Electron paramagnetic resonance investigations of MgSiF₆.6H₂O:Mn²⁺single crystals

Abstract

EPR studies were performed on MgSiFs . 6Hz0 single crystals using MnZt paramagnetic probes substituted for Mg2+ ions. The spin-Hamiltonian parameters (SHPS) are determined accurately for the different phases: the monoclinic phase below T, = 301 K, the rhombohedral phase above To = 370 K and the intermediate phase between T, and To. The sets of SHFS are interpreted in the light of sUuchlral changes between these phases. The EPR line features in the intermediate phase are analysed in the framework of a sWic and an orderdisorder model. We interpret the EPR specwm behaviours around To -20 K as the occurrence of a crossover between static and dynamic disorder in the material.

Study of Structural Phase Transitions in [Mg(H2O)6][SiF6] by Means of Single Crystal 2H NMR

“…The modulational amplitude (∆ ) could be determined by the simulation of the spectra at = 135 using estimated from the simulation at = 90 . In our model, the rotational modulation of [Mg(H 2 O) 6 ] 2+ influenced on the spectra at = 135 not to produce the 'edge singularities' [19,20] but to reduce the linewidth. This effect can be explained as follows: The signal for the different orientation of [Mg(H 2 O) 6 ] 2+ arising from the rotational modulation is added in (8), and the linewidth and the intensity of the spectrum transformed from each signal can be distinct owing to the reorientation of [Mg(H 2 O) 6 ] 2+ .…”

“…Three 2(∆ ) which can be attributed to three unequal water molecules in [Mg(H 2 O) 6 ] 2+ were observed for each rotation. It is found that the electric field gradient (EFG) tensors for two deuterons in a water molecule are averaged by the fast 180 flip of H 2 O resulting in the same tensors [9,12] and that inversion symmetry exists for [Mg(H 2 O) 6 ] 2+ at this temperature [2]. The fitting calculation for the rotation around the axis was performed with the equation [13] 2(∆ ) = 3 4 + ( ) cos (2 ) (1) + 2 sin (2 ) where is the angle between the axis and the external magnetic field 0 .…”

“…Assuming an Arrhenius relation, is given by [9]. We believe that the former is better than the latter, because in this study the spectra were analyzed by separating the effect on the spectral shape of the motion of [Mg(H 2 O) 6 ] 2+ and that of the modulated structure. Figure 5 shows the temperature dependences of the 2 H NMR quadrupole splitting, 2(∆ ), in phases I and II.…”

“…A further transition at 343 K was observed in the incommensurate phase [1, 7 -9]. 0932 6 ] 2+ was estimated. Moreover, the correlation between this motion and II-III transition was obtained.…”

“…The structure of phase II was specified as an arrangement of domains of different octahedra orientation with overall space group P3, although an analysis based on a modulated structure was not performed. The modulated structure was found by ESR spectroscopy to be a vibrational modulation of [Mg(H 2 O) 6 ] 2+ expressed by a plane-wave nearly along the 3 axis [1]. A further transition at 343 K was observed in the incommensurate phase [1, 7 -9].…”