Structural changes in the antifluorite crystal (NH4)2SnBr6

1983 ◽  
Vol 61 (7) ◽  
pp. 997-1002 ◽  
Author(s):  
Robin L. Armstrong ◽  
Rose M. Morra ◽  
Brian M. Powell ◽  
William J. L. Buyers
Keyword(s):  
Nuclear Quadrupole Resonance ◽  
Structural Changes ◽  
Profile Analysis ◽  
Structural Phase ◽  
Unit Cell ◽  
Structural Phase Transitions ◽  
Quadrupole Resonance ◽  
Large Distortion ◽  
Small Distortion ◽  
Nuclear Quadrupole

A study of structural phase transitions in the antifluorite crystal (NH4)2SnBr6 by nuclear quadrupole resonance and neutron powder diffraction techniques is presented. Interpretation of the bromine nuclear quadrupole resonance spectrum leads to a prediction concerning the nature of the distortions that occur as the temperature is lowered. Profile analysis of neutron diffraction data substantiates the prediction by revealing that the 157 K transition involves the antiferro-rotation of halide octahedra accompanied by a relatively small distortion of the unit cell whereas the 144 K transition involves a further ferro-rotation of the octahedra and a relatively large distortion of the unit cell.

Neutron-diffraction study of the structural phase transitions in the antifluorite K2PtBr6

1985 ◽  
Vol 63 (7) ◽  
pp. 988-996 ◽  
Author(s):  
Robin L. Armstrong ◽  
Rose M. Morra ◽  
Brian M. Powell
Keyword(s):  
Phase Transitions ◽  
Nuclear Quadrupole Resonance ◽  
Goodness Of Fit ◽  
Structural Phase ◽  
Spectral Characteristics ◽  
Neutron Diffraction Study ◽  
Structural Phase Transitions ◽  
Quadrupole Resonance ◽  
Nuclear Quadrupole

Neutron-powder-diffraction profiles are reported for the antifluorite K2PtBr6 at temperatures of 296, 160, 141, and 120 K. Bromine nuclear-quadrupole-resonance measurements have shown that each of these temperatures corresponds to a different crystal structure and have provided information on the most probable structures. A variety of structural models with symmetries obtained from a group theoretical classification of structural phase transitions are fitted to the diffraction data. A comparison of goodness-of-fit parameters for the models considered at each temperature allows the space group of each phase to be identified. The results provide (i) a definitive test of the validity of a classification scheme proposed for nuclear-quadrupole-resonance spectral characteristics, and (ii) support for the proposition that the allowable, continuous distortions of a crystal are given by the nondisruptive criterion rather than the more restrictive group–subgroup criterion.

NQR study of the structural phase transitions in a system with reduced effective dimensionality: K2ReCl6

1983 ◽  
Vol 61 (10) ◽  
pp. 1374-1381 ◽  
Author(s):  
Marie D'Iorio ◽  
Robin L. Armstrong
Keyword(s):  
Fourier Transform ◽  
Phase Transitions ◽  
Hydrostatic Pressure ◽  
Magnetic Resonance ◽  
Structural Phase ◽  
Nonlinear Phenomena ◽  
Structural Phase Transitions ◽  
Quadrupole Resonance ◽  
Nuclear Quadrupole ◽  
Effective Dimensionality

Chlorine 35 nuclear quadrupole resonance (NQR) frequency and lineshape data, obtained with a Fourier transform magnetic resonance spectrometer, are presented for the antifluorite crystal K2ReCl6. In the temperature range investigated, 85 to 130 K, two structural phase transitions occur which reduce the crystal symmetry from cubic to tetragonal to monoclinic. The variation of the NQR spectrum with temperature and with hydrostatic pressure up to 2.64 kbar is documented. The structure of the spectrum and the intensity distribution within it are unexpected from previous studies of antifluorite systems. An explanation of the results is given which is based on recent discussions of nonlinear phenomena at phase transitions in crystals exhibiting reduced effective dimensionality.

Effect of hydrostatic pressure on the structural phase transition in K2OsCl6:a35Cl nuclear quadrupole resonance

1987 ◽  
Vol 65 (2) ◽  
pp. 134-137 ◽  
Author(s):  
Marcin Krupski ◽  
Robin L. Armstrong ◽  
Mariusz Maćkowiak ◽  
Maria Zdanowska-Fraczek
Keyword(s):  
Phase Transition ◽  
Nuclear Quadrupole Resonance ◽  
Structural Phase Transition ◽  
Structural Phase ◽  
Pressure Coefficient ◽  
Nuclear Quadrupole Resonance Frequency ◽  
Rigid Sphere ◽  
Cubic Phase ◽  
Quadrupole Resonance ◽  
Nuclear Quadrupole

The 35Cl nuclear quadrupole resonance frequency is reported in K2OsCl6 at temperatures in the vicinity of the structural phase transition for three externally applied hydrostatic pressures. The phase-transition temperature, Tc, decreases linearly with pressure, and the pressure coefficient, (dTc/dP), is measured to be −(2.2 ± 0.3) × 10−2 K∙MPa−1. This value is compared with that measured for K2ReCl6 and is consistent with the prediction of a calculation based on a modified rigid-sphere model. Measurements of (∂v/∂P)T show precursor behaviour as Tc is approached from the high-temperature cubic phase.

35Cl and 37Cl Pure Quadrupole Resonance in α-CH2ClCOOH: Hydrogen Bonding and Chlorine Isotope Effect

1978 ◽  
Vol 33 (1) ◽  
pp. 74-77 ◽  
Author(s):  
Helmut Rager
Keyword(s):  
Crystal Structure ◽  
Hydrogen Bonding ◽  
Temperature Dependence ◽  
Bond Length ◽  
Nuclear Quadrupole Resonance ◽  
Isotope Effect ◽  
Unit Cell ◽  
Chlorine Atoms ◽  
Quadrupole Resonance ◽  
Nuclear Quadrupole

The temperature dependence of the pure nuclear quadrupole resonance of 35Cl and 37Cl in solid α-CH2ClCOOH has been measured from 77 K to 298 K. The NQR spectrum consists of a doublet arising from the two nonequivalent chlorine atoms in the unit cell. The temperature dependence was calculated using the Bayer-Kushida theory which gives a satisfactory fit to the NQR spectra in the temperature range investigated. From the NQR data evidence of an isotope effect was found for both nonequivalent chlorine atoms. The isotope effect depends on the C-Cl bond length and its direction probably on the state of binding of the chlorine atoms. The NQR results and their interpretation are consistent with the crystal structure of α- CH2ClCOOH.

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