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. Corresponding T(r) and n(r)
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 Bmmb space 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-xO x

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

21. Thank You !