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Local structure and disorder in crystalline materials are increasingly recognized as the key to understanding their functional properties. From negative thermal expansion to dielectric response to thermoelectric properties to ionic conductivity, a clear picture of the local atomic arrangements is essential for understanding these phenomena and developing new practical systems. The combination of total scattering and reverse Monte Carlo (RMC) modeling can provide an unprecedented level of structural detail. In this article, we briefly introduce the method and present a short overview of the scientific areas in which RMC has provided important new insights. Finally, we discuss how the RMC algorithm can be used to combine inputs from multiple experimental techniques, thus moving toward a complex modeling paradigm and helping us to fully understand complex functional materials. 429 Annu. Rev. Mater. Res. 2014.44:429-449. Downloaded from www.annualreviews.org by Dalhousie University on 07/08/14. For personal use only.

Abstract

a b s t r a c tIn spite of its influence on a number of physical properties, short-range order in crystalline alloys has received little recent attention, largely due to the complexity of the experimental methods involved. In this work, a novel approach that could be used for the analysis of ordering transitions and short-range order in crystalline alloys using total scattering and reverse Monte Carlo (RMC) refinements is presented. Calculated pair distribution functions representative of different types of short-range order are used to illustrate the level of information contained within these experimentally accessible functions and the insight into ordering which may be obtained using this new method. Key considerations in the acquisition of data of sufficient quality for successful analysis are also discussed. It is shown that the atomistic models obtained from RMC refinements may be analysed to identify directly the Clapp configurations that are present. It is further shown how these configurations can be enhanced compared with a random structure, and how their degradation pathways and the distribution of Warren-Cowley parameters, can then be used to obtain a detailed, quantitative structural description of the short-range order occurring in crystalline alloys.

Cu 3 Au is often cited as a case example of a metallic system exhibiting both short-range order in the solid solution phase and a long-range order-disorder transition. In this work, X-ray total scattering data obtained from the in situ heating of a gas-atomised powder sample of Cu 3 Au are used to demonstrate the suitability of total scattering, in conjunction with large-box modelling, for the analysis of short-range order in alloys. The existence of an ordering transition at c. 400 • is confirmed, and the development of short-range order reminiscent of the L1 2 long-range ordered structure is observed prior to this transition. Furthermore, it is found that a degree of short-range order is present even in quenched samples (usually assumed to be completely random) which throws into question the identification of short-range order in previous ex situ studies. It is demonstrated that total scattering can be used successfully to identify the type and degree of ordering, differences in the bond length distributions in the first coordination shell and to suggest a likely mechanism for the formation of order in the system.

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