Erdong Wu

Modelling Dislocation‐Induced Anisotropic Line Broadening in Rietveld Refinements Using a Voigt Function. II. Application to Neutron Powder Diffraction Data

In paper I [Wu et al. (1998). J. Appl. Cryst. 31, 356–362] an approach was developed to the problem of modelling dislocation-induced X-ray or neutron-diffraction-line broadening. This paper applies those findings to the Rietveld refinement of the neutron powder diffraction profiles of deuterium-cycled LaNi5 and β-PdD0.66. These interstitially modified materials exhibit, respectively, strong and weak anisotropic strain broadening. The broadening in LaNi5 is consistent with a dislocation slip system a/3 (\bar 2110) {0\bar 110}, in agreement with transmission electron microscopy studies. In PdD0.66 the model predicts a regular distribution of screw dislocations, which remains to be confirmed by other techniques.

Modelling dislocation-induced anisotropic line broadening in Rietveld refinements using a Voigt function. I. General principles

The theory of dislocation-induced X-ray or neutron diffraction line broadening developed by Krivoglaz et al. and Wilkens has been adapted for Rietveld refinement of the powder diffraction profile by fitting a Voigt function to each peak. Information on both the type of slip system and the density of dislocations in the crystallites may then be found by evaluating the shape parameter and the index-dependent breadth of the Voigt function.

In-situ neutron powder diffraction study of annealing activated LaNi5

Abstract In-situ neutron powder diffraction measurements and the Rietveld refinement technique have been used to study a fully hydrogen activated LaNi 5 alloy during annealing in the temperature range 293 to 1123 K. Diffraction data were analysed by imposing a dislocation-induced anisotropic line broadening model, to reveal the variation of dislocation density and defect structure as a function of temperature. The activated sample contained predominantly edge dislocations with Burgers vector a /3〈−2110〉 on prismatic {0−110} slip planes associated with medium to long ranged strain fields ( M ∼2–3). A small proportion of dislocations (∼10%) with the same Burger vectors on basal {0001} slip planes are also indicated by the refinements. This second type of dislocation appears to be more resistant to annealing than the dislocations on prismatic slip planes. A progressive decrease in dislocation density from ∼6×10 11 /cm 2 to the limit of the resolution of our measurement (≲10 10 /cm 2 ) was observed between 293 and 973 K. This annealing appears to cause the restoration of the large pressure hysteresis characteristic of unactivated LaNi 5 . The most significant reduction in dislocation density occurs at ∼800 K. In contrast, TGA measurements show the release of trapped hydrogen at ∼500 K. The latter feature is associated with an anomaly in the c / a ratio. The results suggest that, in addition to primary defects in the form of dislocations, a different class of defects such as vacancies and small dislocation loops may also exist in hydrogen activated LaNi 5 . Further, whereas the former are undoubtedly associated with observed changes in the pressure hysteresis during activation, the latter are likely to be the favoured site for hydrogen trapping in activated LaNi 5 .

Ordering of deuterium in PdD0.65 at 54 K

Neutron powder diffraction studies on PdD0.65 demonstrate that the 50 K anomaly is due to an order-disorder transition of deuterium within octahedral interstitial sites. The slow transition to the ordered phase involves diffusion of deuterium from the nearest-neighbour deuterium positions to the second-nearest-neighbour positions. The ordered crystal structure remains cubic, and is accurately described in space group Pm3n by doubling the cell constant relative to the disordered structure (space group Fm3m).

Applications of In Situ Neutron Diffraction to Optimisation of Novel Materials Synthesis

For almost a decade the development of ultra-fast, high-flux neutron diffractometers has largely exceeded the experimental requirements of most users. Fortunately, in recent years the unique capabilities of these instruments have become more widely recognised and they are being applied as a reliable means of kinetic analysis. When combined with PSDs capable of a wide angular range (5–160 ∘ 2θ) and very fine time-resolution ( < 80 ms), high-flux neutron diffractometers begin to emerge as an industrially relevant technique in the design, characterisation and certification of advanced materials. The ability to implement such detailed analysis has been significantly aided through the concurrent development of batch Rietveld data processing suites and the Quantitative Phase Analysis (QPA) technique.