Static and dynamic crystal-field effects in ferrous fluosilicate

1987 ◽  
Vol 65 (10) ◽  
pp. 1280-1293 ◽  
Author(s):  
D. C. Price
Keyword(s):  
Low Temperature ◽  
Crystal Field ◽  
Metal Ion ◽  
Vibronic Coupling ◽  
Spin Lattice Relaxation ◽  
Temperature Phase ◽  
Relaxation Rates ◽  
Zinc Compounds ◽  
Crystal Fields ◽  
Low Temperature Phase

This paper examines the crystal fields acting on Fe2+ ions in ferrous fluosilicate, and the way that they influence the magnetic and spectroscopic properties of the compound. The crystal structures of ferrous fluosilicate and of the structurally similar magnesium, manganese, cobalt, nickel, and zinc compounds are described to establish the point symmetries of the metal-ion sites. In the low-temperature monoclinic (P21/c) phase of the Fe2+, Mg2+, Co2+, and Mn2+ compounds, although the metal-ion site is required crystallographically to have only inversion symmetry, all of the available low-temperature data are consistent with a crystal field of C2h symmetry. In the high-temperature phases of these salts, and for nickel and zinc fluosilicates at all temperatures, the site symmetries are either exactly or approximately C3i. The dominant effect of the phase transition on the Fe2+ crystal field in FeSiF6∙6H2O is to remove the nonaxial terms [Formula: see text] and [Formula: see text] that are present in the low-temperature phase. The other terms appear to be relatively unaffected.Both the time-dependent (spin-lattice relaxation) and time-independent (vibronic coupling) manifestations of the dynamic crystal field at the Fe2+ ions are qualitatively examined. A new interpretation of the 57Fe Mössbauer spectra of FeSiF6∙6H2O in the low-temperature phase is described, from which relaxation rates comparable to those deduced from other measurements are obtained. Evidence that the measured 57Fe Mössbauer quadrupole splittings of Fe2+ ions in FeSiF6∙6H2O and in ZnSiF6∙6H2O show significant effects of vibronic coupling is examined.

Phase transitions in solid cycloheptene

1990 ◽  
Vol 68 (4) ◽  
pp. 604-611 ◽  
Author(s):  
Julian Haines ◽  
D. F. R. Gilson
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Low Temperature ◽  
Lattice Relaxation ◽  
Proton Spin ◽  
Spin Lattice Relaxation ◽  
Temperature Phase ◽  
Scanning Calorimetry ◽  
Spin Lattice ◽  
Low Temperature Phase

The phase transition behaviour of cycloheptene has been investigated by differential scanning calorimetry, proton spin-lattice relaxation, and vibrational spectroscopy (infrared and Raman). Two solid–solid phase transitions were observed, at 154 and 210 K, with transition enthalpies and entropies of 5.28 and 0.71 kJ mol−1 and 34.3 and 3.4 JK−1, respectively. Cycloheptene melted at 217 K with an entropy of melting of 4.5 JK−1 mol−1. The bands in the vibrational spectra of the two high temperature phases were broad and featureless, characteristic of highly disordered phases. The presence of other conformers, in addition to the chair form, was indicated from bands in the spectra. The ring inversion mode was highly phase dependent and exhibited soft mode type behaviour prior to the transition from the low temperature phase. The low frequency Raman spectra (external modes) of these phases indicated that the molecules are undergoing isotropic reorientation. In the low temperature phase, the vibrational bands were narrow; the splitting of the fundamentals into two components and the presence of nine external modes are consistent with unit cell symmetry of either C2 or Cs with two molecules per primitive unit cell. A glassy state can be produced from the intermediate phase and the vibrational spectra were very similar to those of the high temperature phases, indicating that static disorder was present. The barriers to reorientation, as obtained from proton spin-lattice relaxation measurements, are 9.0 kJ mol−1 in both the high temperature phases, and 15.4 kJ mol−1 in the low temperature, ordered phase. Keywords: cycloheptene, phase transition, differential scanning calorimetry, NMR, vibrational spectroscopy.

Deuterium NMR Study of the Solid-Solid Phase Transition in Phenethylammonium Bromide

1991 ◽  
Vol 46 (8) ◽  
pp. 691-696 ◽  
Author(s):  
Marco L. H. Gruwel ◽  
Roderick E. Wasylishen
Keyword(s):  
Phase Transition ◽  
Low Temperature ◽  
Line Shape ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Temperature Phase ◽  
Spin Lattice ◽  
H Nmr ◽  
Line Shapes ◽  
Low Temperature Phase

AbstractUsing 2H NMR, the dynamics of the cation in phenethylammonium bromide were studied in the two solid phases. Line shape and spin-lattice relaxation rate studies of the ammonium headgroups and the adajacent methylene groups indicate the onset of alkyl-chain motion prior to the first order phase transition. In the low-temperature phase the line shape and the spin-lattice relaxation rates of the -ND3 groups are consistent with C3 jumps and an activation energy of 54±4 kJ mol-1. However, in the high-temperature phase the spin-lattice relaxation studies indicate the presence of small-angle diffusion of the -ND3 groups around the C3 symmetry axis. In this phase the -CD2- groups show line shapes typical of large-amplitude two-site jumps occurring at a rate > 107 s-1 . In the low-temperature phase, at temperatures below 295 K, the -CD2- 2H NMR line shapes indicate that the C - D bonds are essentially static

Molecular Motion in Phenpropylammonium Chloride Studied by Deuterium NMR

1992 ◽  
Vol 47 (10) ◽  
pp. 1073-1086
Author(s):  
Marco L. H. Gruwel ◽  
Roderick E. Wasylishen
Keyword(s):  
Low Temperature ◽  
Large Amplitude ◽  
Phenyl Ring ◽  
Rotational Diffusion ◽  
Spin Lattice Relaxation ◽  
Temperature Phase ◽  
H Nmr ◽  
Line Shapes ◽  
Low Temperature Phase ◽  
Methylene Group

AbstractCation dynamics in phenpropylammonium chloride, C6H5(CH2)3NH3Cr, were studied in three different solid phases by means of 2H nmr. Both 2H nmr line shapes and spin-lattice relaxation studies were performed on the ammonium head group, the adjacent methylene group and the phenyl-ring. In the low temperature phase, solid III, the - ND3 group dynamics were dominated by C3 jumps about the C - N axis. From the observed minima in T1Z and T1Q a quadrupole coupling constant of 165 ± 5 kHz was obtained. The 2H nmr line shapes of the methylene group indicate that this group does not execute any large amplitude motion in the low temperature phase. In contrast, the phenyl ring deuterium nuclei give rise to line shapes characteristic of C2 ring flips about the Cpara - Cipso axis.In the solid II phase the 2H nmr line shapes of both the - ND3 and - CD2 - groups are characterized by asymmetric Pake powder patterns. Since one of the principal components of the electric field gradient tensor remains temperature independent for both groups, it was concluded that these groups perform planar large-amplitude motions between two sites. Axially symmetric spectra were obtained for all three groups in the high temperature phase, solid I. The 2H nmr line shapes indicate the presence of whole ion reorientation about a molecular axis. In addition, the - ND3 groups perform C3 jumps about the C - N axis and the - C6D5 group display line shapes characteristic for rotational diffusion about the Cpara - Cipso axis.

Quantum-Mechanical Theory of Orientational Ordering in Solid Methanes

1972 ◽  
Vol 50 (13) ◽  
pp. 1568-1578 ◽  
Author(s):  
S. Alexander ◽  
M. Lerner-Naor
Keyword(s):  
Low Temperature ◽  
Field Approximation ◽  
Quantum Mechanical ◽  
Temperature Phase ◽  
Mechanical Theory ◽  
Crystal Fields ◽  
Quantum Mechanical Theory ◽  
Full Symmetry ◽  
Orientational Ordering ◽  
Low Temperature Phase

The quantum-mechanical theory of the low temperature phase transitions of the solid methanes is discussed. It is shown how the standard techniques for treating the molecular field approximation can be applied to these problems. It is shown how internal and external symmetries should be combined so as to utilize the full symmetry of the problem. This analysis is quite general and applicable to all problems of orientational ordering. Results of detailed calculations for the James–Keenan model of CH4 and CD4 are presented and compared with experimental results. The calculations assume octupole interactions, include no crystal fields, and include all states up to J = 6. It is suggested that the inclusion of crystal field and possibly states of higher J is essential for describing the low temperature behavior.

scholarly journals NQR Parameters in Incommensurate Cs2CdBr4 and Cs2HgBr4 Crystals

1986 ◽  
Vol 41 (1-2) ◽  
pp. 261-264 ◽  
Author(s):  
Hirokazu Nakayama ◽  
Nobuo Nakamura ◽  
Hideaki Chihara
Keyword(s):  
Low Temperature ◽  
Relaxation Times ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Temperature Phase ◽  
Critical Fluctuation ◽  
Spin Lattice ◽  
Commensurate Phase ◽  
Lock In ◽  
Low Temperature Phase

The temperature dependence of the 81Br spin-lattice relaxation times for Cs2CdBr4 and Cs2HgBr4 was measured in the low temperature and the commensurate phases. For the commensurate phase of Cs2CdBr4 rapid shortening of the T1 of νB ~ νC was observed on approaching the “lock-in” transition point. It is probably due to an anisotropic critical fluctuation. On the other hand, T, in the low temperature phase of Cs2HgBr4 behaves like an order parameter but no critical decrease of T1 was observed in the commensurate phase.

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