08-液体结构的XAFS

液体结构的
XAFS 探测
李晨曦
中国科学院物理研究所
1. Introduction
Understanding of the properties of liquids is of fundamental importance for science, technology, production and life itself.
The fundamental problem in the liquid theory is to understand the macroscopic properties and structure in terms of interparticle interactions..
There is no a tractable mathematical method to give an accurate approximation to the liquid state to cover both the structural and thermal properties.
Four types of information:
the equilibrium properties;
the transport properties;
the macroscopic data;
the microscopic data.
Firstly, the positions and energies of the atoms are important.
Secondly, the motion of the atoms
is the significant feature.
Structure of liquids
is a basic issue of liquid state physics.
3. Extraction of message
from measured structure data The measured diffraction data:
(1) to calculate the structure factor S(q);
(2) to calculate pair correlation function g(r) from S(q) by FT;
(3) to recognize the features in S(q) and g(r);
(4) to calculate the coordination number.
Fig.3 Normalized g(r) around Rb atoms in RbCl at 1023K and around Rb atom in Rb2ZnCl4 at 823K
Fig.4 (a) amorphous model pair correlation function (b) obtained from EXAFS data by regularization method
Calculate χc (k )
of the configuration
via g ij (r )
dr r k r g k j a
ij
j
ij c ),()()(χχ∑∫=
New configuration model
Move one atom with random
distance to random direction
satisfying some restriction,
e.g. closest atomic distance
For molecular system
move a whole molecule together
some rotations and individual atomic moves depending on the molecular symmetry
Under convergence condition, a series of independent configuration models are obtained.
statistic properties for the sample
can be carried out.
(1) Bond-angle distribution
Bonds : vectors joining any two atoms
not imply any chemical bounding The bond angle cosine is illustrated of the dominating local symmetry.
(2). Pair correlation function
From the simulated structure models, the averaged pair correlation and partial pair correlation functions can be obtained statistically.
The coordination distribution
All possible positions of atoms around a central atom
The shortest bond length is defined as the first minimum in the distribution function
The coordination
number distribution
of liquid GaSb
heterocoordination in the liquid state coordination number is 4 in the solid state
(6) XANES calculations
use the independent structural models
to calculate the averaged XANES spectrum to explore the electronic structure.
(7) Electronic structure analyzing
Using the structural models, we
can also study the electronic
structures by calculating the
density of states.
5 conclusion
XAFS is resultful for measurement of liquid structure
We should not stop at measured structural data. We can obtain more messages for understanding liquid structure.
We can use the three-dimensional model to get more insight of the properties liquids.
谢谢大家!。