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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. - PowerPoint PPT Presentation
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Reverse Monte Carlo modelling of cation distributions in crystal structures
Hui Qun 惠群
Why Neutron Total Scattering?
max
2
Qr
◆ Bragg+diffuse scatterings
◆ Time-of-flight (TOF) to obtain high Q )
sin4(
◆ High spatial resolution
Quantitatively understand neutron total scattering data 1
VN
dQQrQr
QiQrG
drQrQr
rGrQi
rG
Qi
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and
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240)(
:ansformsFourier tr by the )(function on distributi radial total the torelated
andfactor structure scattering- total theis )(
Quantitatively understand neutron total scattering data 2
nmnnm
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mn
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atomic finding ofty probabliti the tells)(
RMC applied to the study of crystalline materials
Generalising 2
2
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: The quality of the fit of the experimental to the data simulated from the structural model
2
Flowchart of the RMC modelling
Modelling cation distribution –atom swap mode in RMC
Testing
SrTiO3 configurations
Corresponding T
(r) and n(r)
Corresponding T
(r) and n(r)
SrTiO3 configurations
Corresponding T
(r) and n(r)
SrTiO3 configurations
SrTiO3 configurations
Corresponding T
(r) and n(r)
Perovskites ABO3
“A” site: Na+, K+, Pb2+, Ba2+, Mg2+, Ca2+, Sr2+
“B” site: Ti4+, Zr4+, Nb5+
A
B
Structure and cation ordering in Ca0.5Sr0.5TiO3 at room temperature
different Initial configurations: Configurations with Pbnm space group (1 type
of “A” site): (1) Ca and Sr are arranged randomly (2) Ca and Sr are clustered at each half of the
configuration box (3) Ca sheet and Sr sheet are alternatively
arranged. Configurations with Bmmb space group (2
types of “B” site) (1) Ca and Sr are positioned at the two types of
“A” sites, respectively (2) (2) Ca and Sr are arranged randomly at the
two types of “A” sites
Analysis of RMC configurations – the Ca/Sr ordering 1
Calculations of n(r) show Ca/Sr ordering at the “A” site
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)
Analysis of RMC configurations – the Ca/Sr ordering 3
Illustrated via the total distribution function
Ordered config
Disordered config
Comparison
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
More…
Work have done: (1) other compositions of Ca1-xSrxTiO3
(2) PbZr1-xTixO3(PZT) cation distribution at “B” site Applications in wider area(1) More complex system such as (Mg,Fe)(Si,Al)O3
(2) Metal alloys(3) Deduce atom/vacancy short-range ordering in
solid solutions having vacancies in their structures e.g. SrTixFe1-xO5.5+0.5x
Summary
Development, testing and applicationWorks well and gives unique information on
cation distribution