Makoto Sakata,a Takafumi Itsubo,a Eiji Nishibori,a Yutakata Moritomo,a Norimichi Kojima,b Yasuo Ohishi,c and Masaki Takataa,c
aDepartment of Applied Physics, Nagoya
University, Chikusa, Nagoya 464-8603, Japan; bGraduate School of Arts and Science, University
of Tokyo, Tokyo 153-8902, Japan; cJapan Synchrotron Research Institute, Kouto,
Mikazuki-cho, Sayo-gun, Hyougo 679-5198, Japan (sakata@cc.nagoya-u.ac.jp)
It is very well
known that physical properties of materials can be drastically changed by
applying pressure, such as emergence of superconductivity of ladder compounds.
It is also very common that materials undergo structural phase transitions when
they start to show very different physical properties under high pressure. It is, therefore, highly desirable to
determine structural changes accurately, preferably at electron density level,
under high pressure. It is, however, not easy task to carry out accurate
structure analysis under high pressure because of experimental restrictions,
which prevent from the collection of an accurate X-ray diffraction data. The
present state-of-art of structure refinements under high pressure is far from
matured.
In order to
produce high-pressure conditions, we have to use high-pressure cell, such as
diamond anvil cell (DAC), which limit the amount of specimen to be used in the
X-ray diffraction experiment to micro-gram order. Under such a condition, there
is no doubt that third generation SR source has a great advantage. In this
study, an accurate structural analysis of Cs2Au2Br6
by using SR powder data collected at BL10XU, Spring-8 will be described. Gold
atoms in Cs2Au2Br6 are a mixed valence state
at ambient pressure.
In this study, a
DAC with large culet is used to have the powder specimen as much as possible.
Because of this, the highest pressure to be reached by the DAC has to be
limited to a few tenth GPa. The specimen was oscillated about 3.5 degree at all
the pressure points, where the experiments were done. The oscillation was very
effective to have homogeneous intensities along Debye ring, which is essential
for the accurate structure analysis like charge density study. The collected
data at 2.2GPa and 8.1Gpa are analysed by MEM/Rietveld analysis[1] to obtain
the experimental charge density distributions.
In the Rietveld
refinements, which is done as a preliminary analysis before MEM, the R-factors
based on integrated Bragg intensities became as
small as 1.28% and 1.05% at 2.2GPa and 8.1% GPa, respectively. It has to be
admitted that the refinements is done satisfactorily to proceed further MEM
analysis. From the MEM charge density map obtained in this study, it is
revealed that the network structure is formed by covalency between Au and Br
atoms at 8.1GPa, while at 2.2GPa Au and Br atoms form clusters. It is also
recognised that the mixed valence state of Au at 2.2GPa transfers to the charge
order states of Au+ and Au3+ at 8.1GPa. The present results may represent the
present state-of-art of charge density studies under high pressure.
References
1
Takata, M.,
Nishibori, E. and Sakata, M. (2001) Z. Kristallogr. 216, 71.