The relations between structural phase transitions, little groups and orbits and nuclear quadrupole resonance

1982 ◽  
Vol 83 ◽  
pp. 167-180 ◽  
Author(s):  
Robin L. Armstrong ◽  
Mark Sutton
1985 ◽  
Vol 63 (7) ◽  
pp. 988-996 ◽  
Author(s):  
Robin L. Armstrong ◽  
Rose M. Morra ◽  
Brian M. Powell

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.


1983 ◽  
Vol 61 (10) ◽  
pp. 1374-1381 ◽  
Author(s):  
Marie D'Iorio ◽  
Robin L. Armstrong

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.


1983 ◽  
Vol 61 (7) ◽  
pp. 997-1002 ◽  
Author(s):  
Robin L. Armstrong ◽  
Rose M. Morra ◽  
Brian M. Powell ◽  
William J. L. Buyers

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.


1991 ◽  
Vol 46 (4) ◽  
pp. 367-375 ◽  
Author(s):  
Jutta Hartmann ◽  
Alarich Weiss

Abstract The 79Br and 127I nuclear quadrupole resonance, NQR, spectra of ortho-substituted anilinium bromides and iodides 2-RC6H4NH3⊕ X⊖ with X = Br, I and R = CI, CN, C2H5, NH2, and NH3⊕ X⊖ have been investigated in the temperature range 77 < T/K <420. Phase transitions occur in 2-(C2H5)C6H4NH3⊕ Br⊖ ½H2O at Tc= 164 K. in 2-(C2H5)C6H4NH3⊕ I⊖ at Tc = 214 K, in [1,2-C6H4(N(H,D)3)-]2⊕[Br⊖]2 at Tc = 209 K, and in [1,2-C6H4(NH3)2]2⊕[I⊖]2 at Tc = 173 K. The NQR data are discussed and compared with NQR spectra of para-substituted anilinium halides


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