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Reverse Monte Carlo modelling of cation distributions in crystal structures

Reverse Monte Carlo modelling of cation distributions in crystal structures. Hui Qun 惠群. Why Neutron Total Scattering?. ◆ Bragg+diffuse scatterings. ◆ Time-of-flight (TOF) to obtain high. ◆ High spatial resolution. Quantitatively understand neutron total scattering data 1.

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Reverse Monte Carlo modelling of cation distributions in crystal structures

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  1. Reverse Monte Carlo modelling of cation distributions in crystal structures Hui Qun 惠群

  2. Why Neutron Total Scattering? ◆ Bragg+diffuse scatterings ◆Time-of-flight (TOF) to obtain high ◆High spatial resolution

  3. Quantitatively understand neutron total scattering data 1

  4. Quantitatively understand neutron total scattering data 2

  5. RMC applied to the study of crystalline materials : The quality of the fit of the experimental to the data simulated from the structural model Generalising

  6. Flowchart of the RMC modelling

  7. Modelling cation distribution –atom swap mode in RMC

  8. Testing Corresponding T(r) and n(r) SrTiO3 configurations

  9. Corresponding T(r) and n(r) SrTiO3 configurations

  10. Corresponding T(r) and n(r) SrTiO3 configurations

  11. SrTiO3 configurations

  12. Corresponding T(r) and n(r)

  13. Perovskites ABO3 “A” site: Na+, K+, Pb2+, Ba2+, Mg2+, Ca2+, Sr2+ “B” site: Ti4+, Zr4+, Nb5+ A B

  14. Structure and cation ordering in Ca0.5Sr0.5TiO3 at room temperature different Initial configurations: • Configurations with Pbnm space group (1 type of “A” site): • Ca and Sr are arranged randomly • Ca and Sr are clustered at each half of the configuration box • Ca sheet and Sr sheet are alternatively arranged. • Configurations with Bmmbspace group (2 types of “B” site) • Ca and Sr are positioned at the two types of “A” sites, respectively • (2) Ca and Sr are arranged randomly at the two types of “A” sites

  15. Analysis of RMC configurations – the Ca/Sr ordering 1 Calculations of n(r) show Ca/Sr ordering at the “A” site

  16. Analysis of RMC configurations –the Ca/Sr ordering 2 Illustrated via the partial distribution functions which involve Ca and Sr atoms g(r) g(r)

  17. Analysis of RMC configurations – the Ca/Sr ordering 3 Illustrated via the total distribution function Ordered config Comparison Disordered config

  18. Structure of Ca0.5Sr0.5TiO3 Ca and Sr atoms occupy two different types of “A” sites and are ordered. A Ca atom is surrounded by about 6 nearest Sr atom and vice versa. The structure is compatible with the space group P21nm

  19. More… • Work have done: • other compositions of Ca1-xSrxTiO3 • PbZr1-xTixO3(PZT) cation distribution at “B” site • Applications in wider area • More complex system such as (Mg,Fe)(Si,Al)O3 • Metal alloys • Deduce atom/vacancy short-range ordering in solid solutions having vacancies in their structures e.g. SrTixFe1-xO5.5+0.5x

  20. Summary • Development, testing and application • Works well and gives unique information on cation distribution

  21. Thank You !

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