H-1 NMR study of molecular dynamics and phase transitions in (CH3NH3) 2PbBr6 and [N(CH3)(4)](2)SeCl6.

The proton spin-lattice relaxation time (T-1) has been measured in methylammonium hexabromoplumbate, (CH3NH3)(2)PbBr6, in the range 77-450K and tetramethylammonium (TMA) hexachloroselenate, [N(CH3)(4) ](2)SeCl6 in the range 77-430K In (CH3NH3)(2)PbBr6, T-1 shows a slope change at 415K, a minimum at 215K, and a discontinuous jump at 167K In [N(CH3)(4)](2)SeCl6, T-1 shows a slope change at 340K and goes through a narrow symmetric minimum at 194K. Further, it exhibits a shoulder-like structure at 106K and goes through a broad minimum at 85K. Motional parameters such as activation energy and preexponential factor have been evaluated for the reorientational motion of CH3-NH3 groups and the TMA ion in different phases

Structuring from nanoparticles in oil-based ferrofluids

The effect of magnetic field on the structure formation in an oil-based magnetic fluid with various concentrations of magnetite particles was studied. The evaluation of the experimental data obtained from small-angle X-ray scattering and ultrasonic attenuation indicates the formation of chain-like aggregates composed of magnetite particles. The experimental data obtained from ultrasonic spectroscopy fit well with the recent theoretical model by Shliomis, Mond and Morozov but only for a diluted magnetic fluid. In this model it is assumed that a dimer is the main building block of a B-field-induced chain-like structure, thus the estimation of the nematic order parameter does not depend on the actual length of the structure. The scattering method used reveals information about the aggregated structure size and relative changes in the degree of anisotropy in qualitative terms. The coupling constant lambda, concentrations phi, average particle size d and its polydispersity sigma were initially obtained using the vibrating sample magnetometry and these results were further confirmed by rheometry and scattering methods. Both the particles' orientational distribution and the nematic order parameter S were inferred from the ultrasonic measurements. The investigation of SAXS patterns reveals the orientation and sizes of aggregated structures under application of different magnetic-field strengths. In addition, the magnetic-field-dependent yield stress was measured, and a relationship between the yield stress and magnetic-field strength up to 0.5T was established.<p> </p><p>© The Author(s) 2011. This article is published with open access at Springerlink.com</p