Magnetic resonance and neutron scattering studies of structural phase transitions in antifluorite crystals

Many cubic antifluorite (R2MX6) crystals exhibit structural phase transitions. These structural deformations and their dynamics have been studied using nuclear quadrupole resonance (NQR) and neutron scattering techniques. Group theoretical considerations have been employed to develop a classification scheme based on NQR spectral characteristics to identify the possible and most-probable symmetries of the distorted structures with the assumption that the various phases are linked by group-subgroup relationships. Neutron diffraction experiments have been carried out to validate this classification scheme. The effect of hydrostatic pressure on the phase transition temperature Tc has been investigated in K2ReCl6 and K2OsCl6 using NQR and the results interpreted. The soft-mode dynamics of those crystals that undergo a rotative phase transition from the cubic to a tetragonal phase has been studied and the longitudinal rotary-lattice mode has been identified as the soft-mode. Neutron diffraction data have been acquired in both phases in K2OsCl6 and discussed in terms of a rigid-ion lattice dynamical model. Non-linear dynamical effects were first seen in NQR measurements of chlorine spin lattice relaxation in the cubic phase of K2OsCl6. Subsequently these effects have been observed in the NQR frequency spectrum and the neutron powder-diffraction profile. The totality of results suggests that although the development of dynamic short-range order above Tc is an intrinsic effect, the detailed manifestation of this phenomenon is extrinsic in origin, being governed by the presence of crystal defects. (author). 27 refs.; 16 figs.; 1 tab