Connection between Reorientational Motions of NH$\text{}_{3}$ Groups and Phase Transitions in [Ni(NH$\text{}_{3}$)$\text{}_{6}$](NO$\text{}_{3}$)$\text{}_{2}$ and [Mg(NH$\text{}_{3}$)$\text{}_{6}$](NO$\text{}_{3}$)$\text{}_{2}$ Compounds

Abstract

Quasielastic neutron scattering data for [Νi(ΝH$\text{}_{3}$)$\text{}_{6}$](ΝO$\text{}_{3}$)$\text{}_{2}$ and [Mg(ΝH$\text{}_{3}$)$\text{}_{6}$](NO$\text{}_{3}$)$\text{}_{2}$ were subjected to a new analysis. On the basis of the excess of intensity of elastic component observed in these spectra in comparison to the intensity of this component resulting from the applied reorientational model this analysis determined temperature dependence of the new "R" parameter. This parameter gives us information concerning the number of these NH$\text{}_{3}$ groups which in the [Me(NΗ$\text{}_{3}$)$\text{}_{6}$]$\text{}^{2+}$ cation reorientate with the correlation time of order of 10$\text{}^{-12}$ s. It has been ascertained that in phases I and II of both substances under study all NH$\text{}_{3}$ groups reorientate fast (R = 6ΝH$\text{}_{3}$ groups), whereas in phases III and IV only part of these groups reorientate fast (R = 3ΝΗ$\text{}_{3}$ groups - in the case of [Νi(ΝΗ$\text{}_{3}$)$\text{}_{6}$](ΝO$\text{}_{3}$)$\text{}_{2}$ and R = 2ΝΗ$\text{}_{3}$ groups - in the case of [Mg(ΝΗ$\text{}_{3}$)$\text{}_{6}$](NO$\text{}_{3}$)$\text{}_{2}$). Moreover, the temperature dependence of reorientational correlation time τ for these fast reorienting ΝH$\text{}_{3}$ groups were also determined. The remaining NH$\text{}_{3}$ groups reorientate on a much slower time scale. Both temperature dependencies: R vs. Τ and τ vs. Τ register a deep thermal hysteresis of the phase transition: phase II - phase III (Τ$\text{}_{C2}^{h}$ - T$\text{}_{C2}^{c}$ ≈ 95 K and ≈ 35 K respectively for [Ni(ΝΗ$\text{}_{3}$)$\text{}_{6}$](ΝO$\text{}_{3}$)$\text{}_{2}$ and [Mg(ΝΗ$\text{}_{3}$)$\text{}_{6}$](NO$\text{}_{3}$)$\text{}_{2}$)