A1 Refereed original research article in a scientific journal
Deuteron NMR relaxation, spectra, and evidence for the order-disorder phase transition in (ND4)(2)PtCl6
Authors: Birczynski A, Ylinen EE, Punkkinen M, Prager M, Szymocha AM, Lalowicz ZT
Publisher: AMER INST PHYSICS
Publication year: 2008
Journal:: Journal of Chemical Physics
Journal name in source: JOURNAL OF CHEMICAL PHYSICS
Journal acronym: J CHEM PHYS
Volume: 128
Issue: 18
First page : 184510
Number of pages: 9
ISSN: 0021-9606
DOI: https://doi.org/10.1063/1.2919568
Abstract
Deuteron NMR relaxation and spectra were studied at the resonance frequency of 46 MHz in polycrystalline (ND4)(2)PtCl6 between 300-5 K. The relaxation rate maximum near 50 K is about 53% smaller than the calculated maximum related to 120 degrees rotations about the threefold symmetry axes of the ammonium ion. The difference is explained by assuming for a N-D vector a total of 24 equilibrium directions, which in groups of six deviate from the nearest Pt-N vector by a certain angle Theta. So-called limited jumps between the directions of each group take place much more frequently than the large-angle rotations, thus rendering a fraction of the deuteron quadrupole coupling ineffective in relaxation. A motional model is presented, which takes into account both these motions simultaneously. A comparison with experimental data leads to Theta=26.0 degrees, in reasonable agreement with earlier neutron diffraction data. A sharp decrease found in the relaxation rate at the order-disorder phase transition temperature of 27.2 K is related to the fact that one of the six equilibrium directions becomes preferred. This leads to a formation of ordered domains, in which the active motion driving the relaxation is 120 degrees rotations. Two components in the spectra found below 55 K are related to domains (broad) and transition regions between domains (narrow). Reasons for the nonexponentiality observed below 20 K are discussed, the most likely explanation being that limited jumps dominate within transition regions and make the corresponding deuterons relax faster than those in domains. (C) 2008 American Institute of Physics.
Deuteron NMR relaxation and spectra were studied at the resonance frequency of 46 MHz in polycrystalline (ND4)(2)PtCl6 between 300-5 K. The relaxation rate maximum near 50 K is about 53% smaller than the calculated maximum related to 120 degrees rotations about the threefold symmetry axes of the ammonium ion. The difference is explained by assuming for a N-D vector a total of 24 equilibrium directions, which in groups of six deviate from the nearest Pt-N vector by a certain angle Theta. So-called limited jumps between the directions of each group take place much more frequently than the large-angle rotations, thus rendering a fraction of the deuteron quadrupole coupling ineffective in relaxation. A motional model is presented, which takes into account both these motions simultaneously. A comparison with experimental data leads to Theta=26.0 degrees, in reasonable agreement with earlier neutron diffraction data. A sharp decrease found in the relaxation rate at the order-disorder phase transition temperature of 27.2 K is related to the fact that one of the six equilibrium directions becomes preferred. This leads to a formation of ordered domains, in which the active motion driving the relaxation is 120 degrees rotations. Two components in the spectra found below 55 K are related to domains (broad) and transition regions between domains (narrow). Reasons for the nonexponentiality observed below 20 K are discussed, the most likely explanation being that limited jumps dominate within transition regions and make the corresponding deuterons relax faster than those in domains. (C) 2008 American Institute of Physics.
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