A Rietveld-Approach for the Analysis of Neutron Time-of-Flight Transmission Data
A novel technique to obtain crystallographic information on a sample with a temporal resolution in the order of ten seconds is presented. The crystallographic information is derived from neutron time-of-flight (TOF) transmission patterns utilising neutrons traveling through a sample without interaction. The application of a pulsed neutron source provides Bragg-edge transmission patterns over a large wavelength range with sufficient statistics within seconds. This allows to study kinetics of structural phase transitions and chemical reactions that last for much less than one hour. Since no beam path is required for diffracted intensity, equipment for special sample environments, e.g. temperature, atmosphere or pressure, can be designed much easier than for a diffraction experiment. The acquired patterns are naturally based on different laws than conventional diffraction patterns. Thus, the available Rietveld software for powder diffraction data analysis can not be used or even easily adapted for the analysis of transmission data. The large number of spectra acquired during the course of a phase transition, typically in the order of 1000, requires automated analysis. Consequently, a software 'BETMAn' (Bragg-Edge Transmission Analysis) was developed. BETMAn allows for the first time to apply the Rietveld method to neutrons transmission patterns. BETMAn and its application for studying the mechanisms and kinetics of structural phase transitions and chemical reactions are presented in this work.