Phase transition and NH3 dynamics in [Ni(NH3)4](ReO4)2 studied by infrared absorption, X-ray powder diffraction and neutron scattering methods

- Dynamics of NH3 groups probed by IR absorption and quasielastic neutron scattering measurements.
- Splitting of some IR bands and X-ray powder diffraction measurements indicate structural character of the phase transition.
- Reorientational motion of NH3 ligands is not perturbed in the vicinity of phase transition.
- Activation energy for NH3 reorientational motion was estimated from IR band shape analysis.

The phase transition in [Ni(NH3)4](ReO4)2 detected previously by differential scanning calorimetry (DSC) at Tch = 188 K was now investigated by infrared absorption (FT-IR), incoherent inelastic and elastic neutron scattering (IINS, ND), and X-ray powder diffraction (XRPD) methods. The reorientational dynamics of NH3 groups was investigated by infrared band shape analysis (IRBS) and quasielastic neutron scattering (QENS) methods. The infrared data show that some of the bands split in the vicinity of the phase transition temperature, which suggests a change in the crystal structure. The systematic narrowing of particular bands at cooling is also observed, but reorientational dynamics of NH3 is not stopped at the phase transition temperature, which is fully confirmed by the QENS analysis. The broadening of the quasielastic neutron scattering peak is clearly visible below the phase transition temperature. Both NPD and XRPD measurements indicate that a small change of crystal structure is associated with the phase transition.

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