenna-Piggott and co-workers on this system, 88 (and also to their work on Cr II 35,36 and Mn III 32 ) which make use of LFT as well a tour de force analysis of the JahnTeller effect in such complexes.…”

“…24 Another locus of these older theories is the place where they were brought into full flower, Copenhagen, Denmark. The work by Jesper Bendix [25][26][27] and Høgni Weihe 28 and their co-workers in Copenhagen, together with Philip L. W. TregennaPiggott in Switzerland, [29][30][31][32][33][34][35][36][37] represent superb examples of state-of-the-art experimental (not only HFEPR, but also optical and vibrational spectroscopy, and most significantly, inelastic neutron scattering (INS) 31 ) and computational studies. In particular, Bendix has directly compared LFT to the current luminary, DFT.…”

Pressure‐Induced Sequential Orbital Reorientation in a Magnetic Framework Material

“…[2] In a traditional ML 6 system, where the JT-active center is coordinated to identical ligands, the elongation of neighboring centers is uncorrelated unless there are cooperative interactions (e.g., crystal packing influences). However, when the metal is coordinated to a mixture of ligands, the differing crystal field strengths of the ligands will preferentially stabilize one of the possible JT orientations.…”

“…A single pressure-induced JT-switch has been observed in a handful of systems, including the MO 6 octahedra in various Tutton salts [(NH 4 ) 2 M(H 2 O) 6 (SO 4 ) 2 for M = Cr, Cu] [6,7] and the MnF 6 octahedra in Na 3 MnF 6 . [8] In each of these cases only a single ligand-type surrounds the JT-active center, with the ligands subtly differentiated by pressure-dependent secondorder intermolecular interactions.…”