Connection between Reorientational Motions of NH3 Groups and Phase Transitions in [Ni(NH3)6](NO3)2 and [Mg(NH3)6](NO3)2 Compounds

A. Migdał-Mikuli and E. Mikuli Chemical Faculty of the Jagiellonian University, R. Ingardena 3, 30-060 Kraków, Poland

Received: April 27, 1995

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Quasielastic neutron scattering data for [Νi(ΝH3)6](ΝO3)2 and [Mg(ΝH3)6](NO3)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 NH3 groups which in the [Me(NΗ3)6]2+ cation reorientate with the correlation time of order of 10-12 s. It has been ascertained that in phases I and II of both substances under study all NH3 groups reorientate fast (R = 6ΝH3 groups), whereas in phases III and IV only part of these groups reorientate fast (R = 3ΝΗ3 groups - in the case of [Νi(ΝΗ3)6](ΝO3)2 and R = 2ΝΗ3 groups - in the case of [Mg(ΝΗ3)6](NO3)2). Moreover, the temperature dependence of reorientational correlation time τ for these fast reorienting ΝH3 groups were also determined. The remaining NH3 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 (ΤhC2 - TcC2 ≈ 95 K and ≈ 35 K respectively for [Ni(ΝΗ3)6](ΝO3)2 and [Mg(ΝΗ3)6](NO3)2).

DOI: 10.12693/APhysPolA.88.527 PACS numbers: 61.50.Ks, 64.70.Kb, 28.20.Cz, 35.20.Yh