Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

4C/2003G196

Valence fluctuated state in Yb4As3 studied by resonant x-ray scattering
Hironori Nakao, Kouhei Kiyoto, Soukichi Kodama, Daisuke Bizen, Takeshi Matsumura,Kazuaki Iwasa, Youichi Murakami, Akira Ochiai
Department of Physics, Graduate school of Science and Faculty of Science, Tohoku University
Introduction
In rare earth compounds, valence fluctuation (VF) is
one of the anomalous phenomena due to the instability of4f electrons, and has attracted much attention. Yb4As3, anexample of VF compounds, has a cubic anti-Th3P4
structure with space group I-43d and the lattice constant,a = 8.788Å[1]. Only one kind of Yb ion
crystallographically exists in the structure and the valence
should be the mean value, +2.25, although the two kinds
2+3+of valence states, Yb and Yb in the ratio 3:1, haveobserved by the x-ray absorption spectra at Yb L3-edge
and the photoemission [2]. As a result, an inhomogeneousmixed valence state of the thermally fluctuated valence isexpected at room temperature. At TCO ~ 290 K, there is afirst-order structural phase transition from the cubicstructure to a trigonal one with space group R3c. In the3+transition, the magnetic Yb ions occupy one of four
equivalent [111] directions with the trigonal distortion.
The other three directions are shared by the nonmagnetic
2+
Yb ions. The charge ordering of the Yb ion in Yb4As3 hasbeen clearly determined by the resonant x-ray scattering(RXS) technique. The resonant signal at (3 -3 0)reflecting the difference of the atomic scattering factors
3+2+
between Yb and Yb was observed, and the temperaturedependence of the order parameter of the charge ordering
was measured [3]. On the other hand, the VF state above
TCO has never been measured, although the short-rangeorder of the charge ordering (SRO-CO) is expected by thespecific heat [4]. Therefore, we have studied not only thevalence state but also crystal structure in the VF stateabove TCO using RXS and non-resonant x-ray scattering(non-RXS), respectively.
ExperimentHigh-quality Yb4As3 single crystals were grown as inref.[1]. To observe the strong RXS signal at (3 -3 0)reflection positions, (1 1 0) surfaces were polished withfine emery paper. The diffraction experiments wereperformed using a four-circle diffractometer equippedwith a closed-cycle helium refrigerator at beam line 4C of
the Photon Factory. The incident beam wasmonochromatized by a pair of Si(111) crystals, giving anenergy resolution of about 2 eV, and focused by a bent
cylindrical mirror. The x-ray energy near the Yb L3-edge(2p-5d transition energy, ~8.94 keV) was used for theRXS experiments. A Hamamatsu Photonics chargecoupled device (CCD) camera was also used for thisexperiment.

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Users' Report

Results
To elucidate the VF state in Yb4As3, the temperaturedependence of the valence state of Yb ion and the crystal
structure reflecting the valence state were measured by
RXS and non-RXS near the TCO. At the TCO, not only
lattice constants but also the domain structure drasticallychange. The chasing the all peak positions and intensities
using the scintillation counter is difficult, and needs longexperimental time. Therefore, an oscillation photographmethod using the CCD camera was selected to measure
the overview of the temperature dependence.

The oscillation photograph at T=291 K near thefundamental peak (2 2 0) is shown in the figure. The (2 20)H of the cubic structure and (2 2 0)L, (2 –2 0)L of thetrigonal one coexist clearly owing to this first-order phasetransition. We have also measured the RXS at (3 –3 0).
Finally, it becomes clear that the trigonal structure of thecharge-ordering state survives as a SRO-CO in the cubicstructure of the charge disordered state. The SRO-CO is
expected to be the fully charge ordered state, while itaccompanies few atomic displacement due to the chargeordering. The correlation length of the SRO-CO gradually
decreases with increasing temperature like a second ordertransition, and still survives even at 320K. These are firstobservation of the spatial correlation of the VF state, andthis success with typical VF compound, Yb4As3, will be
crucial in the study of the VF states.
References[1] A. Ochiai et al., J. Phys. Soc. Jpn. 59 (1990) 4129.
[2] N. Mori et al., Physica B 230-232 (1997) 630;
S. Suga et al., J. Phys. Soc. Jpn. 67 (1998) 3552.
[3] U. Staub et al., Phys. Rev. B 71 (2005) 075115;
U. Staub et al., Europhys. Lett. 53 (2001) 72.
[4] A. Ochiai et al., Physica B 186-188 (1993) 437.
* nakao@iiyo.phys.tohoku.ac.jp