Molecular and electron-spin dynamics in [M(H2O)6] [AB6] as studied by solid state NMR

Abstract

A method using the 2H-NMR spectra was shown to be useful to study the solid state physics in diamagnetic and paramagnetic compounds and in crystals having a modulated structure. The spectra and T1 of NMR in solids were measured for [M(H2O)6][AB6] crystals to investigate the static and dynamic structure of H2O and [M(H2O)6]2+ as well as the dynamics of the electron spin in the paramagnetic M2+ ion. The physical properties of [M(H2O)6] [AB6] were found to be as follows: (1) The spin-lattice relaxations of the electron spin of M2+ in [M(H2O)6] [SiF6] (M2+ = Fe 2+ , Co2+ , Ni2+) are dominated by the Orbach process, the Orbach process, and the Raman process, respectively. The spin-lattice relaxation of the electron spin of Cu2+ in [Cu(H 2O)6][PtCl6] is caused by jumping between the Jahn-Teller configurations. (2) In [Cu(H2O)6] [PtCl 6], H2O and [Cu(H2O)6]2+ undergo 180° flips and jumping between the different Jahn-Teller configurations, respectively. A weakening of the hydrogen bond O-H⋯Cl upon deuteration results in a lowering of the transition temperature. (3) In [M(H2O)6][SiF6], H2O and [M(H 2O)6]2+ undergo 180° flips and reorientation about the C3, axis, respectively. The order-disorder transition is closely related to a freezing of this reorientation. (4) By elongation of [M(H2O)6]2+ along the C 3 axis, the mobility becomes higher. (5) The disorder of [Fe(H 2O)6]2+ in the high-temperature phase of [Fe(H2O)6][SiF6] is dynamic. Rotational modulation of [Mg(H2O)6]2+ along the C 3 axis exists in the incommensurate phase of [Mg(H2O) 6] [SiF6].